Auto merge of rust-lang#92268 - jswrenn:transmute, r=oli-obk

Initial implementation of transmutability trait.

*T'was the night before Christmas and all through the codebase, not a miri was stirring — no hint of `unsafe`!*

This PR provides an initial, **incomplete** implementation of *[MCP 411: Lang Item for Transmutability](rust-lang/compiler-team#411. The `core::mem::BikeshedIntrinsicFrom` trait provided by this PR is implemented on-the-fly by the compiler for types `Src` and `Dst` when the bits of all possible values of type `Src` are safely reinterpretable as a value of type `Dst`.

What this PR provides is:
- [x] [support for transmutations involving primitives](https://github.com/jswrenn/rust/tree/transmute/src/test/ui/transmutability/primitives)
- [x] [support for transmutations involving arrays](https://github.com/jswrenn/rust/tree/transmute/src/test/ui/transmutability/arrays)
- [x] [support for transmutations involving structs](https://github.com/jswrenn/rust/tree/transmute/src/test/ui/transmutability/structs)
- [x] [support for transmutations involving enums](https://github.com/jswrenn/rust/tree/transmute/src/test/ui/transmutability/enums)
- [x] [support for transmutations involving unions](https://github.com/jswrenn/rust/tree/transmute/src/test/ui/transmutability/unions)
- [x] [support for weaker validity checks](https://github.com/jswrenn/rust/blob/transmute/src/test/ui/transmutability/unions/should_permit_intersecting_if_validity_is_assumed.rs) (i.e., `Assume::VALIDITY`)
- [x] visibility checking

What isn't yet implemented:
- [ ] transmutability options passed using the `Assume` struct
- [ ] [support for references](https://github.com/jswrenn/rust/blob/transmute/src/test/ui/transmutability/references.rs)
- [ ] smarter error messages

These features will be implemented in future PRs.

Author: bors

Cargo.lock

@@ -4553,6 +4553,7 @@ dependencies = [
  "rustc_session",
  "rustc_span",
  "rustc_target",
+ "rustc_transmute",
  "smallvec",
  "tracing",
 ]
@@ -4577,6 +4578,20 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "rustc_transmute"
+version = "0.1.0"
+dependencies = [
+ "itertools",
+ "rustc_data_structures",
+ "rustc_infer",
+ "rustc_macros",
+ "rustc_middle",
+ "rustc_span",
+ "rustc_target",
+ "tracing",
+]
+
 [[package]]
 name = "rustc_ty_utils"
 version = "0.0.0"

compiler/rustc_hir/src/lang_items.rs

@@ -191,6 +191,9 @@ language_item_table! {
     CoerceUnsized,           sym::coerce_unsized,      coerce_unsized_trait,       Target::Trait,          GenericRequirement::Minimum(1);
     DispatchFromDyn,         sym::dispatch_from_dyn,   dispatch_from_dyn_trait,    Target::Trait,          GenericRequirement::Minimum(1);
 
+    // language items relating to transmutability
+    TransmuteTrait,          sym::transmute_trait,     transmute_trait,            Target::Trait,          GenericRequirement::Exact(6);
+
     Add(Op),                 sym::add,                 add_trait,                  Target::Trait,          GenericRequirement::Exact(1);
     Sub(Op),                 sym::sub,                 sub_trait,                  Target::Trait,          GenericRequirement::Exact(1);
     Mul(Op),                 sym::mul,                 mul_trait,                  Target::Trait,          GenericRequirement::Exact(1);