Auto merge of #73905 - matthewjasper:projection-bounds-2, r=nikomatsakis

Separate projection bounds and predicates

Follow up to #72788.

- Rename `projection_predicates` to `item_bounds`
- Separate bounds on associated types (the things after the `:` in `type X: ...`) and opaque types (the things after `impl`)  from predicates.
- Projection candidates now have the correct nested obligations
- Trait object candidates now check that the associated types on the trait object satisfy their bounds as nested obligations
- Type alias impl trait types are now checked (#73035)
- `feature(generic_associated_types)` no longer changes how we handle bounds (#73816)

Opening for a perf and crater runs.

r? `@nikomatsakis`

compiler/rustc_ast_lowering/src/expr.rs

@@ -432,17 +432,25 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.with_catch_scope(body.id, |this| {
             let mut block = this.lower_block_noalloc(body, true);
 
-            let try_span = this.mark_span_with_reason(
-                DesugaringKind::TryBlock,
-                body.span,
-                this.allow_try_trait.clone(),
-            );
-
             // Final expression of the block (if present) or `()` with span at the end of block
-            let tail_expr = block
-                .expr
-                .take()
-                .unwrap_or_else(|| this.expr_unit(this.sess.source_map().end_point(try_span)));
+            let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
+                (
+                    this.mark_span_with_reason(
+                        DesugaringKind::TryBlock,
+                        expr.span,
+                        this.allow_try_trait.clone(),
+                    ),
+                    expr,
+                )
+            } else {
+                let try_span = this.mark_span_with_reason(
+                    DesugaringKind::TryBlock,
+                    this.sess.source_map().end_point(body.span),
+                    this.allow_try_trait.clone(),
+                );
+
+                (try_span, this.expr_unit(try_span))
+            };
 
             let ok_wrapped_span =
                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
@@ -1553,7 +1561,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 hir::LangItem::TryFromError,
                 unstable_span,
                 from_expr,
-                try_span,
+                unstable_span,
             );
             let thin_attrs = ThinVec::from(attrs);
             let catch_scope = self.catch_scopes.last().copied();

compiler/rustc_error_codes/src/error_codes/E0284.md

@@ -5,37 +5,29 @@ as the `collect` method for `Iterator`s.
 For example:
 
 ```compile_fail,E0284
-fn foo() -> Result<bool, ()> {
-    let results = [Ok(true), Ok(false), Err(())].iter().cloned();
-    let v: Vec<bool> = results.collect()?;
-    // Do things with v...
-    Ok(true)
+fn main() {
+    let n: u32 = 1;
+    let mut d: u64 = 2;
+    d = d + n.into();
 }
 ```
 
-Here we have an iterator `results` over `Result<bool, ()>`.
-Hence, `results.collect()` can return any type implementing
-`FromIterator<Result<bool, ()>>`. On the other hand, the
-`?` operator can accept any type implementing `Try`.
+Here we have an addition of `d` and `n.into()`. Hence, `n.into()` can return
+any type `T` where `u64: Add<T>`. On the other hand, the `into` method can
+return any type where `u32: Into<T>`.
 
-The author of this code probably wants `collect()` to return a
-`Result<Vec<bool>, ()>`, but the compiler can't be sure
-that there isn't another type `T` implementing both `Try` and
-`FromIterator<Result<bool, ()>>` in scope such that
-`T::Ok == Vec<bool>`. Hence, this code is ambiguous and an error
-is returned.
+The author of this code probably wants `into()` to return a `u64`, but the
+compiler can't be sure that there isn't another type `T` where both
+`u32: Into<T>` and `u64: Add<T>`.
 
