E0379: Make diagnostic more precise

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