Suggest to add missing feature when using gated const features

src/test/ui/consts/const-eval/assign-to-static-within-other-static-2.stderr

@@ -3,6 +3,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL |     *FOO.0.get() = 5;
    |     ^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to previous error
 

src/test/ui/consts/const-eval/mod-static-with-const-fn.stderr

@@ -3,6 +3,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL |     *FOO.0.get() = 5;
    |     ^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0015]: calls in statics are limited to constant functions, tuple structs and tuple variants
   --> $DIR/mod-static-with-const-fn.rs:21:5

src/test/ui/consts/const_let_assign3.stderr

@@ -3,6 +3,8 @@ error[E0019]: constant function contains unimplemented expression type
    |
 LL |         self.state = x;
    |         ^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0658]: references in constants may only refer to immutable values
   --> $DIR/const_let_assign3.rs:16:5
@@ -27,6 +29,8 @@ error[E0019]: constant contains unimplemented expression type
    |
 LL |     *y = 42;
    |     ^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to 4 previous errors
 

src/test/ui/consts/projection_qualif.stock.stderr

@@ -21,6 +21,8 @@ error[E0019]: constant contains unimplemented expression type
    |
 LL |         unsafe { *b = 5; }
    |                  ^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to 3 previous errors
 

src/test/ui/consts/static_mut_containing_mut_ref2.stock.stderr

@@ -12,6 +12,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
    |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors