From 75cea03e0385b793a640f3f74237e3caf5060641 Mon Sep 17 00:00:00 2001
From: dianne <diannes.gm@gmail.com>
Date: Sat, 28 Jun 2025 01:37:33 -0700
Subject: [PATCH 01/12] de-duplicate condition scoping logic

---
 compiler/rustc_ast_lowering/src/expr.rs       | 45 ++-----------------
 .../rustc_hir_analysis/src/check/region.rs    | 38 +++++++++++-----
 compiler/rustc_hir_typeck/src/lib.rs          |  6 +--
 3 files changed, 33 insertions(+), 56 deletions(-)

diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index c2140514e3117..9942862a25d07 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -532,7 +532,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         then: &Block,
         else_opt: Option<&Expr>,
     ) -> hir::ExprKind<'hir> {
-        let lowered_cond = self.lower_cond(cond);
+        let lowered_cond = self.lower_expr(cond);
         let then_expr = self.lower_block_expr(then);
         if let Some(rslt) = else_opt {
             hir::ExprKind::If(
@@ -545,44 +545,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
-    // Lowers a condition (i.e. `cond` in `if cond` or `while cond`), wrapping it in a terminating scope
-    // so that temporaries created in the condition don't live beyond it.
-    fn lower_cond(&mut self, cond: &Expr) -> &'hir hir::Expr<'hir> {
-        fn has_let_expr(expr: &Expr) -> bool {
-            match &expr.kind {
-                ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
-                ExprKind::Let(..) => true,
-                _ => false,
-            }
-        }
-
-        // We have to take special care for `let` exprs in the condition, e.g. in
-        // `if let pat = val` or `if foo && let pat = val`, as we _do_ want `val` to live beyond the
-        // condition in this case.
-        //
-        // In order to maintain the drop behavior for the non `let` parts of the condition,
-        // we still wrap them in terminating scopes, e.g. `if foo && let pat = val` essentially
-        // gets transformed into `if { let _t = foo; _t } && let pat = val`
-        match &cond.kind {
-            ExprKind::Binary(op @ Spanned { node: ast::BinOpKind::And, .. }, lhs, rhs)
-                if has_let_expr(cond) =>
-            {
-                let op = self.lower_binop(*op);
-                let lhs = self.lower_cond(lhs);
-                let rhs = self.lower_cond(rhs);
-
-                self.arena.alloc(self.expr(cond.span, hir::ExprKind::Binary(op, lhs, rhs)))
-            }
-            ExprKind::Let(..) => self.lower_expr(cond),
-            _ => {
-                let cond = self.lower_expr(cond);
-                let reason = DesugaringKind::CondTemporary;
-                let span_block = self.mark_span_with_reason(reason, cond.span, None);
-                self.expr_drop_temps(span_block, cond)
-            }
-        }
-    }
-
     // We desugar: `'label: while $cond $body` into:
     //
     // ```
@@ -606,7 +568,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         body: &Block,
         opt_label: Option<Label>,
     ) -> hir::ExprKind<'hir> {
-        let lowered_cond = self.with_loop_condition_scope(|t| t.lower_cond(cond));
+        let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
         let then = self.lower_block_expr(body);
         let expr_break = self.expr_break(span);
         let stmt_break = self.stmt_expr(span, expr_break);
@@ -2095,7 +2057,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
     /// In terms of drop order, it has the same effect as wrapping `expr` in
     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
     ///
-    /// The drop order can be important in e.g. `if expr { .. }`.
+    /// The drop order can be important, e.g. to drop temporaries from an `async fn`
+    /// body before its parameters.
     pub(super) fn expr_drop_temps(
         &mut self,
         span: Span,
diff --git a/compiler/rustc_hir_analysis/src/check/region.rs b/compiler/rustc_hir_analysis/src/check/region.rs
index c458878da15b5..0c575430f0e4f 100644
--- a/compiler/rustc_hir_analysis/src/check/region.rs
+++ b/compiler/rustc_hir_analysis/src/check/region.rs
@@ -176,15 +176,31 @@ fn resolve_block<'tcx>(
     visitor.cx = prev_cx;
 }
 
-fn resolve_arm<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
-    fn has_let_expr(expr: &Expr<'_>) -> bool {
-        match &expr.kind {
-            hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
-            hir::ExprKind::Let(..) => true,
-            _ => false,
-        }
-    }
+/// Resolve a condition from an `if` expression or match guard so that it is a terminating scope
+/// if it doesn't contain `let` expressions.
+fn resolve_cond<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, cond: &'tcx hir::Expr<'tcx>) {
+    let terminate = match cond.kind {
+        // Temporaries for `let` expressions must live into the success branch.
+        hir::ExprKind::Let(_) => false,
+        // Logical operator chains are handled in `resolve_expr`. Since logical operator chains in
+        // conditions are lowered to control-flow rather than boolean temporaries, there's no
+        // temporary to drop for logical operators themselves. `resolve_expr` will also recursively
+        // wrap any operands in terminating scopes, other than `let` expressions (which we shouldn't
+        // terminate) and other logical operators (which don't need a terminating scope, since their
+        // operands will be terminated). Any temporaries that would need to be dropped will be
+        // dropped before we leave this operator's scope; terminating them here would be redundant.
+        hir::ExprKind::Binary(
+            source_map::Spanned { node: hir::BinOpKind::And | hir::BinOpKind::Or, .. },
+            _,
+            _,
+        ) => false,
+        // Otherwise, conditions should always drop their temporaries.
+        _ => true,
+    };
+    resolve_expr(visitor, cond, terminate);
+}
 
+fn resolve_arm<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
     let prev_cx = visitor.cx;
 
     visitor.enter_node_scope_with_dtor(arm.hir_id.local_id, true);
@@ -192,7 +208,7 @@ fn resolve_arm<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, arm: &'tcx hir:
 
     resolve_pat(visitor, arm.pat);
     if let Some(guard) = arm.guard {
-        resolve_expr(visitor, guard, !has_let_expr(guard));
+        resolve_cond(visitor, guard);
     }
     resolve_expr(visitor, arm.body, false);
 
@@ -420,7 +436,7 @@ fn resolve_expr<'tcx>(
             };
             visitor.enter_scope(Scope { local_id: then.hir_id.local_id, data });
             visitor.cx.var_parent = visitor.cx.parent;
-            visitor.visit_expr(cond);
+            resolve_cond(visitor, cond);
             resolve_expr(visitor, then, true);
             visitor.cx = expr_cx;
             resolve_expr(visitor, otherwise, true);
@@ -435,7 +451,7 @@ fn resolve_expr<'tcx>(
             };
             visitor.enter_scope(Scope { local_id: then.hir_id.local_id, data });
             visitor.cx.var_parent = visitor.cx.parent;
-            visitor.visit_expr(cond);
+            resolve_cond(visitor, cond);
             resolve_expr(visitor, then, true);
             visitor.cx = expr_cx;
         }
diff --git a/compiler/rustc_hir_typeck/src/lib.rs b/compiler/rustc_hir_typeck/src/lib.rs
index 1cc618e2aeeca..b3de473acdb83 100644
--- a/compiler/rustc_hir_typeck/src/lib.rs
+++ b/compiler/rustc_hir_typeck/src/lib.rs
@@ -428,11 +428,9 @@ fn report_unexpected_variant_res(
                 }
                 hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Binary(..), hir_id, .. }) => {
                     suggestion.push((expr.span.shrink_to_lo(), "(".to_string()));
-                    if let hir::Node::Expr(drop_temps) = tcx.parent_hir_node(*hir_id)
-                        && let hir::ExprKind::DropTemps(_) = drop_temps.kind
-                        && let hir::Node::Expr(parent) = tcx.parent_hir_node(drop_temps.hir_id)
+                    if let hir::Node::Expr(parent) = tcx.parent_hir_node(*hir_id)
                         && let hir::ExprKind::If(condition, block, None) = parent.kind
-                        && condition.hir_id == drop_temps.hir_id
+                        && condition.hir_id == *hir_id
                         && let hir::ExprKind::Block(block, _) = block.kind
                         && block.stmts.is_empty()
                         && let Some(expr) = block.expr

From fc557bc5a5686ba3330c64b8187d883d94ac163f Mon Sep 17 00:00:00 2001
From: dianne <diannes.gm@gmail.com>
Date: Sun, 29 Jun 2025 20:34:33 -0700
Subject: [PATCH 02/12] clippy: conditions are no longer wrapped in `DropTemps`

---
 src/tools/clippy/clippy_lints/src/booleans.rs |  5 +-
 .../clippy/clippy_lints/src/if_not_else.rs    |  1 -
 .../src/implicit_saturating_add.rs            |  5 +-
 .../clippy/clippy_lints/src/let_if_seq.rs     | 11 +----
 src/tools/clippy/clippy_utils/src/higher.rs   | 49 +++++++------------
 src/tools/clippy/tests/ui/author/if.stdout    |  3 +-
 .../clippy/tests/ui/author/struct.stdout      |  3 +-
 7 files changed, 29 insertions(+), 48 deletions(-)

diff --git a/src/tools/clippy/clippy_lints/src/booleans.rs b/src/tools/clippy/clippy_lints/src/booleans.rs
index bf43234ff50f6..61c2fc49bd70d 100644
--- a/src/tools/clippy/clippy_lints/src/booleans.rs
+++ b/src/tools/clippy/clippy_lints/src/booleans.rs
@@ -1,5 +1,6 @@
 use clippy_config::Conf;
 use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_hir_and_then};
+use clippy_utils::higher::has_let_expr;
 use clippy_utils::msrvs::{self, Msrv};
 use clippy_utils::source::SpanRangeExt;
 use clippy_utils::sugg::Sugg;
@@ -646,7 +647,9 @@ impl<'tcx> Visitor<'tcx> for NonminimalBoolVisitor<'_, 'tcx> {
     fn visit_expr(&mut self, e: &'tcx Expr<'_>) {
         if !e.span.from_expansion() {
             match &e.kind {
-                ExprKind::Binary(binop, _, _) if binop.node == BinOpKind::Or || binop.node == BinOpKind::And => {
+                ExprKind::Binary(binop, _, _)
+                    if binop.node == BinOpKind::Or || binop.node == BinOpKind::And && !has_let_expr(e) =>
+                {
                     self.bool_expr(e);
                 },
                 ExprKind::Unary(UnOp::Not, inner) => {
diff --git a/src/tools/clippy/clippy_lints/src/if_not_else.rs b/src/tools/clippy/clippy_lints/src/if_not_else.rs
index ab7a965b3672b..25fed0d4dd1b2 100644
--- a/src/tools/clippy/clippy_lints/src/if_not_else.rs
+++ b/src/tools/clippy/clippy_lints/src/if_not_else.rs
@@ -51,7 +51,6 @@ declare_lint_pass!(IfNotElse => [IF_NOT_ELSE]);
 impl LateLintPass<'_> for IfNotElse {
     fn check_expr(&mut self, cx: &LateContext<'_>, e: &Expr<'_>) {
         if let ExprKind::If(cond, cond_inner, Some(els)) = e.kind
-            && let ExprKind::DropTemps(cond) = cond.kind
             && let ExprKind::Block(..) = els.kind
         {
             let (msg, help) = match cond.kind {
diff --git a/src/tools/clippy/clippy_lints/src/implicit_saturating_add.rs b/src/tools/clippy/clippy_lints/src/implicit_saturating_add.rs
index 185fc2aa2d4ac..0fdbf67973813 100644
--- a/src/tools/clippy/clippy_lints/src/implicit_saturating_add.rs
+++ b/src/tools/clippy/clippy_lints/src/implicit_saturating_add.rs
@@ -41,8 +41,7 @@ declare_lint_pass!(ImplicitSaturatingAdd => [IMPLICIT_SATURATING_ADD]);
 impl<'tcx> LateLintPass<'tcx> for ImplicitSaturatingAdd {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
         if let ExprKind::If(cond, then, None) = expr.kind
-            && let ExprKind::DropTemps(expr1) = cond.kind
-            && let Some((c, op_node, l)) = get_const(cx, expr1)
+            && let Some((c, op_node, l)) = get_const(cx, cond)
             && let BinOpKind::Ne | BinOpKind::Lt = op_node
             && let ExprKind::Block(block, None) = then.kind
             && let Block {
@@ -66,7 +65,7 @@ impl<'tcx> LateLintPass<'tcx> for ImplicitSaturatingAdd {
             && Some(c) == get_int_max(ty)
             && let ctxt = expr.span.ctxt()
             && ex.span.ctxt() == ctxt
-            && expr1.span.ctxt() == ctxt
+            && cond.span.ctxt() == ctxt
             && clippy_utils::SpanlessEq::new(cx).eq_expr(l, target)
             && AssignOpKind::AddAssign == op1.node
             && let ExprKind::Lit(lit) = value.kind
diff --git a/src/tools/clippy/clippy_lints/src/let_if_seq.rs b/src/tools/clippy/clippy_lints/src/let_if_seq.rs
index 5db28e9ae9b84..e480c8fbed53c 100644
--- a/src/tools/clippy/clippy_lints/src/let_if_seq.rs
+++ b/src/tools/clippy/clippy_lints/src/let_if_seq.rs
@@ -62,15 +62,8 @@ impl<'tcx> LateLintPass<'tcx> for LetIfSeq {
             if let hir::StmtKind::Let(local) = stmt.kind
                 && let hir::PatKind::Binding(mode, canonical_id, ident, None) = local.pat.kind
                 && let hir::StmtKind::Expr(if_) = next.kind
-                && let hir::ExprKind::If(
-                    hir::Expr {
-                        kind: hir::ExprKind::DropTemps(cond),
-                        ..
-                    },
-                    then,
-                    else_,
-                ) = if_.kind
-                && !is_local_used(cx, *cond, canonical_id)
+                && let hir::ExprKind::If(cond, then, else_) = if_.kind
+                && !is_local_used(cx, cond, canonical_id)
                 && let hir::ExprKind::Block(then, _) = then.kind
                 && let Some(value) = check_assign(cx, canonical_id, then)
                 && !is_local_used(cx, value, canonical_id)
diff --git a/src/tools/clippy/clippy_utils/src/higher.rs b/src/tools/clippy/clippy_utils/src/higher.rs
index 6971b488013c4..4e0b00df9508d 100644
--- a/src/tools/clippy/clippy_utils/src/higher.rs
+++ b/src/tools/clippy/clippy_utils/src/higher.rs
@@ -54,7 +54,7 @@ impl<'tcx> ForLoop<'tcx> {
     }
 }
 
