Some unstable const features do not indicate the right feature flag in their diagnostics

[RalfJung]

Generally, when one does something on nightly that would work with a feature gate, but the feature gate is missing, the error should indicate the missing gate. For some const features, that is currently not the case, for example:

#![feature(const_raw_ptr_deref)]
// missing gate: #![feature(const_mut_refs)]

const WRITE: () = unsafe {
    *std::ptr::null_mut() = 0;
};

fn main() {
}

error[E0019]: constant contains unimplemented expression type
 --> src/main.rs:5:5
  |
5 |     *std::ptr::null_mut() = 0;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

This got triggered by #71630, but is orthogonal to the discussion around the unleash flag in #71631.

(I did not try to exhaustively explore this issue for all const gates.)

Cc @rust-lang/wg-const-eval

This issue has been assigned to @mibac138 via this comment.

[mibac138]

@rustbot claim