Rollup merge of #129199 - RalfJung:writes_through_immutable_pointer, r=compiler-errors

make writes_through_immutable_pointer a hard error

This turns the lint added in https://github.com/rust-lang/rust/pull/118324 into a hard error. This has been reported in cargo's future-compat reports since Rust 1.76 (released in February). Given that const_mut_refs is still unstable, it should be impossible to even hit this error on stable: we did accidentally stabilize some functions that can cause this error, but that got reverted in https://github.com/rust-lang/rust/pull/117905. Still, let's do a crater run just to be sure.

Given that this should only affect unstable code, I don't think it needs an FCP, but let's Cc ``@rust-lang/lang`` anyway -- any objection to making this unambiguous UB into a hard error during const-eval? This can be viewed as part of https://github.com/rust-lang/rust/pull/129195 which is already nominated for discussion.

compiler/rustc_const_eval/src/errors.rs

@@ -407,13 +407,6 @@ pub struct ConstEvalError {
     pub frame_notes: Vec<FrameNote>,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(const_eval_write_through_immutable_pointer)]
-pub struct WriteThroughImmutablePointer {
-    #[subdiagnostic]
-    pub frames: Vec<FrameNote>,
-}
-
 #[derive(Diagnostic)]
 #[diag(const_eval_nullary_intrinsic_fail)]
 pub struct NullaryIntrinsicError {