-/// An `if` expression without `DropTemps`
+/// An `if` expression without `let`
 pub struct If<'hir> {
     /// `if` condition
     pub cond: &'hir Expr<'hir>,
@@ -66,16 +66,10 @@ pub struct If<'hir> {
 
 impl<'hir> If<'hir> {
     #[inline]
-    /// Parses an `if` expression
+    /// Parses an `if` expression without `let`
     pub const fn hir(expr: &Expr<'hir>) -> Option<Self> {
-        if let ExprKind::If(
-            Expr {
-                kind: ExprKind::DropTemps(cond),
-                ..
-            },
-            then,
-            r#else,
-        ) = expr.kind
+        if let ExprKind::If(cond, then, r#else) = expr.kind
+            && !has_let_expr(cond)
         {
             Some(Self { cond, then, r#else })
         } else {
@@ -198,18 +192,10 @@ impl<'hir> IfOrIfLet<'hir> {
     /// Parses an `if` or `if let` expression
     pub const fn hir(expr: &Expr<'hir>) -> Option<Self> {
         if let ExprKind::If(cond, then, r#else) = expr.kind {
-            if let ExprKind::DropTemps(new_cond) = cond.kind {
-                return Some(Self {
-                    cond: new_cond,
-                    then,
-                    r#else,
-                });
-            }
-            if let ExprKind::Let(..) = cond.kind {
-                return Some(Self { cond, then, r#else });
-            }
+            Some(Self { cond, then, r#else })
+        } else {
+            None
         }
-        None
     }
 }
 
@@ -343,15 +329,7 @@ impl<'hir> While<'hir> {
             Block {
                 expr:
                     Some(Expr {
-                        kind:
-                            ExprKind::If(
-                                Expr {
-                                    kind: ExprKind::DropTemps(condition),
-                                    ..
-                                },
-                                body,
-                                _,
-                            ),
+                        kind: ExprKind::If(condition, body, _),
                         ..
                     }),
                 ..
@@ -360,6 +338,7 @@ impl<'hir> While<'hir> {
             LoopSource::While,
             span,
         ) = expr.kind
+            && !has_let_expr(condition)
         {
             return Some(Self { condition, body, span });
         }
@@ -493,3 +472,13 @@ pub fn get_vec_init_kind<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -
     }
     None
 }
+
+/// Checks that a condition doesn't have a `let` expression, to keep `If` and `While` from accepting
+/// `if let` and `while let`.
+pub const fn has_let_expr<'tcx>(cond: &'tcx Expr<'tcx>) -> bool {
+    match &cond.kind {
+        ExprKind::Let(_) => true,
+        ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
+        _ => false,
+    }
+}
diff --git a/src/tools/clippy/tests/ui/author/if.stdout b/src/tools/clippy/tests/ui/author/if.stdout
index da359866bffc6..dbff55634ea27 100644
--- a/src/tools/clippy/tests/ui/author/if.stdout
+++ b/src/tools/clippy/tests/ui/author/if.stdout
@@ -1,8 +1,7 @@
 if let StmtKind::Let(local) = stmt.kind
     && let Some(init) = local.init
     && let ExprKind::If(cond, then, Some(else_expr)) = init.kind
-    && let ExprKind::DropTemps(expr) = cond.kind
-    && let ExprKind::Lit(ref lit) = expr.kind
+    && let ExprKind::Lit(ref lit) = cond.kind
     && let LitKind::Bool(true) = lit.node
     && let ExprKind::Block(block, None) = then.kind
     && block.stmts.len() == 1
diff --git a/src/tools/clippy/tests/ui/author/struct.stdout b/src/tools/clippy/tests/ui/author/struct.stdout
index 1e8fbafd30c5c..2dedab56dce7d 100644
--- a/src/tools/clippy/tests/ui/author/struct.stdout
+++ b/src/tools/clippy/tests/ui/author/struct.stdout
@@ -2,8 +2,7 @@ if let ExprKind::Struct(qpath, fields, None) = expr.kind
     && fields.len() == 1
     && fields[0].ident.as_str() == "field"
     && let ExprKind::If(cond, then, Some(else_expr)) = fields[0].expr.kind
-    && let ExprKind::DropTemps(expr1) = cond.kind
-    && let ExprKind::Lit(ref lit) = expr1.kind
+    && let ExprKind::Lit(ref lit) = cond.kind
     && let LitKind::Bool(true) = lit.node
     && let ExprKind::Block(block, None) = then.kind
     && block.stmts.is_empty()

From 68d860f8be6ce5de68cb0e9969e8426ceb3ad532 Mon Sep 17 00:00:00 2001
From: dianne <diannes.gm@gmail.com>
Date: Sun, 29 Jun 2025 22:09:30 -0700
Subject: [PATCH 03/12] remove `DesugaringKind::CondTemporary`

---
 compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs |  6 ------
 compiler/rustc_hir_typeck/src/pat.rs           | 12 +-----------
 compiler/rustc_span/src/hygiene.rs             |  7 -------
 3 files changed, 1 insertion(+), 24 deletions(-)

diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
index 58751f232d032..af1fc045ac87f 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
@@ -51,12 +51,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         };
 
         match span.desugaring_kind() {
-            // If span arose from a desugaring of `if` or `while`, then it is the condition
-            // itself, which diverges, that we are about to lint on. This gives suboptimal
-            // diagnostics. Instead, stop here so that the `if`- or `while`-expression's
-            // block is linted instead.
-            Some(DesugaringKind::CondTemporary) => return,
-
             // Don't lint if the result of an async block or async function is `!`.
             // This does not affect the unreachable lints *within* the body.
             Some(DesugaringKind::Async) => return,
diff --git a/compiler/rustc_hir_typeck/src/pat.rs b/compiler/rustc_hir_typeck/src/pat.rs
index 349e72090d3ea..d014883d8acc7 100644
--- a/compiler/rustc_hir_typeck/src/pat.rs
+++ b/compiler/rustc_hir_typeck/src/pat.rs
@@ -24,7 +24,6 @@ use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
 use rustc_session::parse::feature_err;
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::edition::Edition;
-use rustc_span::hygiene::DesugaringKind;
 use rustc_span::source_map::Spanned;
 use rustc_span::{BytePos, DUMMY_SP, Ident, Span, kw, sym};
 use rustc_trait_selection::infer::InferCtxtExt;
@@ -902,16 +901,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // then that's equivalent to there existing a LUB.
         let cause = self.pattern_cause(ti, span);
         if let Err(err) = self.demand_suptype_with_origin(&cause, expected, pat_ty) {
-            err.emit_unless(
-                ti.span
-                    .filter(|&s| {
-                        // In the case of `if`- and `while`-expressions we've already checked
-                        // that `scrutinee: bool`. We know that the pattern is `true`,
-                        // so an error here would be a duplicate and from the wrong POV.
-                        s.is_desugaring(DesugaringKind::CondTemporary)
-                    })
-                    .is_some(),
-            );
+            err.emit();
         }
 
         pat_ty
diff --git a/compiler/rustc_span/src/hygiene.rs b/compiler/rustc_span/src/hygiene.rs
index 29be3b73ee99d..c3080875da803 100644
--- a/compiler/rustc_span/src/hygiene.rs
+++ b/compiler/rustc_span/src/hygiene.rs
@@ -1191,11 +1191,6 @@ impl AstPass {
 /// The kind of compiler desugaring.
 #[derive(Clone, Copy, PartialEq, Debug, Encodable, Decodable, HashStable_Generic)]
 pub enum DesugaringKind {
-    /// We desugar `if c { i } else { e }` to `match $ExprKind::Use(c) { true => i, _ => e }`.
-    /// However, we do not want to blame `c` for unreachability but rather say that `i`
-    /// is unreachable. This desugaring kind allows us to avoid blaming `c`.
-    /// This also applies to `while` loops.
-    CondTemporary,
     QuestionMark,
     TryBlock,
     YeetExpr,
@@ -1230,7 +1225,6 @@ impl DesugaringKind {
     /// The description wording should combine well with "desugaring of {}".
     pub fn descr(self) -> &'static str {
         match self {
-            DesugaringKind::CondTemporary => "`if` or `while` condition",
             DesugaringKind::Async => "`async` block or function",
             DesugaringKind::Await => "`await` expression",
             DesugaringKind::QuestionMark => "operator `?`",
@@ -1253,7 +1247,6 @@ impl DesugaringKind {
     /// like `from_desugaring = "QuestionMark"`
     pub fn matches(&self, value: &str) -> bool {
         match self {
-            DesugaringKind::CondTemporary => value == "CondTemporary",
             DesugaringKind::Async => value == "Async",
             DesugaringKind::Await => value == "Await",
             DesugaringKind::QuestionMark => value == "QuestionMark",

From c39a187c04cf045cf1128a75d0ccb62d0705b347 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 9 Jul 2025 20:00:59 +0000
Subject: [PATCH 04/12] Be a bit more careful around exotic cycles in in the
 inliner

---
 .../rustc_mir_transform/src/inline/cycle.rs   | 44 ++++++++++-------
 ...ne_double_cycle.a.Inline.panic-unwind.diff | 48 +++++++++++++++++++
 ...ne_double_cycle.b.Inline.panic-unwind.diff | 48 +++++++++++++++++++
 tests/mir-opt/inline_double_cycle.rs          | 22 +++++++++
 4 files changed, 146 insertions(+), 16 deletions(-)
 create mode 100644 tests/mir-opt/inline_double_cycle.a.Inline.panic-unwind.diff
 create mode 100644 tests/mir-opt/inline_double_cycle.b.Inline.panic-unwind.diff
 create mode 100644 tests/mir-opt/inline_double_cycle.rs

diff --git a/compiler/rustc_mir_transform/src/inline/cycle.rs b/compiler/rustc_mir_transform/src/inline/cycle.rs
index 93a81f0dca510..7f9234d1dc891 100644
--- a/compiler/rustc_mir_transform/src/inline/cycle.rs
+++ b/compiler/rustc_mir_transform/src/inline/cycle.rs
@@ -64,15 +64,15 @@ fn process<'tcx>(
     typing_env: ty::TypingEnv<'tcx>,
     caller: ty::Instance<'tcx>,
     target: LocalDefId,
-    seen: &mut FxHashSet<ty::Instance<'tcx>>,
+    seen: &mut FxHashMap<ty::Instance<'tcx>, bool>,
     involved: &mut FxHashSet<LocalDefId>,
     recursion_limiter: &mut FxHashMap<DefId, usize>,
     recursion_limit: Limit,
 ) -> bool {
     trace!(%caller);
-    let mut cycle_found = false;
+    let mut reaches_root = false;
 