 To resolve this error, use a concrete type for the intermediate expression:
 
 ```
-fn foo() -> Result<bool, ()> {
-    let results = [Ok(true), Ok(false), Err(())].iter().cloned();
-    let v = {
-        let temp: Result<Vec<bool>, ()> = results.collect();
-        temp?
-    };
-    // Do things with v...
-    Ok(true)
+fn main() {
+    let n: u32 = 1;
+    let mut d: u64 = 2;
+    let m: u64 = n.into();
+    d = d + m;
 }
 ```
 

compiler/rustc_infer/src/infer/higher_ranked/mod.rs

@@ -2,7 +2,7 @@
 //! the end of the file for details.
 
 use super::combine::CombineFields;
-use super::{HigherRankedType, InferCtxt, PlaceholderMap};
+use super::{HigherRankedType, InferCtxt};
 
 use crate::infer::CombinedSnapshot;
 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
@@ -33,7 +33,7 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
         self.infcx.commit_if_ok(|_| {
             // First, we instantiate each bound region in the supertype with a
             // fresh placeholder region.
-            let (b_prime, _) = self.infcx.replace_bound_vars_with_placeholders(&b);
+            let b_prime = self.infcx.replace_bound_vars_with_placeholders(&b);
 
             // Next, we instantiate each bound region in the subtype
             // with a fresh region variable. These region variables --
@@ -66,10 +66,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// the [rustc dev guide].
     ///
     /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
-    pub fn replace_bound_vars_with_placeholders<T>(
-        &self,
-        binder: &ty::Binder<T>,
-    ) -> (T, PlaceholderMap<'tcx>)
+    pub fn replace_bound_vars_with_placeholders<T>(&self, binder: &ty::Binder<T>) -> T
     where
         T: TypeFoldable<'tcx>,
     {
@@ -122,7 +119,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             next_universe, binder, result, map,
         );
 
-        (result, map)
+        result
     }
 
     /// See `infer::region_constraints::RegionConstraintCollector::leak_check`.

compiler/rustc_infer/src/infer/mod.rs

@@ -351,11 +351,6 @@ pub struct InferCtxt<'a, 'tcx> {
     universe: Cell<ty::UniverseIndex>,
 }
 
-/// A map returned by `replace_bound_vars_with_placeholders()`
-/// indicating the placeholder region that each late-bound region was
-/// replaced with.
-pub type PlaceholderMap<'tcx> = BTreeMap<ty::BoundRegion, ty::Region<'tcx>>;
-
 /// See the `error_reporting` module for more details.
 #[derive(Clone, Debug, PartialEq, Eq, TypeFoldable)]
 pub enum ValuePairs<'tcx> {
@@ -992,7 +987,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
 
         Some(self.commit_if_ok(|_snapshot| {
-            let (ty::SubtypePredicate { a_is_expected, a, b }, _) =
+            let ty::SubtypePredicate { a_is_expected, a, b } =
                 self.replace_bound_vars_with_placeholders(&predicate);
 
             let ok = self.at(cause, param_env).sub_exp(a_is_expected, a, b)?;
@@ -1007,7 +1002,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         predicate: ty::PolyRegionOutlivesPredicate<'tcx>,
     ) -> UnitResult<'tcx> {
         self.commit_if_ok(|_snapshot| {
-            let (ty::OutlivesPredicate(r_a, r_b), _) =
+            let ty::OutlivesPredicate(r_a, r_b) =
                 self.replace_bound_vars_with_placeholders(&predicate);
             let origin = SubregionOrigin::from_obligation_cause(cause, || {
                 RelateRegionParamBound(cause.span)

compiler/rustc_infer/src/infer/outlives/verify.rs

@@ -328,8 +328,8 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         assoc_item_def_id: DefId,
     ) -> impl Iterator<Item = ty::Region<'tcx>> {
         let tcx = self.tcx;
-        let predicates = tcx.projection_predicates(assoc_item_def_id);
-        predicates
+        let bounds = tcx.item_bounds(assoc_item_def_id);
+        bounds
             .into_iter()
             .filter_map(|p| p.to_opt_type_outlives())
             .filter_map(|p| p.no_bound_vars())

compiler/rustc_infer/src/traits/util.rs

@@ -4,7 +4,6 @@ use crate::traits::{Obligation, ObligationCause, PredicateObligation};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::ty::outlives::Component;
 use rustc_middle::ty::{self, ToPredicate, TyCtxt, WithConstness};
-use rustc_span::Span;
 
 pub fn anonymize_predicate<'tcx>(
     tcx: TyCtxt<'tcx>,
@@ -94,7 +93,11 @@ pub fn elaborate_predicates<'tcx>(
     tcx: TyCtxt<'tcx>,
     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 ) -> Elaborator<'tcx> {
-    let obligations = predicates.map(|predicate| predicate_obligation(predicate, None)).collect();
+    let obligations = predicates
+        .map(|predicate| {
+            predicate_obligation(predicate, ty::ParamEnv::empty(), ObligationCause::dummy())
+        })
+        .collect();
     elaborate_obligations(tcx, obligations)
 }
 
