Tweak unevaluated constant in pattern error

Silence errors that are implied by the errors in the `const` item definition.

Add a primary span label.

2 files changed, 11 insertions, 1 deletions

diff --git a/compiler/rustc_mir_build/src/errors.rs b/compiler/rustc_mir_build/src/errors.rs
index 58487a48184..226c469b7d7 100644
--- a/compiler/rustc_mir_build/src/errors.rs
+++ b/compiler/rustc_mir_build/src/errors.rs
@@ -702,6 +702,7 @@ pub(crate) struct ConstPatternDependsOnGenericParameter {
 #[diag(mir_build_could_not_eval_const_pattern)]
 pub(crate) struct CouldNotEvalConstPattern {
     #[primary_span]
+    #[label]
     pub(crate) span: Span,
 }
 
diff --git a/compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs b/compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs
index aa98874654e..3690459bf51 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs
+++ b/compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs
@@ -122,7 +122,16 @@ impl<'tcx> ConstToPat<'tcx> {
             Ok(Ok(c)) => c,
             Err(ErrorHandled::Reported(_, _)) => {
                 // Let's tell the use where this failing const occurs.
-                let err = self.tcx.dcx().create_err(CouldNotEvalConstPattern { span: self.span });
+                let mut err =
+                    self.tcx.dcx().create_err(CouldNotEvalConstPattern { span: self.span });
+                // We've emitted an error on the original const, it would be redundant to complain
+                // on its use as well.
+                if let ty::ConstKind::Unevaluated(uv) = self.c.kind()
+                    && let hir::def::DefKind::Const | hir::def::DefKind::AssocConst =
+                        self.tcx.def_kind(uv.def)
+                {
+                    err.downgrade_to_delayed_bug();
+                }
                 return self.mk_err(err, ty);
             }
             Err(ErrorHandled::TooGeneric(_)) => {