-    for &(callee, args) in tcx.mir_inliner_callees(caller.def) {
+    for &(callee_def_id, args) in tcx.mir_inliner_callees(caller.def) {
         let Ok(args) = caller.try_instantiate_mir_and_normalize_erasing_regions(
             tcx,
             typing_env,
@@ -81,14 +81,17 @@ fn process<'tcx>(
             trace!(?caller, ?typing_env, ?args, "cannot normalize, skipping");
             continue;
         };
-        let Ok(Some(callee)) = ty::Instance::try_resolve(tcx, typing_env, callee, args) else {
-            trace!(?callee, "cannot resolve, skipping");
+        let Ok(Some(callee)) = ty::Instance::try_resolve(tcx, typing_env, callee_def_id, args)
+        else {
+            trace!(?callee_def_id, "cannot resolve, skipping");
             continue;
         };
 
         // Found a path.
         if callee.def_id() == target.to_def_id() {
-            cycle_found = true;
+            reaches_root = true;
+            seen.insert(callee, true);
+            continue;
         }
 
         if tcx.is_constructor(callee.def_id()) {
@@ -101,10 +104,17 @@ fn process<'tcx>(
             continue;
         }
 
-        if seen.insert(callee) {
+        let callee_reaches_root = if let Some(&c) = seen.get(&callee) {
+            // Even if we have seen this callee before, and thus don't need
+            // to recurse into it, we still need to propagate whether it reaches
+            // the root so that we can mark all the involved callers, in case we
+            // end up reaching that same recursive callee through some *other* cycle.
+            c
+        } else {
+            seen.insert(callee, false);
             let recursion = recursion_limiter.entry(callee.def_id()).or_default();
             trace!(?callee, recursion = *recursion);
-            let found_recursion = if recursion_limit.value_within_limit(*recursion) {
+            let callee_reaches_root = if recursion_limit.value_within_limit(*recursion) {
                 *recursion += 1;
                 ensure_sufficient_stack(|| {
                     process(
@@ -122,17 +132,19 @@ fn process<'tcx>(
                 // Pessimistically assume that there could be recursion.
                 true
             };
-            if found_recursion {
-                if let Some(callee) = callee.def_id().as_local() {
-                    // Calling `optimized_mir` of a non-local definition cannot cycle.
-                    involved.insert(callee);
-                }
-                cycle_found = true;
+            seen.insert(callee, callee_reaches_root);
+            callee_reaches_root
+        };
+        if callee_reaches_root {
+            if let Some(callee_def_id) = callee.def_id().as_local() {
+                // Calling `optimized_mir` of a non-local definition cannot cycle.
+                involved.insert(callee_def_id);
             }
+            reaches_root = true;
         }
     }
 
-    cycle_found
+    reaches_root
 }
 
 #[instrument(level = "debug", skip(tcx), ret)]
@@ -166,7 +178,7 @@ pub(crate) fn mir_callgraph_cyclic<'tcx>(
         typing_env,
         root_instance,
         root,
-        &mut FxHashSet::default(),
+        &mut FxHashMap::default(),
         &mut involved,
         &mut FxHashMap::default(),
         recursion_limit,
diff --git a/tests/mir-opt/inline_double_cycle.a.Inline.panic-unwind.diff b/tests/mir-opt/inline_double_cycle.a.Inline.panic-unwind.diff
new file mode 100644
index 0000000000000..55da685a3d430
--- /dev/null
+++ b/tests/mir-opt/inline_double_cycle.a.Inline.panic-unwind.diff
@@ -0,0 +1,48 @@
+- // MIR for `a` before Inline
++ // MIR for `a` after Inline
+  
+  fn a() -> () {
+      let mut _0: ();
+      let _1: ();
+      let mut _2: ();
+      let _3: ();
+      let mut _4: ();
++     let mut _5: fn() {a};
++     let mut _6: fn() {b};
++     scope 1 (inlined <fn() {a} as FnOnce<()>>::call_once - shim(fn() {a})) {
++     }
++     scope 2 (inlined <fn() {b} as FnOnce<()>>::call_once - shim(fn() {b})) {
++     }
+  
+      bb0: {
+          StorageLive(_1);
+          StorageLive(_2);
+          _2 = ();
+-         _1 = <fn() {a} as FnOnce<()>>::call_once(a, move _2) -> [return: bb1, unwind continue];
++         StorageLive(_5);
++         _5 = a;
++         _1 = move _5() -> [return: bb1, unwind continue];
+      }
+  
+      bb1: {
++         StorageDead(_5);
+          StorageDead(_2);
+          StorageDead(_1);
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = ();
+-         _3 = <fn() {b} as FnOnce<()>>::call_once(b, move _4) -> [return: bb2, unwind continue];
++         StorageLive(_6);
++         _6 = b;
++         _3 = move _6() -> [return: bb2, unwind continue];
+      }
+  
+      bb2: {
++         StorageDead(_6);
+          StorageDead(_4);
+          StorageDead(_3);
+          _0 = const ();
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/inline_double_cycle.b.Inline.panic-unwind.diff b/tests/mir-opt/inline_double_cycle.b.Inline.panic-unwind.diff
new file mode 100644
index 0000000000000..9e6eef1fa3049
--- /dev/null
+++ b/tests/mir-opt/inline_double_cycle.b.Inline.panic-unwind.diff
@@ -0,0 +1,48 @@
+- // MIR for `b` before Inline
++ // MIR for `b` after Inline
+  
+  fn b() -> () {
+      let mut _0: ();
+      let _1: ();
+      let mut _2: ();
+      let _3: ();
+      let mut _4: ();
++     let mut _5: fn() {b};
++     let mut _6: fn() {a};
++     scope 1 (inlined <fn() {b} as FnOnce<()>>::call_once - shim(fn() {b})) {
++     }
++     scope 2 (inlined <fn() {a} as FnOnce<()>>::call_once - shim(fn() {a})) {
++     }
+  
+      bb0: {
+          StorageLive(_1);
+          StorageLive(_2);
+          _2 = ();
+-         _1 = <fn() {b} as FnOnce<()>>::call_once(b, move _2) -> [return: bb1, unwind continue];
++         StorageLive(_5);
++         _5 = b;
++         _1 = move _5() -> [return: bb1, unwind continue];
+      }
+  
+      bb1: {
++         StorageDead(_5);
+          StorageDead(_2);
+          StorageDead(_1);
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = ();
+-         _3 = <fn() {a} as FnOnce<()>>::call_once(a, move _4) -> [return: bb2, unwind continue];
++         StorageLive(_6);
++         _6 = a;
++         _3 = move _6() -> [return: bb2, unwind continue];
+      }
+  
+      bb2: {
++         StorageDead(_6);
+          StorageDead(_4);
+          StorageDead(_3);
+          _0 = const ();
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/inline_double_cycle.rs b/tests/mir-opt/inline_double_cycle.rs
new file mode 100644
index 0000000000000..cf3b87cf0ad30
--- /dev/null
+++ b/tests/mir-opt/inline_double_cycle.rs
@@ -0,0 +1,22 @@
+// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
+// skip-filecheck
+//@ test-mir-pass: Inline
+//@ edition: 2021
+//@ compile-flags: -Zinline-mir --crate-type=lib
+
+// EMIT_MIR inline_double_cycle.a.Inline.diff
+// EMIT_MIR inline_double_cycle.b.Inline.diff
+
+#![feature(fn_traits)]
+
+#[inline]
+pub fn a() {
+    FnOnce::call_once(a, ());
+    FnOnce::call_once(b, ());
+}
+
+#[inline]
+pub fn b() {
+    FnOnce::call_once(b, ());
+    FnOnce::call_once(a, ());
+}

From 889582e704e516ab22a685711ef593a98fd9021a Mon Sep 17 00:00:00 2001
From: Mu001999 <mu001999@outlook.com>
Date: Sun, 6 Jul 2025 14:07:25 +0800
Subject: [PATCH 05/12] Check assoc consts and tys later like assoc fns

---
 compiler/rustc_passes/src/dead.rs             | 70 ++++++++++++-------
 .../equality-unused-issue-126729.rs           | 44 ++++++++++++
 .../ui/const-generics/cross_crate_complex.rs  |  1 +
 .../missing-bounds.fixed                      |  2 +
 .../missing-bounds.rs                         |  2 +
 .../missing-bounds.stderr                     | 18 ++---
 .../unused-trait-with-assoc-const.rs          | 13 ++++
 .../unused-trait-with-assoc-const.stderr      | 20 ++++++
 .../dead-code/unused-trait-with-assoc-ty.rs   | 11 +++
 .../unused-trait-with-assoc-ty.stderr         | 20 ++++++
 tests/ui/pattern/issue-22546.rs               |  2 +-
 tests/ui/pattern/issue-22546.stderr           | 10 ---
 12 files changed, 169 insertions(+), 44 deletions(-)
 create mode 100644 tests/ui/associated-consts/equality-unused-issue-126729.rs
 create mode 100644 tests/ui/lint/dead-code/unused-trait-with-assoc-const.rs
 create mode 100644 tests/ui/lint/dead-code/unused-trait-with-assoc-const.stderr
 create mode 100644 tests/ui/lint/dead-code/unused-trait-with-assoc-ty.rs
 create mode 100644 tests/ui/lint/dead-code/unused-trait-with-assoc-ty.stderr
 delete mode 100644 tests/ui/pattern/issue-22546.stderr

diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs
index cdfd1a2b07e34..35d6f22fd3b16 100644
--- a/compiler/rustc_passes/src/dead.rs
+++ b/compiler/rustc_passes/src/dead.rs
@@ -18,7 +18,7 @@ use rustc_hir::{self as hir, ImplItem, ImplItemKind, Node, PatKind, QPath, TyKin
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::middle::privacy::Level;
 use rustc_middle::query::Providers;
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::ty::{self, AssocTag, TyCtxt};
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint::builtin::DEAD_CODE;
 use rustc_session::lint::{self, LintExpectationId};
@@ -115,7 +115,10 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
 
     fn handle_res(&mut self, res: Res) {
         match res {
-            Res::Def(DefKind::Const | DefKind::AssocConst | DefKind::TyAlias, def_id) => {
+            Res::Def(
+                DefKind::Const | DefKind::AssocConst | DefKind::AssocTy | DefKind::TyAlias,
+                def_id,
+            ) => {
                 self.check_def_id(def_id);
             }
             _ if self.in_pat => {}
@@ -482,7 +485,7 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
     ) -> bool {
         let trait_def_id = match self.tcx.def_kind(local_def_id) {
             // assoc impl items of traits are live if the corresponding trait items are live
-            DefKind::AssocFn => self
+            DefKind::AssocConst | DefKind::AssocTy | DefKind::AssocFn => self
                 .tcx
                 .associated_item(local_def_id)
                 .trait_item_def_id
@@ -647,6 +650,31 @@ impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
 
         self.in_pat = in_pat;
     }
+
+    fn visit_trait_ref(&mut self, t: &'tcx hir::TraitRef<'tcx>) {
+        if let Some(trait_def_id) = t.path.res.opt_def_id()
+            && let Some(segment) = t.path.segments.last()
+            && let Some(args) = segment.args
+        {
+            for constraint in args.constraints {
+                if let Some(local_def_id) = self
+                    .tcx
+                    .associated_items(trait_def_id)
+                    .find_by_ident_and_kind(
+                        self.tcx,
+                        constraint.ident,
+                        AssocTag::Const,
+                        trait_def_id,
+                    )
+                    .and_then(|item| item.def_id.as_local())
+                {
+                    self.worklist.push((local_def_id, ComesFromAllowExpect::No));
+                }
+            }
+        }
+
+        intravisit::walk_trait_ref(self, t);
+    }
 }
 
 fn has_allow_dead_code_or_lang_attr(
@@ -744,18 +772,12 @@ fn check_item<'tcx>(
                 {
                     worklist.push((local_def_id, comes_from_allow));
                 } else if of_trait {
-                    // FIXME: This condition can be removed
-                    // if we support dead check for assoc consts and tys.
-                    if !matches!(tcx.def_kind(local_def_id), DefKind::AssocFn) {
-                        worklist.push((local_def_id, ComesFromAllowExpect::No));
-                    } else {
-                        // We only care about associated items of traits,
-                        // because they cannot be visited directly,
-                        // so we later mark them as live if their corresponding traits
-                        // or trait items and self types are both live,
-                        // but inherent associated items can be visited and marked directly.
-                        unsolved_items.push((id, local_def_id));
-                    }
+                    // We only care about associated items of traits,
+                    // because they cannot be visited directly,
+                    // so we later mark them as live if their corresponding traits
+                    // or trait items and self types are both live,
+                    // but inherent associated items can be visited and marked directly.
+                    unsolved_items.push((id, local_def_id));
                 }
             }
         }
@@ -791,15 +813,14 @@ fn check_trait_item(
     worklist: &mut Vec<(LocalDefId, ComesFromAllowExpect)>,
     id: hir::TraitItemId,
 ) {
-    use hir::TraitItemKind::{Const, Fn};
-    if matches!(tcx.def_kind(id.owner_id), DefKind::AssocConst | DefKind::AssocFn) {
-        let trait_item = tcx.hir_trait_item(id);
-        if matches!(trait_item.kind, Const(_, Some(_)) | Fn(..))
-            && let Some(comes_from_allow) =
-                has_allow_dead_code_or_lang_attr(tcx, trait_item.owner_id.def_id)
-        {
-            worklist.push((trait_item.owner_id.def_id, comes_from_allow));
-        }
+    use hir::TraitItemKind::{Const, Fn, Type};
+
+    let trait_item = tcx.hir_trait_item(id);
+    if matches!(trait_item.kind, Const(_, Some(_)) | Type(_, Some(_)) | Fn(..))
+        && let Some(comes_from_allow) =
+            has_allow_dead_code_or_lang_attr(tcx, trait_item.owner_id.def_id)
+    {
+        worklist.push((trait_item.owner_id.def_id, comes_from_allow));
     }
 }
 