@@ -109,15 +112,10 @@ pub fn elaborate_obligations<'tcx>(
 
 fn predicate_obligation<'tcx>(
     predicate: ty::Predicate<'tcx>,
-    span: Option<Span>,
+    param_env: ty::ParamEnv<'tcx>,
+    cause: ObligationCause<'tcx>,
 ) -> PredicateObligation<'tcx> {
-    let cause = if let Some(span) = span {
-        ObligationCause::dummy_with_span(span)
-    } else {
-        ObligationCause::dummy()
-    };
-
-    Obligation { cause, param_env: ty::ParamEnv::empty(), recursion_depth: 0, predicate }
+    Obligation { cause, param_env, recursion_depth: 0, predicate }
 }
 
 impl Elaborator<'tcx> {
@@ -133,10 +131,11 @@ impl Elaborator<'tcx> {
                 // Get predicates declared on the trait.
                 let predicates = tcx.super_predicates_of(data.def_id());
 
-                let obligations = predicates.predicates.iter().map(|&(pred, span)| {
+                let obligations = predicates.predicates.iter().map(|&(pred, _)| {
                     predicate_obligation(
                         pred.subst_supertrait(tcx, &ty::Binder::bind(data.trait_ref)),
-                        Some(span),
+                        obligation.param_env,
+                        obligation.cause.clone(),
                     )
                 });
                 debug!("super_predicates: data={:?}", data);
@@ -233,7 +232,13 @@ impl Elaborator<'tcx> {
                         })
                         .map(|predicate_kind| predicate_kind.to_predicate(tcx))
                         .filter(|&predicate| visited.insert(predicate))
-                        .map(|predicate| predicate_obligation(predicate, None)),
+                        .map(|predicate| {
+                            predicate_obligation(
+                                predicate,
+                                obligation.param_env,
+                                obligation.cause.clone(),
+                            )
+                        }),
                 );
             }
             ty::PredicateAtom::TypeWellFormedFromEnv(..) => {

compiler/rustc_lint/src/unused.rs

@@ -200,7 +200,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                 ty::Adt(def, _) => check_must_use_def(cx, def.did, span, descr_pre, descr_post),
                 ty::Opaque(def, _) => {
                     let mut has_emitted = false;
-                    for (predicate, _) in cx.tcx.predicates_of(def).predicates {
+                    for &(predicate, _) in cx.tcx.explicit_item_bounds(def) {
                         // We only look at the `DefId`, so it is safe to skip the binder here.
                         if let ty::PredicateAtom::Trait(ref poly_trait_predicate, _) =
                             predicate.skip_binders()

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -937,7 +937,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .tables
             .inferred_outlives
             .get(self, item_id)
-            .map(|predicates| predicates.decode((self, tcx)))
+            .map(|predicates| tcx.arena.alloc_from_iter(predicates.decode((self, tcx))))
             .unwrap_or_default()
     }
 
@@ -949,6 +949,19 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         self.root.tables.super_predicates.get(self, item_id).unwrap().decode((self, tcx))
     }
 
+    fn get_explicit_item_bounds(
+        &self,
+        item_id: DefIndex,
+        tcx: TyCtxt<'tcx>,
+    ) -> &'tcx [(ty::Predicate<'tcx>, Span)] {
+        self.root
+            .tables
+            .explicit_item_bounds
+            .get(self, item_id)
+            .map(|bounds| tcx.arena.alloc_from_iter(bounds.decode((self, tcx))))
+            .unwrap_or_default()
+    }
+
     fn get_generics(&self, item_id: DefIndex, sess: &Session) -> ty::Generics {
         self.root.tables.generics.get(self, item_id).unwrap().decode((self, sess))
     }

