Fix "unstable in stable" error

The "otherwise" note is printed before the suggestion currently.

Author: ecstatic-morse

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

@@ -33,7 +33,12 @@ pub fn non_const<O: NonConstOp>(ccx: &ConstCx<'_, '_>, op: O, span: Span) -> boo
                         concat!(r#"#[rustc_const_unstable(feature = "...", issue = "...")]"#, '\n').to_owned(),
                         Applicability::HasPlaceholders,
                     )
-                    .note("otherwise `#[allow_internal_unstable]` can be used to bypass stability checks")
+                    .span_suggestion(
+                        ccx.body.span,
+                        "otherwise `#[allow_internal_unstable]` can be used to bypass stability checks",
+                        format!("#[allow_internal_unstable({})]", gate),
+                        Applicability:: MaybeIncorrect,
+                    )
                     .emit();
             }
 

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

@@ -4,11 +4,14 @@ error: const-stable function cannot use `#[feature(const_fn_fn_ptr_basics)]`
 LL | const fn error(_: fn()) {}
    |                ^
    |
-   = note: otherwise `#[allow_internal_unstable]` can be used to bypass stability checks
 help: if it is not part of the public API, make this function unstably const
    |
 LL | #[rustc_const_unstable(feature = "...", issue = "...")]
    |
+help: otherwise `#[allow_internal_unstable]` can be used to bypass stability checks
+   |
+LL | #[allow_internal_unstable(const_fn_fn_ptr_basics)]
+   |
 
 error: aborting due to previous error
 

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

@@ -20,11 +20,14 @@ error: const-stable function cannot use `#[feature(const_fn_floating_point_arith
 LL | const fn bar3() -> u32 { (5f32 + 6f32) as u32 }
    |                          ^^^^^^^^^^^^^
    |
-   = note: otherwise `#[allow_internal_unstable]` can be used to bypass stability checks
 help: if it is not part of the public API, make this function unstably const
    |
 LL | #[rustc_const_unstable(feature = "...", issue = "...")]
    |
+help: otherwise `#[allow_internal_unstable]` can be used to bypass stability checks
+   |
+LL | #[allow_internal_unstable(const_fn_floating_point_arithmetic)]
+   |
 
 error: `foo2_gated` is not yet stable as a const fn
   --> $DIR/min_const_fn_libstd_stability.rs:39:32

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

@@ -20,11 +20,14 @@ error: const-stable function cannot use `#[feature(const_fn_floating_point_arith
 LL | const unsafe fn bar3() -> u32 { (5f32 + 6f32) as u32 }
    |                                 ^^^^^^^^^^^^^
    |
-   = note: otherwise `#[allow_internal_unstable]` can be used to bypass stability checks
 help: if it is not part of the public API, make this function unstably const
    |
 LL | #[rustc_const_unstable(feature = "...", issue = "...")]
    |
+help: otherwise `#[allow_internal_unstable]` can be used to bypass stability checks
+   |
+LL | #[allow_internal_unstable(const_fn_floating_point_arithmetic)]
+   |
 
 error: `foo2_gated` is not yet stable as a const fn
   --> $DIR/min_const_unsafe_fn_libstd_stability.rs:39:48