@@ -1163,6 +1184,7 @@ impl<'tcx> DeadVisitor<'tcx> {
         }
         match self.tcx.def_kind(def_id) {
             DefKind::AssocConst
+            | DefKind::AssocTy
             | DefKind::AssocFn
             | DefKind::Fn
             | DefKind::Static { .. }
diff --git a/tests/ui/associated-consts/equality-unused-issue-126729.rs b/tests/ui/associated-consts/equality-unused-issue-126729.rs
new file mode 100644
index 0000000000000..1482b874b9d69
--- /dev/null
+++ b/tests/ui/associated-consts/equality-unused-issue-126729.rs
@@ -0,0 +1,44 @@
+//@ check-pass
+
+#![feature(associated_const_equality)]
+#![deny(dead_code)]
+
+trait Tr {
+    const I: i32;
+}
+
+impl Tr for () {
+    const I: i32 = 1;
+}
+
+fn foo() -> impl Tr<I = 1> {}
+
+trait Tr2 {
+    const J: i32;
+    const K: i32;
+}
+
+impl Tr2 for () {
+    const J: i32 = 1;
+    const K: i32 = 1;
+}
+
+fn foo2() -> impl Tr2<J = 1, K = 1> {}
+
+mod t {
+    pub trait Tr3 {
+        const L: i32;
+    }
+
+    impl Tr3 for () {
+        const L: i32 = 1;
+    }
+}
+
+fn foo3() -> impl t::Tr3<L = 1> {}
+
+fn main() {
+    foo();
+    foo2();
+    foo3();
+}
diff --git a/tests/ui/const-generics/cross_crate_complex.rs b/tests/ui/const-generics/cross_crate_complex.rs
index d13b69aa0cfb4..b44d889f5e99e 100644
--- a/tests/ui/const-generics/cross_crate_complex.rs
+++ b/tests/ui/const-generics/cross_crate_complex.rs
@@ -11,6 +11,7 @@ async fn foo() {
     async_in_foo(async_out_foo::<4>().await).await;
 }
 
+#[allow(dead_code)]
 struct Faz<const N: usize>;
 
 impl<const N: usize> Foo<N> for Faz<N> {}
diff --git a/tests/ui/generic-associated-types/missing-bounds.fixed b/tests/ui/generic-associated-types/missing-bounds.fixed
index 703d3c1e0fb17..15cdd44d7f1f9 100644
--- a/tests/ui/generic-associated-types/missing-bounds.fixed
+++ b/tests/ui/generic-associated-types/missing-bounds.fixed
@@ -1,5 +1,7 @@
 //@ run-rustfix
 
+#![allow(dead_code)]
+
 use std::ops::Add;
 
 struct A<B>(B);
diff --git a/tests/ui/generic-associated-types/missing-bounds.rs b/tests/ui/generic-associated-types/missing-bounds.rs
index f40b422887311..dad111c8c15cf 100644
--- a/tests/ui/generic-associated-types/missing-bounds.rs
+++ b/tests/ui/generic-associated-types/missing-bounds.rs
@@ -1,5 +1,7 @@
 //@ run-rustfix
 
+#![allow(dead_code)]
+
 use std::ops::Add;
 
 struct A<B>(B);
diff --git a/tests/ui/generic-associated-types/missing-bounds.stderr b/tests/ui/generic-associated-types/missing-bounds.stderr
index 13e4d2498760e..97b88c26e3b38 100644
--- a/tests/ui/generic-associated-types/missing-bounds.stderr
+++ b/tests/ui/generic-associated-types/missing-bounds.stderr
@@ -1,5 +1,5 @@
 error: equality constraints are not yet supported in `where` clauses
-  --> $DIR/missing-bounds.rs:37:33
+  --> $DIR/missing-bounds.rs:39:33
    |
 LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B {
    |                                 ^^^^^^^^^^^^^^^^^^^^^^ not supported
@@ -12,7 +12,7 @@ LL + impl<B: Add> Add for E<B> where B: Add<Output = B> {
    |
 
 error[E0308]: mismatched types
-  --> $DIR/missing-bounds.rs:11:11
+  --> $DIR/missing-bounds.rs:13:11
    |
 LL | impl<B> Add for A<B> where B: Add {
    |      - expected this type parameter
@@ -25,14 +25,14 @@ LL |         A(self.0 + rhs.0)
    = note: expected type parameter `B`
              found associated type `<B as Add>::Output`
 help: the type constructed contains `<B as Add>::Output` due to the type of the argument passed
-  --> $DIR/missing-bounds.rs:11:9
+  --> $DIR/missing-bounds.rs:13:9
    |
 LL |         A(self.0 + rhs.0)
    |         ^^--------------^
    |           |
    |           this argument influences the type of `A`
 note: tuple struct defined here
-  --> $DIR/missing-bounds.rs:5:8
+  --> $DIR/missing-bounds.rs:7:8
    |
 LL | struct A<B>(B);
    |        ^
@@ -42,7 +42,7 @@ LL | impl<B> Add for A<B> where B: Add<Output = B> {
    |                                  ++++++++++++
 
 error[E0308]: mismatched types
-  --> $DIR/missing-bounds.rs:21:14
+  --> $DIR/missing-bounds.rs:23:14
    |
 LL | impl<B: Add> Add for C<B> {
    |      - expected this type parameter
@@ -55,7 +55,7 @@ LL |         Self(self.0 + rhs.0)
    = note: expected type parameter `B`
              found associated type `<B as Add>::Output`
 note: tuple struct defined here
-  --> $DIR/missing-bounds.rs:15:8
+  --> $DIR/missing-bounds.rs:17:8
    |
 LL | struct C<B>(B);
    |        ^
@@ -65,7 +65,7 @@ LL | impl<B: Add<Output = B>> Add for C<B> {
    |            ++++++++++++
 
 error[E0369]: cannot add `B` to `B`
-  --> $DIR/missing-bounds.rs:31:21
+  --> $DIR/missing-bounds.rs:33:21
    |
 LL |         Self(self.0 + rhs.0)
    |              ------ ^ ----- B
@@ -78,7 +78,7 @@ LL | impl<B: std::ops::Add<Output = B>> Add for D<B> {
    |       +++++++++++++++++++++++++++
 
 error[E0308]: mismatched types
-  --> $DIR/missing-bounds.rs:42:14
+  --> $DIR/missing-bounds.rs:44:14
    |
 LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B {
    |      - expected this type parameter
@@ -91,7 +91,7 @@ LL |         Self(self.0 + rhs.0)
    = note: expected type parameter `B`
              found associated type `<B as Add>::Output`
 note: tuple struct defined here
-  --> $DIR/missing-bounds.rs:35:8
+  --> $DIR/missing-bounds.rs:37:8
    |
 LL | struct E<B>(B);
    |        ^
diff --git a/tests/ui/lint/dead-code/unused-trait-with-assoc-const.rs b/tests/ui/lint/dead-code/unused-trait-with-assoc-const.rs
new file mode 100644
index 0000000000000..8259e932c640d
--- /dev/null
+++ b/tests/ui/lint/dead-code/unused-trait-with-assoc-const.rs
@@ -0,0 +1,13 @@
+#![deny(dead_code)]
+
+trait Tr { //~ ERROR trait `Tr` is never used
+    const I: Self;
+}
+
+struct Foo; //~ ERROR struct `Foo` is never constructed
+
+impl Tr for Foo {
+    const I: Self = Foo;
+}
+
+fn main() {}
diff --git a/tests/ui/lint/dead-code/unused-trait-with-assoc-const.stderr b/tests/ui/lint/dead-code/unused-trait-with-assoc-const.stderr
new file mode 100644
index 0000000000000..ec894210f7153
--- /dev/null
+++ b/tests/ui/lint/dead-code/unused-trait-with-assoc-const.stderr
@@ -0,0 +1,20 @@
+error: trait `Tr` is never used
+  --> $DIR/unused-trait-with-assoc-const.rs:3:7
+   |
+LL | trait Tr {
+   |       ^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-trait-with-assoc-const.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: struct `Foo` is never constructed
+  --> $DIR/unused-trait-with-assoc-const.rs:7:8
+   |
+LL | struct Foo;
+   |        ^^^
+
+error: aborting due to 2 previous errors
+
diff --git a/tests/ui/lint/dead-code/unused-trait-with-assoc-ty.rs b/tests/ui/lint/dead-code/unused-trait-with-assoc-ty.rs
new file mode 100644
index 0000000000000..e8116d83ebf1c
--- /dev/null
+++ b/tests/ui/lint/dead-code/unused-trait-with-assoc-ty.rs
@@ -0,0 +1,11 @@
+#![deny(dead_code)]
+
+struct T1; //~ ERROR struct `T1` is never constructed
+
+trait Foo { type Unused; } //~ ERROR trait `Foo` is never used
+impl Foo for T1 { type Unused = Self; }
+
+pub trait Bar { type Used; }
+impl Bar for T1 { type Used = Self; }
+
+fn main() {}
diff --git a/tests/ui/lint/dead-code/unused-trait-with-assoc-ty.stderr b/tests/ui/lint/dead-code/unused-trait-with-assoc-ty.stderr
new file mode 100644
index 0000000000000..ab73c64063431
--- /dev/null
+++ b/tests/ui/lint/dead-code/unused-trait-with-assoc-ty.stderr
@@ -0,0 +1,20 @@
+error: struct `T1` is never constructed
+  --> $DIR/unused-trait-with-assoc-ty.rs:3:8
+   |
+LL | struct T1;
+   |        ^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-trait-with-assoc-ty.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: trait `Foo` is never used
+  --> $DIR/unused-trait-with-assoc-ty.rs:5:7
+   |
+LL | trait Foo { type Unused; }
+   |       ^^^
+
+error: aborting due to 2 previous errors
+
diff --git a/tests/ui/pattern/issue-22546.rs b/tests/ui/pattern/issue-22546.rs
index 81908017b4e64..cf128d185df96 100644
--- a/tests/ui/pattern/issue-22546.rs
+++ b/tests/ui/pattern/issue-22546.rs
@@ -15,7 +15,7 @@ impl<T: std::fmt::Display> Foo<T> {
     }
 }
 
-trait Tr { //~ WARN trait `Tr` is never used
+trait Tr {
     type U;
 }
 
diff --git a/tests/ui/pattern/issue-22546.stderr b/tests/ui/pattern/issue-22546.stderr
deleted file mode 100644
index e067a95e4226c..0000000000000
--- a/tests/ui/pattern/issue-22546.stderr
+++ /dev/null
@@ -1,10 +0,0 @@
-warning: trait `Tr` is never used
-  --> $DIR/issue-22546.rs:18:7
-   |
-LL | trait Tr {
-   |       ^^
-   |
-   = note: `#[warn(dead_code)]` on by default
-
-warning: 1 warning emitted
-

