Rollup merge of #118201 - compiler-errors:obligation-causes, r=cjgillot

Miscellaneous `ObligationCauseCode` cleanups

Remove some dead code/unused `ObligationCauseCode`s.

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

@@ -2909,9 +2909,6 @@ impl<'tcx> ObligationCauseExt<'tcx> for ObligationCause<'tcx> {
             CompareImplItemObligation { kind: ty::AssocKind::Const, .. } => {
                 "const is compatible with trait"
             }
-            ExprAssignable => "expression is assignable",
-            IfExpression { .. } => "`if` and `else` have incompatible types",
-            IfExpressionWithNoElse => "`if` missing an `else` returns `()`",
             MainFunctionType => "`main` function has the correct type",
             StartFunctionType => "`#[start]` function has the correct type",
             LangFunctionType(_) => "lang item function has the correct type",
@@ -2932,9 +2929,6 @@ impl IntoDiagnosticArg for ObligationCauseAsDiagArg<'_> {
             CompareImplItemObligation { kind: ty::AssocKind::Fn, .. } => "method_compat",
             CompareImplItemObligation { kind: ty::AssocKind::Type, .. } => "type_compat",
             CompareImplItemObligation { kind: ty::AssocKind::Const, .. } => "const_compat",
-            ExprAssignable => "expr_assignable",
-            IfExpression { .. } => "if_else_different",
-            IfExpressionWithNoElse => "no_else",
             MainFunctionType => "fn_main_correct_type",
             StartFunctionType => "fn_start_correct_type",
             LangFunctionType(_) => "fn_lang_correct_type",

compiler/rustc_middle/src/traits/mod.rs

@@ -250,9 +250,6 @@ pub enum ObligationCauseCode<'tcx> {
     /// A tuple is WF only if its middle elements are `Sized`.
     TupleElem,
 
-    /// This is the trait reference from the given projection.
-    ProjectionWf(ty::AliasTy<'tcx>),
-
     /// Must satisfy all of the where-clause predicates of the
     /// given item.
     ItemObligation(DefId),
@@ -343,7 +340,8 @@ pub enum ObligationCauseCode<'tcx> {
         parent_code: InternedObligationCauseCode<'tcx>,
     },
 
-    /// Error derived when matching traits/impls; see ObligationCause for more details
+    /// Error derived when checking an impl item is compatible with
+    /// its corresponding trait item's definition
     CompareImplItemObligation {
         impl_item_def_id: LocalDefId,
         trait_item_def_id: DefId,
@@ -372,9 +370,6 @@ pub enum ObligationCauseCode<'tcx> {
         origin_expr: bool,
     },
 
-    /// Constants in patterns must have `Structural` type.
-    ConstPatternStructural,
-
     /// Computing common supertype in an if expression
     IfExpression(Box<IfExpressionCause<'tcx>>),
 
@@ -407,9 +402,6 @@ pub enum ObligationCauseCode<'tcx> {
     /// `return` with an expression
     ReturnValue(hir::HirId),
 
-    /// Return type of this function
-    ReturnType,
-
     /// Opaque return type of this function
     OpaqueReturnType(Option<(Ty<'tcx>, Span)>),
 
@@ -419,9 +411,6 @@ pub enum ObligationCauseCode<'tcx> {
     /// #[feature(trivial_bounds)] is not enabled
     TrivialBound,
 
-    /// If `X` is the concrete type of an opaque type `impl Y`, then `X` must implement `Y`
-    OpaqueType,
-
     AwaitableExpr(hir::HirId),
 
     ForLoopIterator,

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2592,11 +2592,9 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             | ObligationCauseCode::MethodReceiver
             | ObligationCauseCode::ReturnNoExpression
             | ObligationCauseCode::UnifyReceiver(..)
-            | ObligationCauseCode::OpaqueType
             | ObligationCauseCode::MiscObligation
             | ObligationCauseCode::WellFormed(..)
             | ObligationCauseCode::MatchImpl(..)
-            | ObligationCauseCode::ReturnType
             | ObligationCauseCode::ReturnValue(_)
             | ObligationCauseCode::BlockTailExpression(..)
             | ObligationCauseCode::AwaitableExpr(_)
@@ -2607,7 +2605,9 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             | ObligationCauseCode::BinOp { .. }
             | ObligationCauseCode::AscribeUserTypeProvePredicate(..)
             | ObligationCauseCode::DropImpl
-            | ObligationCauseCode::ConstParam(_) => {}
+            | ObligationCauseCode::ConstParam(_)
+            | ObligationCauseCode::ReferenceOutlivesReferent(..)
+            | ObligationCauseCode::ObjectTypeBound(..) => {}
             ObligationCauseCode::RustCall => {
                 if let Some(pred) = predicate.to_opt_poly_trait_pred()
                     && Some(pred.def_id()) == self.tcx.lang_items().sized_trait()
@@ -2621,19 +2621,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             ObligationCauseCode::TupleElem => {
                 err.note("only the last element of a tuple may have a dynamically sized type");
             }
-            ObligationCauseCode::ProjectionWf(data) => {
-                err.note(format!("required so that the projection `{data}` is well-formed"));
-            }
-            ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
-                err.note(format!(
-                    "required so that reference `{ref_ty}` does not outlive its referent"
-                ));
-            }
-            ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
-                err.note(format!(
-                    "required so that the lifetime bound of `{region}` for `{object_ty}` is satisfied",
-                ));
-            }
             ObligationCauseCode::ItemObligation(_)
             | ObligationCauseCode::ExprItemObligation(..) => {
                 // We hold the `DefId` of the item introducing the obligation, but displaying it
@@ -2985,9 +2972,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     "all values live across `{what}` must have a statically known size"
                 ));
             }
-            ObligationCauseCode::ConstPatternStructural => {
-                err.note("constants used for pattern-matching must derive `PartialEq` and `Eq`");
-            }
             ObligationCauseCode::SharedStatic => {
                 err.note("shared static variables must have a type that implements `Sync`");
             }

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -1448,7 +1448,6 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         | ObligationCauseCode::ExprItemObligation(..)
                         | ObligationCauseCode::ExprBindingObligation(..)
                         | ObligationCauseCode::Coercion { .. }
-                        | ObligationCauseCode::OpaqueType
                 );
 
                 // constrain inference variables a bit more to nested obligations from normalize so