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -89,6 +89,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
     explicit_predicates_of => { cdata.get_explicit_predicates(def_id.index, tcx) }
     inferred_outlives_of => { cdata.get_inferred_outlives(def_id.index, tcx) }
     super_predicates_of => { cdata.get_super_predicates(def_id.index, tcx) }
+    explicit_item_bounds => { cdata.get_explicit_item_bounds(def_id.index, tcx) }
     trait_def => { cdata.get_trait_def(def_id.index, tcx.sess) }
     adt_def => { cdata.get_adt_def(def_id.index, tcx) }
     adt_destructor => {

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -965,6 +965,14 @@ impl EncodeContext<'a, 'tcx> {
         record!(self.tables.super_predicates[def_id] <- self.tcx.super_predicates_of(def_id));
     }
 
+    fn encode_explicit_item_bounds(&mut self, def_id: DefId) {
+        debug!("EncodeContext::encode_explicit_item_bounds({:?})", def_id);
+        let bounds = self.tcx.explicit_item_bounds(def_id);
+        if !bounds.is_empty() {
+            record!(self.tables.explicit_item_bounds[def_id] <- bounds);
+        }
+    }
+
     fn encode_info_for_trait_item(&mut self, def_id: DefId) {
         debug!("EncodeContext::encode_info_for_trait_item({:?})", def_id);
         let tcx = self.tcx;
@@ -1017,7 +1025,10 @@ impl EncodeContext<'a, 'tcx> {
                     has_self: trait_item.fn_has_self_parameter,
                 }))
             }
-            ty::AssocKind::Type => EntryKind::AssocType(container),
+            ty::AssocKind::Type => {
+                self.encode_explicit_item_bounds(def_id);
+                EntryKind::AssocType(container)
+            }
         });
         record!(self.tables.visibility[def_id] <- trait_item.vis);
         record!(self.tables.span[def_id] <- ast_item.span);
@@ -1255,7 +1266,10 @@ impl EncodeContext<'a, 'tcx> {
             hir::ItemKind::ForeignMod(_) => EntryKind::ForeignMod,
             hir::ItemKind::GlobalAsm(..) => EntryKind::GlobalAsm,
             hir::ItemKind::TyAlias(..) => EntryKind::Type,
-            hir::ItemKind::OpaqueTy(..) => EntryKind::OpaqueTy,
+            hir::ItemKind::OpaqueTy(..) => {
+                self.encode_explicit_item_bounds(def_id);
+                EntryKind::OpaqueTy
+            }
             hir::ItemKind::Enum(..) => EntryKind::Enum(self.tcx.adt_def(def_id).repr),
             hir::ItemKind::Struct(ref struct_def, _) => {
                 let adt_def = self.tcx.adt_def(def_id);

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -295,13 +295,11 @@ define_tables! {
     generics: Table<DefIndex, Lazy<ty::Generics>>,
     explicit_predicates: Table<DefIndex, Lazy!(ty::GenericPredicates<'tcx>)>,
     expn_that_defined: Table<DefIndex, Lazy<ExpnId>>,
-    // FIXME(eddyb) this would ideally be `Lazy<[...]>` but `ty::Predicate`
-    // doesn't handle shorthands in its own (de)serialization impls,
-    // as it's an `enum` for which we want to derive (de)serialization,
-    // so the `ty::codec` APIs handle the whole `&'tcx [...]` at once.
-    // Also, as an optimization, a missing entry indicates an empty `&[]`.
-    inferred_outlives: Table<DefIndex, Lazy!(&'tcx [(ty::Predicate<'tcx>, Span)])>,
+    // As an optimization, a missing entry indicates an empty `&[]`.
+    inferred_outlives: Table<DefIndex, Lazy!([(ty::Predicate<'tcx>, Span)])>,
     super_predicates: Table<DefIndex, Lazy!(ty::GenericPredicates<'tcx>)>,
+    // As an optimization, a missing entry indicates an empty `&[]`.
+    explicit_item_bounds: Table<DefIndex, Lazy!([(ty::Predicate<'tcx>, Span)])>,
     mir: Table<DefIndex, Lazy!(mir::Body<'tcx>)>,
     promoted_mir: Table<DefIndex, Lazy!(IndexVec<mir::Promoted, mir::Body<'tcx>>)>,
     mir_abstract_consts: Table<DefIndex, Lazy!(&'tcx [mir::abstract_const::Node<'tcx>])>,