From f8e7813a27f87ab85406bb57e1144acc537ebc7f Mon Sep 17 00:00:00 2001
From: Folkert de Vries <folkert@folkertdev.nl>
Date: Fri, 4 Jul 2025 23:56:16 +0200
Subject: [PATCH 06/12] make `cfg_select` a builtin macro

---
 compiler/rustc_builtin_macros/messages.ftl    |  6 ++
 .../rustc_builtin_macros/src/cfg_select.rs    | 63 ++++++++++++++++
 compiler/rustc_builtin_macros/src/errors.rs   | 18 +++++
 compiler/rustc_builtin_macros/src/lib.rs      |  2 +
 compiler/rustc_expand/src/base.rs             | 20 +++++
 compiler/rustc_expand/src/mbe/macro_rules.rs  | 65 ++++++++++-------
 compiler/rustc_parse/src/parser/cfg_select.rs | 73 +++++++++++++++++++
 compiler/rustc_parse/src/parser/mod.rs        |  6 +-
 compiler/rustc_span/src/symbol.rs             |  1 +
 library/core/src/macros/mod.rs                | 26 ++-----
 library/coretests/tests/macros.rs             | 10 ++-
 tests/auxiliary/minicore.rs                   |  4 +
 tests/ui/macros/cfg_select.rs                 | 27 +++++++
 tests/ui/macros/cfg_select.stderr             | 25 +++++++
 14 files changed, 295 insertions(+), 51 deletions(-)
 create mode 100644 compiler/rustc_builtin_macros/src/cfg_select.rs
 create mode 100644 compiler/rustc_parse/src/parser/cfg_select.rs
 create mode 100644 tests/ui/macros/cfg_select.rs
 create mode 100644 tests/ui/macros/cfg_select.stderr

diff --git a/compiler/rustc_builtin_macros/messages.ftl b/compiler/rustc_builtin_macros/messages.ftl
index 3594c7ec21045..183927edb0220 100644
--- a/compiler/rustc_builtin_macros/messages.ftl
+++ b/compiler/rustc_builtin_macros/messages.ftl
@@ -81,6 +81,12 @@ builtin_macros_cfg_accessible_literal_path = `cfg_accessible` path cannot be a l
 builtin_macros_cfg_accessible_multiple_paths = multiple `cfg_accessible` paths are specified
 builtin_macros_cfg_accessible_unspecified_path = `cfg_accessible` path is not specified
 
+builtin_macros_cfg_select_no_matches = none of the rules in this `cfg_select` evaluated to true
+
+builtin_macros_cfg_select_unreachable = unreachable rule
+    .label = always matches
+    .label2 = this rules is never reached
+
 builtin_macros_coerce_pointee_requires_maybe_sized = `derive(CoercePointee)` requires `{$name}` to be marked `?Sized`
 
 builtin_macros_coerce_pointee_requires_one_field = `CoercePointee` can only be derived on `struct`s with at least one field
diff --git a/compiler/rustc_builtin_macros/src/cfg_select.rs b/compiler/rustc_builtin_macros/src/cfg_select.rs
new file mode 100644
index 0000000000000..2dc387d5866aa
--- /dev/null
+++ b/compiler/rustc_builtin_macros/src/cfg_select.rs
@@ -0,0 +1,63 @@
+use rustc_ast::tokenstream::TokenStream;
+use rustc_attr_parsing as attr;
+use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacroExpanderResult};
+use rustc_parse::parser::cfg_select::{CfgSelectBranches, CfgSelectRule, parse_cfg_select};
+use rustc_span::{Ident, Span, sym};
+
+use crate::errors::{CfgSelectNoMatches, CfgSelectUnreachable};
+
+/// Selects the first arm whose rule evaluates to true.
+fn select_arm(ecx: &ExtCtxt<'_>, branches: CfgSelectBranches) -> Option<(TokenStream, Span)> {
+    for (cfg, tt, arm_span) in branches.reachable {
+        if attr::cfg_matches(
+            &cfg,
+            &ecx.sess,
+            ecx.current_expansion.lint_node_id,
+            Some(ecx.ecfg.features),
+        ) {
+            return Some((tt, arm_span));
+        }
+    }
+
+    branches.wildcard.map(|(_, tt, span)| (tt, span))
+}
+
+pub(super) fn expand_cfg_select<'cx>(
+    ecx: &'cx mut ExtCtxt<'_>,
+    sp: Span,
+    tts: TokenStream,
+) -> MacroExpanderResult<'cx> {
+    ExpandResult::Ready(match parse_cfg_select(&mut ecx.new_parser_from_tts(tts)) {
+        Ok(branches) => {
+            if let Some((underscore, _, _)) = branches.wildcard {
+                // Warn for every unreachable rule. We store the fully parsed branch for rustfmt.
+                for (rule, _, _) in &branches.unreachable {
+                    let span = match rule {
+                        CfgSelectRule::Wildcard(underscore) => underscore.span,
+                        CfgSelectRule::Cfg(cfg) => cfg.span(),
+                    };
+                    let err = CfgSelectUnreachable { span, wildcard_span: underscore.span };
+                    ecx.dcx().emit_warn(err);
+                }
+            }
+
+            if let Some((tts, arm_span)) = select_arm(ecx, branches) {
+                return ExpandResult::from_tts(
+                    ecx,
+                    tts,
+                    sp,
+                    arm_span,
+                    Ident::with_dummy_span(sym::cfg_select),
+                );
+            } else {
+                // Emit a compiler error when none of the rules matched.
+                let guar = ecx.dcx().emit_err(CfgSelectNoMatches { span: sp });
+                DummyResult::any(sp, guar)
+            }
+        }
+        Err(err) => {
+            let guar = err.emit();
+            DummyResult::any(sp, guar)
+        }
+    })
+}
diff --git a/compiler/rustc_builtin_macros/src/errors.rs b/compiler/rustc_builtin_macros/src/errors.rs
index a5ee7349fc68b..6bcf4d3e0a2ee 100644
--- a/compiler/rustc_builtin_macros/src/errors.rs
+++ b/compiler/rustc_builtin_macros/src/errors.rs
@@ -954,3 +954,21 @@ pub(crate) struct AsmExpectedOther {
     pub(crate) span: Span,
     pub(crate) is_inline_asm: bool,
 }
+
+#[derive(Diagnostic)]
+#[diag(builtin_macros_cfg_select_no_matches)]
+pub(crate) struct CfgSelectNoMatches {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(builtin_macros_cfg_select_unreachable)]
+pub(crate) struct CfgSelectUnreachable {
+    #[primary_span]
+    #[label(builtin_macros_label2)]
+    pub span: Span,
+
+    #[label]
+    pub wildcard_span: Span,
+}
diff --git a/compiler/rustc_builtin_macros/src/lib.rs b/compiler/rustc_builtin_macros/src/lib.rs
index 6bf590df5c9da..7bc448a9acb8b 100644
--- a/compiler/rustc_builtin_macros/src/lib.rs
+++ b/compiler/rustc_builtin_macros/src/lib.rs
@@ -33,6 +33,7 @@ mod autodiff;
 mod cfg;
 mod cfg_accessible;
 mod cfg_eval;
+mod cfg_select;
 mod compile_error;
 mod concat;
 mod concat_bytes;
@@ -79,6 +80,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         asm: asm::expand_asm,
         assert: assert::expand_assert,
         cfg: cfg::expand_cfg,
+        cfg_select: cfg_select::expand_cfg_select,
         column: source_util::expand_column,
         compile_error: compile_error::expand_compile_error,
         concat: concat::expand_concat,
diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
index d6d898088395e..757a7e1c8e77f 100644
--- a/compiler/rustc_expand/src/base.rs
+++ b/compiler/rustc_expand/src/base.rs
@@ -34,6 +34,7 @@ use thin_vec::ThinVec;
 use crate::base::ast::MetaItemInner;
 use crate::errors;
 use crate::expand::{self, AstFragment, Invocation};
+use crate::mbe::macro_rules::ParserAnyMacro;
 use crate::module::DirOwnership;
 use crate::stats::MacroStat;
 
@@ -262,6 +263,25 @@ impl<T, U> ExpandResult<T, U> {
     }
 }
 
+impl<'cx> MacroExpanderResult<'cx> {
+    /// Creates a [`MacroExpanderResult::Ready`] from a [`TokenStream`].
+    ///
+    /// The `TokenStream` is forwarded without any expansion.
+    pub fn from_tts(
+        cx: &'cx mut ExtCtxt<'_>,
+        tts: TokenStream,
+        site_span: Span,
+        arm_span: Span,
+        macro_ident: Ident,
+    ) -> Self {
+        // Emit the SEMICOLON_IN_EXPRESSIONS_FROM_MACROS deprecation lint.
+        let is_local = true;
+
+        let parser = ParserAnyMacro::from_tts(cx, tts, site_span, arm_span, is_local, macro_ident);
+        ExpandResult::Ready(Box::new(parser))
+    }
+}
+
 pub trait MultiItemModifier {
     /// `meta_item` is the attribute, and `item` is the item being modified.
     fn expand(
diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
index 89547088f501f..2d792355b2788 100644
--- a/compiler/rustc_expand/src/mbe/macro_rules.rs
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -96,6 +96,30 @@ impl<'a> ParserAnyMacro<'a> {
         ensure_complete_parse(parser, &path, kind.name(), site_span);
         fragment
     }
+
+    #[instrument(skip(cx, tts))]
+    pub(crate) fn from_tts<'cx>(
+        cx: &'cx mut ExtCtxt<'a>,
+        tts: TokenStream,
+        site_span: Span,
+        arm_span: Span,
+        is_local: bool,
+        macro_ident: Ident,
+    ) -> Self {
+        Self {
+            parser: Parser::new(&cx.sess.psess, tts, None),
+
+            // Pass along the original expansion site and the name of the macro
+            // so we can print a useful error message if the parse of the expanded
+            // macro leaves unparsed tokens.
+            site_span,
+            macro_ident,
+            lint_node_id: cx.current_expansion.lint_node_id,
+            is_trailing_mac: cx.current_expansion.is_trailing_mac,
+            arm_span,
+            is_local,
+        }
+    }
 }
 
 pub(super) struct MacroRule {
@@ -207,9 +231,6 @@ fn expand_macro<'cx>(
     rules: &[MacroRule],
 ) -> Box<dyn MacResult + 'cx> {
     let psess = &cx.sess.psess;
-    // Macros defined in the current crate have a real node id,
-    // whereas macros from an external crate have a dummy id.
-    let is_local = node_id != DUMMY_NODE_ID;
 
     if cx.trace_macros() {
         let msg = format!("expanding `{}! {{ {} }}`", name, pprust::tts_to_string(&arg));
@@ -220,7 +241,7 @@ fn expand_macro<'cx>(
     let try_success_result = try_match_macro(psess, name, &arg, rules, &mut NoopTracker);
 
     match try_success_result {
-        Ok((i, rule, named_matches)) => {
+        Ok((rule_index, rule, named_matches)) => {
             let mbe::TokenTree::Delimited(rhs_span, _, ref rhs) = rule.rhs else {
                 cx.dcx().span_bug(sp, "malformed macro rhs");
             };
@@ -241,27 +262,13 @@ fn expand_macro<'cx>(
                 trace_macros_note(&mut cx.expansions, sp, msg);
             }
 
-            let p = Parser::new(psess, tts, None);
-
+            let is_local = is_defined_in_current_crate(node_id);
             if is_local {
-                cx.resolver.record_macro_rule_usage(node_id, i);
+                cx.resolver.record_macro_rule_usage(node_id, rule_index);
             }
 
-            // Let the context choose how to interpret the result.
-            // Weird, but useful for X-macros.
-            Box::new(ParserAnyMacro {
-                parser: p,
-
-                // Pass along the original expansion site and the name of the macro
-                // so we can print a useful error message if the parse of the expanded
-                // macro leaves unparsed tokens.
-                site_span: sp,
-                macro_ident: name,
-                lint_node_id: cx.current_expansion.lint_node_id,
-                is_trailing_mac: cx.current_expansion.is_trailing_mac,
-                arm_span,
-                is_local,
-            })
+            // Let the context choose how to interpret the result. Weird, but useful for X-macros.
+            Box::new(ParserAnyMacro::from_tts(cx, tts, sp, arm_span, is_local, name))
         }
         Err(CanRetry::No(guar)) => {
             debug!("Will not retry matching as an error was emitted already");
@@ -373,9 +380,9 @@ pub fn compile_declarative_macro(
     node_id: NodeId,
     edition: Edition,
 ) -> (SyntaxExtension, usize) {
-    let is_local = node_id != DUMMY_NODE_ID;
     let mk_syn_ext = |expander| {
         let kind = SyntaxExtensionKind::LegacyBang(expander);
+        let is_local = is_defined_in_current_crate(node_id);
         SyntaxExtension::new(sess, kind, span, Vec::new(), edition, ident.name, attrs, is_local)
     };
     let dummy_syn_ext = |guar| (mk_syn_ext(Arc::new(DummyExpander(guar))), 0);
@@ -439,7 +446,7 @@ pub fn compile_declarative_macro(
     }
 
