E0379: Make diagnostic more precise

compiler/rustc_ast_passes/messages.ftl

@@ -233,8 +233,14 @@ ast_passes_tilde_const_disallowed = `~const` is not allowed here
     .item = this item cannot have `~const` trait bounds
 
 ast_passes_trait_fn_const =
-    functions in traits cannot be declared const
-    .label = functions in traits cannot be const
+    functions in {$in_impl ->
+        [true] trait impls
+        *[false] traits
+    } cannot be declared const
+    .label = functions in {$in_impl ->
+        [true] trait impls
+        *[false] traits
+    } cannot be const
 
 ast_passes_trait_object_single_bound = only a single explicit lifetime bound is permitted
 

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -293,7 +293,7 @@ impl<'a> AstValidator<'a> {
 
     fn check_trait_fn_not_const(&self, constness: Const) {
         if let Const::Yes(span) = constness {
-            self.dcx().emit_err(errors::TraitFnConst { span });
+            self.dcx().emit_err(errors::TraitFnConst { span, in_impl: self.in_trait_impl });
         }
     }
 

compiler/rustc_ast_passes/src/errors.rs

@@ -49,6 +49,7 @@ pub struct TraitFnConst {
     #[primary_span]
     #[label]
     pub span: Span,
+    pub in_impl: bool,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_error_codes/src/error_codes/E0379.md

@@ -6,6 +6,10 @@ Erroneous code example:
 trait Foo {
     const fn bar() -> u32; // error!
 }
+
+impl Foo for () {
+    const fn bar() -> u32 { 0 } // error!
+}
 ```
 
 Trait methods cannot be declared `const` by design. For more information, see