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

tests/ui/consts/const-fn-mismatch.rs

@@ -9,7 +9,7 @@ trait Foo {
 
 impl Foo for u32 {
     const fn f() -> u32 {
-        //~^ ERROR functions in traits cannot be declared const
+        //~^ ERROR functions in trait impls cannot be declared const
         22
     }
 }

tests/ui/consts/const-fn-mismatch.stderr

@@ -1,8 +1,8 @@
-error[E0379]: functions in traits cannot be declared const
+error[E0379]: functions in trait impls cannot be declared const
   --> $DIR/const-fn-mismatch.rs:11:5
    |
 LL |     const fn f() -> u32 {
-   |     ^^^^^ functions in traits cannot be const
+   |     ^^^^^ functions in trait impls cannot be const
 
 error: aborting due to 1 previous error
 

tests/ui/feature-gates/feature-gate-min_const_fn.rs

@@ -8,7 +8,7 @@ trait Foo {
 }
 
 impl Foo for u32 {
-    const fn foo() -> u32 { 0 } //~ ERROR functions in traits cannot be declared const
+    const fn foo() -> u32 { 0 } //~ ERROR functions in trait impls cannot be declared const
 }
 
 trait Bar {}

tests/ui/feature-gates/feature-gate-min_const_fn.stderr

@@ -10,11 +10,11 @@ error[E0379]: functions in traits cannot be declared const
 LL |     const fn bar() -> u32 { 0 }
    |     ^^^^^ functions in traits cannot be const
 
-error[E0379]: functions in traits cannot be declared const
+error[E0379]: functions in trait impls cannot be declared const
   --> $DIR/feature-gate-min_const_fn.rs:11:5
    |
 LL |     const fn foo() -> u32 { 0 }
-   |     ^^^^^ functions in traits cannot be const
+   |     ^^^^^ functions in trait impls cannot be const
 
 error: aborting due to 3 previous errors
 

tests/ui/mismatched_types/const-fn-in-trait.stderr

@@ -4,11 +4,11 @@ error[E0379]: functions in traits cannot be declared const
 LL |     const fn g();
    |     ^^^^^ functions in traits cannot be const
 
-error[E0379]: functions in traits cannot be declared const
+error[E0379]: functions in trait impls cannot be declared const
   --> $DIR/const-fn-in-trait.rs:7:5
    |
 LL |     const fn f() -> u32 { 22 }
-   |     ^^^^^ functions in traits cannot be const
+   |     ^^^^^ functions in trait impls cannot be const
 
 error: aborting due to 2 previous errors
 

tests/ui/parser/fn-header-semantic-fail.rs

@@ -26,10 +26,10 @@ fn main() {
     impl X for Y {
         async fn ft1() {} // OK.
         unsafe fn ft2() {} // OK.
-        const fn ft3() {} //~ ERROR functions in traits cannot be declared const
+        const fn ft3() {} //~ ERROR functions in trait impls cannot be declared const
         extern "C" fn ft4() {}
         const async unsafe extern "C" fn ft5() {}
-        //~^ ERROR functions in traits cannot be declared const
+        //~^ ERROR functions in trait impls cannot be declared const
         //~| ERROR functions cannot be both `const` and `async`
     }
 

tests/ui/parser/fn-header-semantic-fail.stderr

@@ -28,17 +28,17 @@ LL |         const async unsafe extern "C" fn ft5();
    |         |     `async` because of this
    |         `const` because of this
 
-error[E0379]: functions in traits cannot be declared const
+error[E0379]: functions in trait impls cannot be declared const
   --> $DIR/fn-header-semantic-fail.rs:29:9
    |
 LL |         const fn ft3() {}
-   |         ^^^^^ functions in traits cannot be const
+   |         ^^^^^ functions in trait impls cannot be const
 
-error[E0379]: functions in traits cannot be declared const
+error[E0379]: functions in trait impls cannot be declared const
   --> $DIR/fn-header-semantic-fail.rs:31:9
    |
 LL |         const async unsafe extern "C" fn ft5() {}
-   |         ^^^^^ functions in traits cannot be const
+   |         ^^^^^ functions in trait impls cannot be const
 
 error: functions cannot be both `const` and `async`
   --> $DIR/fn-header-semantic-fail.rs:31:9

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/trait-fn-const.rs

@@ -7,7 +7,7 @@ trait Trait {
 }
 
 impl const Trait for () {
-    const fn fun() {}  //~ ERROR functions in traits cannot be declared const
+    const fn fun() {}  //~ ERROR functions in trait impls cannot be declared const
 }
 
 fn main() {}

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/trait-fn-const.stderr

@@ -4,11 +4,11 @@ error[E0379]: functions in traits cannot be declared const
 LL |     const fn fun();
    |     ^^^^^ functions in traits cannot be const
 
-error[E0379]: functions in traits cannot be declared const
+error[E0379]: functions in trait impls cannot be declared const
   --> $DIR/trait-fn-const.rs:10:5
    |
 LL |     const fn fun() {}
-   |     ^^^^^ functions in traits cannot be const
+   |     ^^^^^ functions in trait impls cannot be const
 
 error: aborting due to 2 previous errors