     // Return the number of rules for unused rule linting, if this is a local macro.
-    let nrules = if is_local { rules.len() } else { 0 };
+    let nrules = if is_defined_in_current_crate(node_id) { rules.len() } else { 0 };
 
     let expander =
         Arc::new(MacroRulesMacroExpander { name: ident, span, node_id, transparency, rules });
@@ -1034,9 +1041,7 @@ fn check_matcher_core<'tt>(
                     // definition of this macro_rules, not while (re)parsing
                     // the macro when compiling another crate that is using the
                     // macro. (See #86567.)
-                    // Macros defined in the current crate have a real node id,
-                    // whereas macros from an external crate have a dummy id.
-                    if node_id != DUMMY_NODE_ID
+                    if is_defined_in_current_crate(node_id)
                         && matches!(kind, NonterminalKind::Pat(PatParam { inferred: true }))
                         && matches!(
                             next_token,
@@ -1296,6 +1301,12 @@ fn quoted_tt_to_string(tt: &mbe::TokenTree) -> String {
     }
 }
 
+fn is_defined_in_current_crate(node_id: NodeId) -> bool {
+    // Macros defined in the current crate have a real node id,
+    // whereas macros from an external crate have a dummy id.
+    node_id != DUMMY_NODE_ID
+}
+
 pub(super) fn parser_from_cx(
     psess: &ParseSess,
     mut tts: TokenStream,
diff --git a/compiler/rustc_parse/src/parser/cfg_select.rs b/compiler/rustc_parse/src/parser/cfg_select.rs
new file mode 100644
index 0000000000000..24a05afff4ac9
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/cfg_select.rs
@@ -0,0 +1,73 @@
+use rustc_ast::token::Token;
+use rustc_ast::tokenstream::{TokenStream, TokenTree};
+use rustc_ast::{MetaItemInner, token};
+use rustc_errors::PResult;
+use rustc_span::Span;
+
+use crate::exp;
+use crate::parser::Parser;
+
+pub enum CfgSelectRule {
+    Cfg(MetaItemInner),
+    Wildcard(Token),
+}
+
+#[derive(Default)]
+pub struct CfgSelectBranches {
+    /// All the conditional branches.
+    pub reachable: Vec<(MetaItemInner, TokenStream, Span)>,
+    /// The first wildcard `_ => { ... }` branch.
+    pub wildcard: Option<(Token, TokenStream, Span)>,
+    /// All branches after the first wildcard, including further wildcards.
+    /// These branches are kept for formatting.
+    pub unreachable: Vec<(CfgSelectRule, TokenStream, Span)>,
+}
+
+/// Parses a `TokenTree` that must be of the form `{ /* ... */ }`, and returns a `TokenStream` where
+/// the surrounding braces are stripped.
+fn parse_token_tree<'a>(p: &mut Parser<'a>) -> PResult<'a, TokenStream> {
+    // Generate an error if the `=>` is not followed by `{`.
+    if p.token != token::OpenBrace {
+        p.expect(exp!(OpenBrace))?;
+    }
+
+    // Strip the outer '{' and '}'.
+    match p.parse_token_tree() {
+        TokenTree::Token(..) => unreachable!("because of the expect above"),
+        TokenTree::Delimited(.., tts) => Ok(tts),
+    }
+}
+
+pub fn parse_cfg_select<'a>(p: &mut Parser<'a>) -> PResult<'a, CfgSelectBranches> {
+    let mut branches = CfgSelectBranches::default();
+
+    while p.token != token::Eof {
+        if p.eat_keyword(exp!(Underscore)) {
+            let underscore = p.prev_token;
+            p.expect(exp!(FatArrow))?;
+
+            let tts = parse_token_tree(p)?;
+            let span = underscore.span.to(p.token.span);
+
+            match branches.wildcard {
+                None => branches.wildcard = Some((underscore, tts, span)),
+                Some(_) => {
+                    branches.unreachable.push((CfgSelectRule::Wildcard(underscore), tts, span))
+                }
+            }
+        } else {
+            let meta_item = p.parse_meta_item_inner()?;
+            p.expect(exp!(FatArrow))?;
+
+            let tts = parse_token_tree(p)?;
+            let span = meta_item.span().to(p.token.span);
+
+            match branches.wildcard {
+                None => branches.reachable.push((meta_item, tts, span)),
+                Some(_) => branches.unreachable.push((CfgSelectRule::Cfg(meta_item), tts, span)),
+            }
+        }
+    }
+
+    Ok(branches)
+}
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index cfc0399b0ca96..bc939b8f5a79d 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -1,4 +1,3 @@
-pub mod asm;
 pub mod attr;
 mod attr_wrapper;
 mod diagnostics;
@@ -12,6 +11,11 @@ mod stmt;
 pub mod token_type;
 mod ty;
 
+// Parsers for non-functionlike builtin macros are defined in rustc_parse so they can be used by
+// both rustc_builtin_macros and rustfmt.
+pub mod asm;
+pub mod cfg_select;
+
 use std::assert_matches::debug_assert_matches;
 use std::{fmt, mem, slice};
 
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 09f01d8704e2a..930c245168a3d 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -624,6 +624,7 @@ symbols! {
         cfg_relocation_model,
         cfg_sanitize,
         cfg_sanitizer_cfi,
+        cfg_select,
         cfg_target_abi,
         cfg_target_compact,
         cfg_target_feature,
diff --git a/library/core/src/macros/mod.rs b/library/core/src/macros/mod.rs
index 8035dccc632d4..6b9cbb0643586 100644
--- a/library/core/src/macros/mod.rs
+++ b/library/core/src/macros/mod.rs
@@ -230,32 +230,16 @@ pub macro assert_matches {
 /// ```
 /// #![feature(cfg_select)]
 ///
-/// let _some_string = cfg_select! {{
+/// let _some_string = cfg_select! {
 ///     unix => { "With great power comes great electricity bills" }
 ///     _ => { "Behind every successful diet is an unwatched pizza" }
-/// }};
+/// };
 /// ```
 #[unstable(feature = "cfg_select", issue = "115585")]
 #[rustc_diagnostic_item = "cfg_select"]
-#[rustc_macro_transparency = "semitransparent"]
-pub macro cfg_select {
-    ({ $($tt:tt)* }) => {{
-        $crate::cfg_select! { $($tt)* }
-    }},
-    (_ => { $($output:tt)* }) => {
-        $($output)*
-    },
-    (
-        $cfg:meta => $output:tt
-        $($( $rest:tt )+)?
-    ) => {
-        #[cfg($cfg)]
-        $crate::cfg_select! { _ => $output }
-        $(
-            #[cfg(not($cfg))]
-            $crate::cfg_select! { $($rest)+ }
-        )?
-    },
+#[rustc_builtin_macro]
+pub macro cfg_select($($tt:tt)*) {
+    /* compiler built-in */
 }
 
 /// Asserts that a boolean expression is `true` at runtime.
diff --git a/library/coretests/tests/macros.rs b/library/coretests/tests/macros.rs
index d220e628d7339..fafd3d29aef57 100644
--- a/library/coretests/tests/macros.rs
+++ b/library/coretests/tests/macros.rs
@@ -183,6 +183,12 @@ fn _accepts_expressions() -> i32 {
     }
 }
 
+fn _accepts_only_wildcard() -> i32 {
+    cfg_select! {
+        _ => { 1 }
+    }
+}
+
 // The current implementation expands to a macro call, which allows the use of expression
 // statements.
 fn _allows_stmt_expr_attributes() {
@@ -195,12 +201,12 @@ fn _allows_stmt_expr_attributes() {
 }
 
 fn _expression() {
-    let _ = cfg_select!({
+    let _ = cfg_select!(
         windows => {
             " XP"
         }
         _ => {
             ""
         }
-    });
+    );
 }
diff --git a/tests/auxiliary/minicore.rs b/tests/auxiliary/minicore.rs
index 392ad1cee1425..47dadd51ce0fb 100644
--- a/tests/auxiliary/minicore.rs
+++ b/tests/auxiliary/minicore.rs
@@ -142,6 +142,10 @@ pub macro naked_asm("assembly template", $(operands,)* $(options($(option),*))?)
 pub macro global_asm("assembly template", $(operands,)* $(options($(option),*))?) {
     /* compiler built-in */
 }
+#[rustc_builtin_macro]
+pub macro cfg_select($($tt:tt)*) {
+    /* compiler built-in */
+}
 
 #[rustc_builtin_macro]
 #[macro_export]
diff --git a/tests/ui/macros/cfg_select.rs b/tests/ui/macros/cfg_select.rs
new file mode 100644
index 0000000000000..a4d94836a09a1
--- /dev/null
+++ b/tests/ui/macros/cfg_select.rs
@@ -0,0 +1,27 @@
+#![feature(cfg_select)]
+#![crate_type = "lib"]
+
+fn print() {
+    println!(cfg_select! {
+        unix => { "unix" }
+        _ => { "not unix" }
+    });
+}
+
+fn arm_rhs_must_be_in_braces() -> i32 {
+    cfg_select! {
+        true => 1
+        //~^ ERROR: expected `{`, found `1`
+    }
+}
+
+cfg_select! {
+    _ => {}
+    true => {}
+    //~^ WARN unreachable rule
+}
+
+cfg_select! {
+    //~^ ERROR none of the rules in this `cfg_select` evaluated to true
+    false => {}
+}
diff --git a/tests/ui/macros/cfg_select.stderr b/tests/ui/macros/cfg_select.stderr
new file mode 100644
index 0000000000000..fef5e95a6bce9
--- /dev/null
+++ b/tests/ui/macros/cfg_select.stderr
@@ -0,0 +1,25 @@
+error: expected `{`, found `1`
+  --> $DIR/cfg_select.rs:13:17
+   |
+LL |         true => 1
+   |                 ^ expected `{`
+
+warning: unreachable rule
+  --> $DIR/cfg_select.rs:20:5
+   |
+LL |     _ => {}
+   |     - always matches
+LL |     true => {}
+   |     ^^^^ this rules is never reached
+
+error: none of the rules in this `cfg_select` evaluated to true
+  --> $DIR/cfg_select.rs:24:1
+   |
+LL | / cfg_select! {
+LL | |
+LL | |     false => {}
+LL | | }
+   | |_^
+
+error: aborting due to 2 previous errors; 1 warning emitted
+

From e681d1a9731897951ea4c2d68c7d8f4a322b9904 Mon Sep 17 00:00:00 2001
From: Oli Scherer <github333195615777966@oli-obk.de>
Date: Fri, 11 Jul 2025 08:17:58 +0000
Subject: [PATCH 07/12] constify `From` and `Into`

---
 library/core/src/convert/mod.rs               | 29 +++++++++++++-----
 tests/ui/specialization/issue-111232.rs       |  9 ++++++
 tests/ui/specialization/issue-111232.stderr   |  9 ++++--
 .../non-const-op-in-closure-in-const.rs       |  4 +--
 .../non-const-op-in-closure-in-const.stderr   | 30 -------------------
 5 files changed, 38 insertions(+), 43 deletions(-)
 delete mode 100644 tests/ui/traits/const-traits/non-const-op-in-closure-in-const.stderr

diff --git a/library/core/src/convert/mod.rs b/library/core/src/convert/mod.rs
index 7132e712ec52f..38381dbdf2303 100644
--- a/library/core/src/convert/mod.rs
+++ b/library/core/src/convert/mod.rs
@@ -445,6 +445,8 @@ pub trait AsMut<T: PointeeSized>: PointeeSized {
 #[rustc_diagnostic_item = "Into"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(search_unbox)]
+#[rustc_const_unstable(feature = "const_from", issue = "143773")]
+#[const_trait]
 pub trait Into<T>: Sized {
     /// Converts this type into the (usually inferred) input type.
     #[must_use]
@@ -580,6 +582,8 @@ pub trait Into<T>: Sized {
     note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
 ))]
 #[doc(search_unbox)]
+#[rustc_const_unstable(feature = "const_from", issue = "143773")]
+#[const_trait]
 pub trait From<T>: Sized {
     /// Converts to this type from the input type.
     #[rustc_diagnostic_item = "from_fn"]
@@ -607,6 +611,8 @@ pub trait From<T>: Sized {
 /// [`Into`], see there for details.
 #[rustc_diagnostic_item = "TryInto"]
 #[stable(feature = "try_from", since = "1.34.0")]
+#[rustc_const_unstable(feature = "const_from", issue = "143773")]
+#[const_trait]
 pub trait TryInto<T>: Sized {
     /// The type returned in the event of a conversion error.
     #[stable(feature = "try_from", since = "1.34.0")]
@@ -685,6 +691,8 @@ pub trait TryInto<T>: Sized {
 /// [`try_from`]: TryFrom::try_from
 #[rustc_diagnostic_item = "TryFrom"]
 #[stable(feature = "try_from", since = "1.34.0")]
+#[rustc_const_unstable(feature = "const_from", issue = "143773")]
+#[const_trait]
 pub trait TryFrom<T>: Sized {
     /// The type returned in the event of a conversion error.
     #[stable(feature = "try_from", since = "1.34.0")]
@@ -754,9 +762,10 @@ where
 
 // From implies Into
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, U> Into<U> for T
+#[rustc_const_unstable(feature = "const_from", issue = "143773")]
+impl<T, U> const Into<U> for T
 where
-    U: From<T>,
+    U: ~const From<T>,
 {
     /// Calls `U::from(self)`.
     ///
@@ -771,7 +780,8 @@ where
 
 // From (and thus Into) is reflexive
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> From<T> for T {
+#[rustc_const_unstable(feature = "const_from", issue = "143773")]
+impl<T> const From<T> for T {
     /// Returns the argument unchanged.
     #[inline(always)]
     fn from(t: T) -> T {
@@ -787,7 +797,8 @@ impl<T> From<T> for T {
 #[stable(feature = "convert_infallible", since = "1.34.0")]
 #[rustc_reservation_impl = "permitting this impl would forbid us from adding \
                             `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
-impl<T> From<!> for T {
+#[rustc_const_unstable(feature = "const_from", issue = "143773")]
+impl<T> const From<!> for T {
     fn from(t: !) -> T {
         t
     }
@@ -795,9 +806,10 @@ impl<T> From<!> for T {
 
 // TryFrom implies TryInto
 #[stable(feature = "try_from", since = "1.34.0")]
-impl<T, U> TryInto<U> for T
+#[rustc_const_unstable(feature = "const_from", issue = "143773")]
+impl<T, U> const TryInto<U> for T
 where
-    U: TryFrom<T>,
+    U: ~const TryFrom<T>,
 {
     type Error = U::Error;
 
@@ -810,9 +822,10 @@ where
 // Infallible conversions are semantically equivalent to fallible conversions
 // with an uninhabited error type.
 #[stable(feature = "try_from", since = "1.34.0")]
-impl<T, U> TryFrom<U> for T
+#[rustc_const_unstable(feature = "const_from", issue = "143773")]
+impl<T, U> const TryFrom<U> for T
 where
-    U: Into<T>,
+    U: ~const Into<T>,
 {
     type Error = Infallible;
 
diff --git a/tests/ui/specialization/issue-111232.rs b/tests/ui/specialization/issue-111232.rs
index 3ed3c580e6d12..fa00f01886fb5 100644
--- a/tests/ui/specialization/issue-111232.rs
+++ b/tests/ui/specialization/issue-111232.rs
@@ -1,4 +1,13 @@
 #![feature(min_specialization)]
+#![feature(const_trait_impl)]
+
+trait From<T> {
+    fn from(t: T) -> Self;
+}
+
+impl<T> From<T> for T {
+    fn from(t: T) -> T { t }
+}
 
 struct S;
 
diff --git a/tests/ui/specialization/issue-111232.stderr b/tests/ui/specialization/issue-111232.stderr
index ed392e4f9152b..5f169f0bb36c5 100644
--- a/tests/ui/specialization/issue-111232.stderr
+++ b/tests/ui/specialization/issue-111232.stderr
@@ -1,10 +1,13 @@
 error[E0520]: `from` specializes an item from a parent `impl`, but that item is not marked `default`
-  --> $DIR/issue-111232.rs:6:5
+  --> $DIR/issue-111232.rs:15:5
    |
+LL | impl<T> From<T> for T {
+   | --------------------- parent `impl` is here
+...
 LL |     fn from(s: S) -> S {
-   |     ^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^ cannot specialize default item `from`
    |
-   = note: parent implementation is in crate `core`
+   = note: to specialize, `from` in the parent `impl` must be marked `default`
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/traits/const-traits/non-const-op-in-closure-in-const.rs b/tests/ui/traits/const-traits/non-const-op-in-closure-in-const.rs
index 86e3e5f769f8a..d5f80acc15b54 100644
--- a/tests/ui/traits/const-traits/non-const-op-in-closure-in-const.rs
+++ b/tests/ui/traits/const-traits/non-const-op-in-closure-in-const.rs
@@ -1,6 +1,6 @@
-//@ known-bug: #110395
+#![feature(const_trait_impl, const_from)]
 
-#![feature(const_trait_impl)]
+//@ check-pass
 
 #[const_trait]
 trait Convert<T> {
diff --git a/tests/ui/traits/const-traits/non-const-op-in-closure-in-const.stderr b/tests/ui/traits/const-traits/non-const-op-in-closure-in-const.stderr
deleted file mode 100644
index e7f10e73c6924..0000000000000
--- a/tests/ui/traits/const-traits/non-const-op-in-closure-in-const.stderr
+++ /dev/null
@@ -1,30 +0,0 @@
-error: `[const]` can only be applied to `#[const_trait]` traits
-  --> $DIR/non-const-op-in-closure-in-const.rs:10:44
-   |
-LL | impl<A, B> const Convert<B> for A where B: [const] From<A> {
-   |                                            ^^^^^^^ can't be applied to `From`
-   |
-note: `From` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
-  --> $SRC_DIR/core/src/convert/mod.rs:LL:COL
-
-error: `[const]` can only be applied to `#[const_trait]` traits
-  --> $DIR/non-const-op-in-closure-in-const.rs:10:44
-   |
-LL | impl<A, B> const Convert<B> for A where B: [const] From<A> {
-   |                                            ^^^^^^^ can't be applied to `From`
-   |
-note: `From` can't be used with `[const]` because it isn't annotated with `#[const_trait]`
-  --> $SRC_DIR/core/src/convert/mod.rs:LL:COL
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
-
-error[E0015]: cannot call non-const associated function `<B as From<A>>::from` in constant functions
-  --> $DIR/non-const-op-in-closure-in-const.rs:12:9
-   |
-LL |         B::from(self)
-   |         ^^^^^^^^^^^^^
-   |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-
-error: aborting due to 3 previous errors
-
-For more information about this error, try `rustc --explain E0015`.

From 34426dce3d05e2e5c8c7116809291403ba4cbc00 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov@redhat.com>
Date: Fri, 11 Jul 2025 15:04:13 +0200
Subject: [PATCH 08/12] Fix fallback for CI_JOB_NAME

If CI_JOB_NAME is not specified, it's supposed to fall back to
the image name, which is `$image`, not `$IMAGE`.

Failing to set the correct CI_JOB_NAME causes failures when running
`dist-ohos-*` images locally.
---
 src/ci/docker/run.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/ci/docker/run.sh b/src/ci/docker/run.sh
index 4e69fb2f37057..da7d084d48d8f 100755
--- a/src/ci/docker/run.sh
+++ b/src/ci/docker/run.sh
@@ -361,7 +361,7 @@ docker \
   --env TOOLSTATE_REPO \
   --env TOOLSTATE_PUBLISH \
   --env RUST_CI_OVERRIDE_RELEASE_CHANNEL \
-  --env CI_JOB_NAME="${CI_JOB_NAME-$IMAGE}" \
+  --env CI_JOB_NAME="${CI_JOB_NAME-$image}" \
   --env CI_JOB_DOC_URL="${CI_JOB_DOC_URL}" \
   --env BASE_COMMIT="$BASE_COMMIT" \
   --env DIST_TRY_BUILD \

From 2f05fa6fffb857c08edd0af2dc69f05fa1f02703 Mon Sep 17 00:00:00 2001
From: Jonathan Brouwer <jonathantbrouwer@gmail.com>
Date: Fri, 11 Jul 2025 17:02:24 +0200
Subject: [PATCH 09/12] Fix ICE for parsed attributes with longer path not
 handled by CheckAttrVisitor

Signed-off-by: Jonathan Brouwer <jonathantbrouwer@gmail.com>
---
 Cargo.lock                                        |  1 +
 compiler/rustc_attr_parsing/src/context.rs        |  5 +++++
 compiler/rustc_passes/Cargo.toml                  |  1 +
 compiler/rustc_passes/src/check_attr.rs           | 11 ++++++++++-
 tests/ui/attributes/builtin-attribute-prefix.rs   |  8 ++++++++
 .../ui/attributes/builtin-attribute-prefix.stderr | 15 +++++++++++++++
 6 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/attributes/builtin-attribute-prefix.rs
 create mode 100644 tests/ui/attributes/builtin-attribute-prefix.stderr

diff --git a/Cargo.lock b/Cargo.lock
index 271a2f7962cac..98a78b8495bcb 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4349,6 +4349,7 @@ dependencies = [
  "rustc_ast_lowering",
  "rustc_ast_pretty",
  "rustc_attr_data_structures",
+ "rustc_attr_parsing",
  "rustc_data_structures",
  "rustc_errors",
  "rustc_expand",
diff --git a/compiler/rustc_attr_parsing/src/context.rs b/compiler/rustc_attr_parsing/src/context.rs
index dbe1a5b2ad03d..737fe6ba0817f 100644
--- a/compiler/rustc_attr_parsing/src/context.rs
+++ b/compiler/rustc_attr_parsing/src/context.rs
@@ -718,6 +718,11 @@ impl<'sess, S: Stage> AttributeParser<'sess, S> {
         attributes
     }
 
+    /// Returns whether there is a parser for an attribute with this name
+    pub fn is_parsed_attribute(path: &[Symbol]) -> bool {
+        Late::parsers().0.contains_key(path)
+    }
+
     fn lower_attr_args(&self, args: &ast::AttrArgs, lower_span: impl Fn(Span) -> Span) -> AttrArgs {
         match args {
             ast::AttrArgs::Empty => AttrArgs::Empty,
diff --git a/compiler/rustc_passes/Cargo.toml b/compiler/rustc_passes/Cargo.toml
index b91674890765a..503fc98da76dd 100644
--- a/compiler/rustc_passes/Cargo.toml
+++ b/compiler/rustc_passes/Cargo.toml
@@ -10,6 +10,7 @@ rustc_ast = { path = "../rustc_ast" }
 rustc_ast_lowering = { path = "../rustc_ast_lowering" }
 rustc_ast_pretty = { path = "../rustc_ast_pretty" }
 rustc_attr_data_structures = { path = "../rustc_attr_data_structures" }
+rustc_attr_parsing = { path = "../rustc_attr_parsing" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_expand = { path = "../rustc_expand" }
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index 0aa6a2b41cfb6..3a22b19683cb7 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -7,10 +7,12 @@
 
 use std::cell::Cell;
 use std::collections::hash_map::Entry;
+use std::slice;
 
