Auto merge of #130540 - veera-sivarajan:fix-87525, r=estebank

Add a Lint for Pointer to Integer Transmutes in Consts

Fixes #87525

This PR adds a MirLint for pointer to integer transmutes in const functions and associated consts. The implementation closely follows this comment: https://github.com/rust-lang/rust/pull/85769#issuecomment-880969112. More details about the implementation can be found in the comments.

Note: This could break some sound code as mentioned by RalfJung in https://github.com/rust-lang/rust/pull/85769#issuecomment-886491680:

> ... technically const-code could transmute/cast an int to a ptr and then transmute it back and that would be correct -- so the lint will deny some sound code. Does not seem terribly likely though.

References:
1. https://doc.rust-lang.org/std/mem/fn.transmute.html
2. https://doc.rust-lang.org/reference/items/associated-items.html#associated-constants

compiler/rustc_mir_transform/src/lib.rs

@@ -51,6 +51,7 @@ mod add_subtyping_projections;
 mod check_alignment;
 mod check_const_item_mutation;
 mod check_packed_ref;
+mod check_undefined_transmutes;
 // This pass is public to allow external drivers to perform MIR cleanup
 pub mod cleanup_post_borrowck;
 mod copy_prop;
@@ -298,6 +299,7 @@ fn mir_built(tcx: TyCtxt<'_>, def: LocalDefId) -> &Steal<Body<'_>> {
             &Lint(check_packed_ref::CheckPackedRef),
             &Lint(check_const_item_mutation::CheckConstItemMutation),
             &Lint(function_item_references::FunctionItemReferences),
+            &Lint(check_undefined_transmutes::CheckUndefinedTransmutes),
             // What we need to do constant evaluation.
             &simplify::SimplifyCfg::Initial,
             &Lint(sanity_check::SanityCheck),