Suggest feature for const_mut_refs errors

Author: pvdrz

src/librustc_mir/transform/check_consts/ops.rs

@@ -197,9 +197,13 @@ impl NonConstOp for MutBorrow {
     }
 
     fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
-        let mut err = struct_span_err!(item.tcx.sess, span, E0017,
-                                       "references in {}s may only refer \
-                                        to immutable values", item.const_kind());
+        let mut err = feature_err(
+            &item.tcx.sess.parse_sess,
+            sym::const_mut_refs,
+            span,
+            &format!("references in {}s may only refer \
+                      to immutable values", item.const_kind())
+        );
         err.span_label(span, format!("{}s require immutable values",
                                             item.const_kind()));
         if item.tcx.sess.teach(&err.get_code().unwrap()) {

src/test/ui/consts/const-eval/issue-65394.stderr

@@ -1,8 +1,11 @@
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/issue-65394.rs:8:13
    |
 LL |     let r = &mut x;
    |             ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0493]: destructors cannot be evaluated at compile-time
   --> $DIR/issue-65394.rs:7:9
@@ -12,5 +15,5 @@ LL |     let mut x = Vec::<i32>::new();
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0017, E0493.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0493, E0658.
+For more information about an error, try `rustc --explain E0493`.

src/test/ui/consts/const-multi-ref.stderr

@@ -1,8 +1,11 @@
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/const-multi-ref.rs:6:13
    |
 LL |     let p = &mut a;
    |             ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
   --> $DIR/const-multi-ref.rs:16:13
@@ -12,5 +15,5 @@ LL |     let p = &a;
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0017, E0492.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0492, E0658.
+For more information about an error, try `rustc --explain E0492`.

src/test/ui/consts/const_let_assign3.stderr

@@ -4,17 +4,23 @@ error[E0019]: constant function contains unimplemented expression type
 LL |         self.state = x;
    |         ^^^^^^^^^^^^^^
 
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/const_let_assign3.rs:16:5
    |
 LL |     s.foo(3);
    |     ^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/const_let_assign3.rs:22:13
    |
 LL |     let y = &mut x;
    |             ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0019]: constant contains unimplemented expression type
   --> $DIR/const_let_assign3.rs:24:5
@@ -24,5 +30,5 @@ LL |     *y = 42;
 
 error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0017, E0019.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0019, E0658.
+For more information about an error, try `rustc --explain E0019`.

src/test/ui/consts/projection_qualif.stderr

@@ -1,8 +1,11 @@
-error[E0017]: references in constants may only refer to immutable values
+error[E0658]: references in constants may only refer to immutable values
   --> $DIR/projection_qualif.rs:6:27
    |
 LL |         let b: *mut u32 = &mut a;
    |                           ^^^^^^ constants require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0658]: dereferencing raw pointers in constants is unstable
   --> $DIR/projection_qualif.rs:7:18
@@ -21,5 +24,5 @@ LL |         unsafe { *b = 5; }
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0017, E0019, E0658.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0019, E0658.
+For more information about an error, try `rustc --explain E0019`.

src/test/ui/consts/static_mut_containing_mut_ref2.stderr

@@ -1,8 +1,11 @@
-error[E0017]: references in statics may only refer to immutable values
+error[E0658]: references in statics may only refer to immutable values
   --> $DIR/static_mut_containing_mut_ref2.rs:3:46
    |
 LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^ statics require immutable values
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/57349
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0019]: static contains unimplemented expression type
   --> $DIR/static_mut_containing_mut_ref2.rs:3:45
@@ -12,5 +15,5 @@ LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 4
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0017, E0019.
-For more information about an error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0019, E0658.
+For more information about an error, try `rustc --explain E0019`.