Highlight the const function if error happened because of a bound on …

…the impl block

Currently, for the following code, the compiler produces the errors like the
following error:
```rust
struct Type<T>

impl<T: Clone> Type<T> {
	fn const f() {}
}
```
```text
error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
 --> ./test.rs:3:6
  |
3 | impl<T: Clone> Type<T> {
  |      ^
  |
  = note: see issue #57563 <#57563> for more information
  = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
```

This can be confusing (especially to newcomers) since the error mentions
"const fn parameters", but highlights only the impl.

This commits adds function highlighting, changing the error to the following:

```text
error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
 --> ./test.rs:3:6
  |
3 | impl<T: Clone> Type<T> {
  |      ^
4 |     pub const fn f() {}
  |     ---------------- function declared as const here
  |
  = note: see issue #57563 <#57563> for more information
  = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
```

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -599,12 +599,21 @@ pub mod ty {
         }
 
         fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-            feature_err(
+            let mut builder = feature_err(
                 &ccx.tcx.sess.parse_sess,
                 sym::const_fn_trait_bound,
                 span,
                 "trait bounds other than `Sized` on const fn parameters are unstable",
-            )
+            );
+
+            match ccx.fn_sig() {
+                Some(fn_sig) if !fn_sig.span.contains(span) => {
+                    builder.span_label(fn_sig.span, "function declared as const here");
+                }
+                _ => {}
+            }
+
+            builder
         }
     }
 

src/test/ui/consts/min_const_fn/min_const_fn.stderr

@@ -214,6 +214,9 @@ error[E0658]: trait bounds other than `Sized` on const fn parameters are unstabl
    |
 LL | impl<T: std::fmt::Debug> Foo<T> {
    |      ^
+LL |
+LL |     const fn foo(&self) {}
+   |     ------------------- function declared as const here
    |
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
@@ -223,6 +226,9 @@ error[E0658]: trait bounds other than `Sized` on const fn parameters are unstabl
    |
 LL | impl<T: std::fmt::Debug + Sized> Foo<T> {
    |      ^
+LL |
+LL |     const fn foo2(&self) {}
+   |     -------------------- function declared as const here
    |
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
@@ -232,6 +238,9 @@ error[E0658]: trait bounds other than `Sized` on const fn parameters are unstabl
    |
 LL | impl<T: Sync + Sized> Foo<T> {
    |      ^
+LL |
+LL |     const fn foo3(&self) {}
+   |     -------------------- function declared as const here
    |
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
@@ -292,7 +301,9 @@ error[E0658]: trait bounds other than `Sized` on const fn parameters are unstabl
   --> $DIR/min_const_fn.rs:139:41
    |
 LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
-   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | -------------------------------------   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | |
+   | function declared as const here
    |
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
@@ -301,7 +312,9 @@ error[E0658]: trait bounds other than `Sized` on const fn parameters are unstabl
   --> $DIR/min_const_fn.rs:139:42
    |
 LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
-   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | -------------------------------------    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | |
+   | function declared as const here
    |
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable
@@ -310,7 +323,9 @@ error[E0658]: trait bounds other than `Sized` on const fn parameters are unstabl
   --> $DIR/min_const_fn.rs:139:42
    |
 LL | const fn really_no_traits_i_mean_it() { (&() as &dyn std::fmt::Debug, ()).1 }
-   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | -------------------------------------    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | |
+   | function declared as const here
    |
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable

src/test/ui/consts/min_const_fn/min_const_fn_dyn.stderr

@@ -1,6 +1,8 @@
 error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> $DIR/min_const_fn_dyn.rs:9:5
    |
+LL | const fn no_inner_dyn_trait2(x: Hide) {
+   | ------------------------------------- function declared as const here
 LL |     x.0.field;
    |     ^^^^^^^^^
    |
@@ -11,7 +13,9 @@ error[E0658]: trait bounds other than `Sized` on const fn parameters are unstabl
   --> $DIR/min_const_fn_dyn.rs:12:66
    |
 LL | const fn no_inner_dyn_trait_ret() -> Hide { Hide(HasDyn { field: &0 }) }
-   |                                                                  ^^
+   | -----------------------------------------                        ^^
+   | |
+   | function declared as const here
    |
    = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
    = help: add `#![feature(const_fn_trait_bound)]` to the crate attributes to enable