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

Miscellaneous `ObligationCauseCode` cleanups

Remove some dead code/unused `ObligationCauseCode`s.

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