 use rustc_abi::{Align, ExternAbi, Size};
 use rustc_ast::{AttrStyle, LitKind, MetaItemInner, MetaItemKind, ast};
 use rustc_attr_data_structures::{AttributeKind, InlineAttr, ReprAttr, find_attr};
+use rustc_attr_parsing::{AttributeParser, Late};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::{Applicability, DiagCtxtHandle, IntoDiagArg, MultiSpan, StashKey};
 use rustc_feature::{AttributeDuplicates, AttributeType, BUILTIN_ATTRIBUTE_MAP, BuiltinAttribute};
@@ -373,11 +375,18 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                             | sym::custom_mir,
                             ..
                         ] => {}
-                        [name, ..] => {
+                        [name, rest@..] => {
                             match BUILTIN_ATTRIBUTE_MAP.get(name) {
                                 // checked below
                                 Some(BuiltinAttribute { type_: AttributeType::CrateLevel, .. }) => {}
                                 Some(_) => {
+                                    if rest.len() > 0 && AttributeParser::<Late>::is_parsed_attribute(slice::from_ref(name)) {
+                                        // Check if we tried to use a builtin attribute as an attribute namespace, like `#[must_use::skip]`.
+                                        // This check is here to solve https://github.com/rust-lang/rust/issues/137590
+                                        // An error is already produced for this case elsewhere
+                                        continue
+                                    }
+
                                     // FIXME: differentiate between unstable and internal attributes just
                                     // like we do with features instead of just accepting `rustc_`
                                     // attributes by name. That should allow trimming the above list, too.
diff --git a/tests/ui/attributes/builtin-attribute-prefix.rs b/tests/ui/attributes/builtin-attribute-prefix.rs
new file mode 100644
index 0000000000000..d184c6d008df6
--- /dev/null
+++ b/tests/ui/attributes/builtin-attribute-prefix.rs
@@ -0,0 +1,8 @@
+// Regression test for https://github.com/rust-lang/rust/issues/143789
+#[must_use::skip]
+//~^ ERROR failed to resolve: use of unresolved module or unlinked crate `must_use`
+fn main() { }
+
+// Regression test for https://github.com/rust-lang/rust/issues/137590
+struct S(#[stable::skip] u8, u16, u32);
+//~^ ERROR failed to resolve: use of unresolved module or unlinked crate `stable`
diff --git a/tests/ui/attributes/builtin-attribute-prefix.stderr b/tests/ui/attributes/builtin-attribute-prefix.stderr
new file mode 100644
index 0000000000000..a16080c003fb4
--- /dev/null
+++ b/tests/ui/attributes/builtin-attribute-prefix.stderr
@@ -0,0 +1,15 @@
+error[E0433]: failed to resolve: use of unresolved module or unlinked crate `stable`
+  --> $DIR/builtin-attribute-prefix.rs:7:12
+   |
+LL | struct S(#[stable::skip] u8, u16, u32);
+   |            ^^^^^^ use of unresolved module or unlinked crate `stable`
+
+error[E0433]: failed to resolve: use of unresolved module or unlinked crate `must_use`
+  --> $DIR/builtin-attribute-prefix.rs:2:3
+   |
+LL | #[must_use::skip]
+   |   ^^^^^^^^ use of unresolved module or unlinked crate `must_use`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0433`.

From cdbe44d8069d2f352c6dea560574a6fa9d9df5ae Mon Sep 17 00:00:00 2001
From: bit-aloo <sshourya17@gmail.com>
Date: Fri, 11 Jul 2025 20:33:36 +0530
Subject: [PATCH 10/12] Use short command method directly from fingerprint

---
 src/bootstrap/src/utils/tracing.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/bootstrap/src/utils/tracing.rs b/src/bootstrap/src/utils/tracing.rs
index 99849341dc3b9..58d127dfb791d 100644
--- a/src/bootstrap/src/utils/tracing.rs
+++ b/src/bootstrap/src/utils/tracing.rs
@@ -58,7 +58,7 @@ macro_rules! trace_cmd {
                 target: "COMMAND",
                 ::tracing::Level::TRACE,
                 "executing command",
-                cmd = $cmd.format_short_cmd(),
+                cmd = $cmd.fingerprint().format_short_cmd(),
                 full_cmd = ?$cmd
             ).entered()
         }

From d004abb4d3d7a4c50c54ffb3c4f227ff0d689f03 Mon Sep 17 00:00:00 2001
From: bit-aloo <sshourya17@gmail.com>
Date: Fri, 11 Jul 2025 20:36:13 +0530
Subject: [PATCH 11/12] remove format short command and push format short
 command method inside fingerprint impl

---
 src/bootstrap/src/utils/exec.rs    | 29 ++++-------------------------
 src/bootstrap/src/utils/tracing.rs |  2 --
 2 files changed, 4 insertions(+), 27 deletions(-)

diff --git a/src/bootstrap/src/utils/exec.rs b/src/bootstrap/src/utils/exec.rs
index c5bafda88c459..ebc18f11a05ae 100644
--- a/src/bootstrap/src/utils/exec.rs
+++ b/src/bootstrap/src/utils/exec.rs
@@ -76,8 +76,10 @@ pub struct CommandFingerprint {
     cwd: Option<PathBuf>,
 }
 
-impl FormatShortCmd for CommandFingerprint {
-    fn format_short_cmd(&self) -> String {
+impl CommandFingerprint {
+    /// Helper method to format both Command and BootstrapCommand as a short execution line,
+    /// without all the other details (e.g. environment variables).
+    pub fn format_short_cmd(&self) -> String {
         let program = Path::new(&self.program);
         let mut line = vec![program.file_name().unwrap().to_str().unwrap().to_owned()];
         line.extend(self.args.iter().map(|arg| arg.to_string_lossy().into_owned()));
@@ -545,29 +547,6 @@ impl Default for CommandOutput {
     }
 }
 
-/// Helper trait to format both Command and BootstrapCommand as a short execution line,
-/// without all the other details (e.g. environment variables).
-pub trait FormatShortCmd {
-    fn format_short_cmd(&self) -> String;
-}
-
-#[cfg(feature = "tracing")]
-impl FormatShortCmd for BootstrapCommand {
-    fn format_short_cmd(&self) -> String {
-        self.command.format_short_cmd()
-    }
-}
-
-#[cfg(feature = "tracing")]
-impl FormatShortCmd for Command {
-    fn format_short_cmd(&self) -> String {
-        let program = Path::new(self.get_program());
-        let mut line = vec![program.file_name().unwrap().to_str().unwrap()];
-        line.extend(self.get_args().map(|arg| arg.to_str().unwrap()));
-        line.join(" ")
-    }
-}
-
 #[derive(Clone, Default)]
 pub struct ExecutionContext {
     dry_run: DryRun,
diff --git a/src/bootstrap/src/utils/tracing.rs b/src/bootstrap/src/utils/tracing.rs
index 58d127dfb791d..109407bc5f235 100644
--- a/src/bootstrap/src/utils/tracing.rs
+++ b/src/bootstrap/src/utils/tracing.rs
@@ -52,8 +52,6 @@ macro_rules! error {
 macro_rules! trace_cmd {
     ($cmd:expr) => {
         {
-            use $crate::utils::exec::FormatShortCmd;
-
             ::tracing::span!(
                 target: "COMMAND",
                 ::tracing::Level::TRACE,

From 3751e133bca55dbec521ccb36132fa0bdbf10b88 Mon Sep 17 00:00:00 2001
From: okaneco <47607823+okaneco@users.noreply.github.com>
Date: Sat, 5 Jul 2025 22:26:40 -0400
Subject: [PATCH 12/12] slice: Mark `rotate_left`, `rotate_right` unstably
 const

---
 library/core/src/slice/mod.rs    |  6 ++++--
 library/core/src/slice/rotate.rs | 24 +++++++++++++++++-------
 2 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs
index dc09ba8d788fc..7df717751f522 100644
--- a/library/core/src/slice/mod.rs
+++ b/library/core/src/slice/mod.rs
@@ -3668,7 +3668,8 @@ impl<T> [T] {
     /// assert_eq!(a, ['a', 'c', 'd', 'e', 'b', 'f']);
     /// ```
     #[stable(feature = "slice_rotate", since = "1.26.0")]
-    pub fn rotate_left(&mut self, mid: usize) {
+    #[rustc_const_unstable(feature = "const_slice_rotate", issue = "143812")]
+    pub const fn rotate_left(&mut self, mid: usize) {
         assert!(mid <= self.len());
         let k = self.len() - mid;
         let p = self.as_mut_ptr();
@@ -3713,7 +3714,8 @@ impl<T> [T] {
     /// assert_eq!(a, ['a', 'e', 'b', 'c', 'd', 'f']);
     /// ```
     #[stable(feature = "slice_rotate", since = "1.26.0")]
-    pub fn rotate_right(&mut self, k: usize) {
+    #[rustc_const_unstable(feature = "const_slice_rotate", issue = "143812")]
+    pub const fn rotate_right(&mut self, k: usize) {
         assert!(k <= self.len());
         let mid = self.len() - k;
         let p = self.as_mut_ptr();
diff --git a/library/core/src/slice/rotate.rs b/library/core/src/slice/rotate.rs
index 80178f297eaae..b3b64422884d5 100644
--- a/library/core/src/slice/rotate.rs
+++ b/library/core/src/slice/rotate.rs
@@ -1,5 +1,5 @@
 use crate::mem::{MaybeUninit, SizedTypeProperties};
-use crate::{cmp, ptr};
+use crate::ptr;
 
 type BufType = [usize; 32];
 
@@ -11,7 +11,7 @@ type BufType = [usize; 32];
 ///
 /// The specified range must be valid for reading and writing.
 #[inline]
-pub(super) unsafe fn ptr_rotate<T>(left: usize, mid: *mut T, right: usize) {
+pub(super) const unsafe fn ptr_rotate<T>(left: usize, mid: *mut T, right: usize) {
     if T::IS_ZST {
         return;
     }
@@ -21,7 +21,8 @@ pub(super) unsafe fn ptr_rotate<T>(left: usize, mid: *mut T, right: usize) {
     }
     // `T` is not a zero-sized type, so it's okay to divide by its size.
     if !cfg!(feature = "optimize_for_size")
-        && cmp::min(left, right) <= size_of::<BufType>() / size_of::<T>()
+        // FIXME(const-hack): Use cmp::min when available in const
+        && const_min(left, right) <= size_of::<BufType>() / size_of::<T>()
     {
         // SAFETY: guaranteed by the caller
         unsafe { ptr_rotate_memmove(left, mid, right) };
@@ -45,7 +46,7 @@ pub(super) unsafe fn ptr_rotate<T>(left: usize, mid: *mut T, right: usize) {
 ///
 /// The specified range must be valid for reading and writing.
 #[inline]
-unsafe fn ptr_rotate_memmove<T>(left: usize, mid: *mut T, right: usize) {
+const unsafe fn ptr_rotate_memmove<T>(left: usize, mid: *mut T, right: usize) {
     // The `[T; 0]` here is to ensure this is appropriately aligned for T
     let mut rawarray = MaybeUninit::<(BufType, [T; 0])>::uninit();
     let buf = rawarray.as_mut_ptr() as *mut T;
@@ -117,7 +118,7 @@ unsafe fn ptr_rotate_memmove<T>(left: usize, mid: *mut T, right: usize) {
 ///
 /// The specified range must be valid for reading and writing.
 #[inline]
-unsafe fn ptr_rotate_gcd<T>(left: usize, mid: *mut T, right: usize) {
+const unsafe fn ptr_rotate_gcd<T>(left: usize, mid: *mut T, right: usize) {
     // Algorithm 2
     // Microbenchmarks indicate that the average performance for random shifts is better all
     // the way until about `left + right == 32`, but the worst case performance breaks even
@@ -175,7 +176,9 @@ unsafe fn ptr_rotate_gcd<T>(left: usize, mid: *mut T, right: usize) {
         }
     }
     // finish the chunk with more rounds
-    for start in 1..gcd {
+    // FIXME(const-hack): Use `for start in 1..gcd` when available in const
+    let mut start = 1;
+    while start < gcd {
         // SAFETY: `gcd` is at most equal to `right` so all values in `1..gcd` are valid for
         // reading and writing as per the function's safety contract, see [long-safety-expl]
         // above
@@ -201,6 +204,8 @@ unsafe fn ptr_rotate_gcd<T>(left: usize, mid: *mut T, right: usize) {
                 i += right;
             }
         }
+
+        start += 1;
     }
 }
 
@@ -222,7 +227,7 @@ unsafe fn ptr_rotate_gcd<T>(left: usize, mid: *mut T, right: usize) {
 ///
 /// The specified range must be valid for reading and writing.
 #[inline]
-unsafe fn ptr_rotate_swap<T>(mut left: usize, mut mid: *mut T, mut right: usize) {
+const unsafe fn ptr_rotate_swap<T>(mut left: usize, mut mid: *mut T, mut right: usize) {
     loop {
         if left >= right {
             // Algorithm 3
@@ -265,3 +270,8 @@ unsafe fn ptr_rotate_swap<T>(mut left: usize, mut mid: *mut T, mut right: usize)
         }
     }
 }
+
+// FIXME(const-hack): Use cmp::min when available in const
+const fn const_min(left: usize, right: usize) -> usize {
+    if right < left { right } else { left }
+}