Suggest to add missing feature when using gated const features

src/librustc_mir/transform/check_consts/ops.rs

@@ -39,6 +39,9 @@ fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
             "{} contains unimplemented expression type",
             ccx.const_kind()
         );
+        if let Some(feat) = Self::feature_gate() {
+            err.help(&format!("add `#![feature({})]` to the crate attributes to enable", feat));
+        }
         if ccx.tcx.sess.teach(&err.get_code().unwrap()) {
             err.note(
                 "A function call isn't allowed in the const's initialization expression \

src/test/ui/check-static-values-constraints.stderr

@@ -18,6 +18,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL | static STATIC11: Box<MyOwned> = box MyOwned;
    |                                     ^^^^^^^
+   |
+   = 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/check-static-values-constraints.rs:90:32
@@ -36,6 +38,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL |     box MyOwned,
    |         ^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0010]: allocations are not allowed in statics
   --> $DIR/check-static-values-constraints.rs:97:5
@@ -48,6 +52,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL |     box MyOwned,
    |         ^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0010]: allocations are not allowed in statics
   --> $DIR/check-static-values-constraints.rs:102:6
@@ -60,6 +66,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL |     &box MyOwned,
    |          ^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0010]: allocations are not allowed in statics
   --> $DIR/check-static-values-constraints.rs:104:6
@@ -72,6 +80,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL |     &box MyOwned,
    |          ^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0010]: allocations are not allowed in statics
   --> $DIR/check-static-values-constraints.rs:111:5
@@ -84,6 +94,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL |     box 3;
    |         ^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0507]: cannot move out of static item `x`
   --> $DIR/check-static-values-constraints.rs:116:45
@@ -105,6 +117,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL |     let y = { static x: Box<isize> = box 3; x };
    |                                          ^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to 17 previous errors
 

src/test/ui/const-suggest-feature.rs

+const WRITE: () = unsafe {
+    *std::ptr::null_mut() = 0;
+    //~^ ERROR dereferencing raw pointers in constants is unstable
+    //~| HELP add `#![feature(const_raw_ptr_deref)]` to the crate attributes to enable
+    //~| ERROR constant contains unimplemented expression type
+    //~| HELP add `#![feature(const_mut_refs)]` to the crate attributes to enable
+};
+
+fn main() {}

src/test/ui/const-suggest-feature.stderr

+error[E0658]: dereferencing raw pointers in constants is unstable
+  --> $DIR/const-suggest-feature.rs:2:5
+   |
+LL |     *std::ptr::null_mut() = 0;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #51911 <https://github.com/rust-lang/rust/issues/51911> for more information
+   = help: add `#![feature(const_raw_ptr_deref)]` to the crate attributes to enable
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/const-suggest-feature.rs:2:5
+   |
+LL |     *std::ptr::null_mut() = 0;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0019, E0658.
+For more information about an error, try `rustc --explain E0019`.

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
 

src/test/ui/error-codes/E0010-teach.stderr

@@ -12,6 +12,7 @@ error[E0019]: constant contains unimplemented expression type
 LL | const CON : Box<i32> = box 0;
    |                            ^
    |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
    = note: A function call isn't allowed in the const's initialization expression because the expression's value must be known at compile-time.
    = note: Remember: you can't use a function call inside a const's initialization expression! However, you can use it anywhere else.
 

src/test/ui/error-codes/E0010.stderr

@@ -9,6 +9,8 @@ error[E0019]: constant contains unimplemented expression type
    |
 LL | const CON : Box<i32> = box 0;
    |                            ^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 

src/test/ui/error-codes/E0017.stderr

@@ -12,6 +12,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0658]: references in statics may only refer to immutable values
   --> $DIR/E0017.rs:6:39

src/test/ui/error-codes/E0388.stderr

@@ -12,6 +12,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0658]: references in statics may only refer to immutable values
   --> $DIR/E0388.rs:5:39

src/test/ui/issues/issue-7364.stderr

@@ -9,6 +9,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL | static boxed: Box<RefCell<isize>> = box RefCell::new(0);
    |                                         ^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error[E0277]: `std::cell::RefCell<isize>` cannot be shared between threads safely
   --> $DIR/issue-7364.rs:6:1

src/test/ui/static/static-mut-not-constant.stderr

@@ -9,6 +9,8 @@ error[E0019]: static contains unimplemented expression type
    |
 LL | static mut a: Box<isize> = box 3;
    |                                ^
+   |
+   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors