Auto merge of #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.

core/src/mem/mod.rs

@@ -27,6 +27,10 @@ mod valid_align;
 // alignment as a parameter, such as `Layout::padding_needed_for`.
 pub(crate) use valid_align::ValidAlign;
 
+mod transmutability;
+#[unstable(feature = "transmutability", issue = "99571")]
+pub use transmutability::{Assume, BikeshedIntrinsicFrom};
+
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(inline)]
 pub use crate::intrinsics::transmute;

core/src/mem/transmutability.rs

@@ -0,0 +1,43 @@
+/// Are values of a type transmutable into values of another type?
+///
+/// This trait is implemented on-the-fly by the compiler for types `Src` and `Self` when the bits of
+/// any value of type `Self` are safely transmutable into a value of type `Dst`, in a given `Context`,
+/// notwithstanding whatever safety checks you have asked the compiler to [`Assume`] are satisfied.
+#[unstable(feature = "transmutability", issue = "99571")]
+#[cfg_attr(not(bootstrap), lang = "transmute_trait")]
+#[rustc_on_unimplemented(
+    message = "`{Src}` cannot be safely transmuted into `{Self}` in the defining scope of `{Context}`.",
+    label = "`{Src}` cannot be safely transmuted into `{Self}` in the defining scope of `{Context}`."
+)]
+pub unsafe trait BikeshedIntrinsicFrom<
+    Src,
+    Context,
+    const ASSUME_ALIGNMENT: bool,
+    const ASSUME_LIFETIMES: bool,
+    const ASSUME_VALIDITY: bool,
+    const ASSUME_VISIBILITY: bool,
+> where
+    Src: ?Sized,
+{
+}
+
+/// What transmutation safety conditions shall the compiler assume that *you* are checking?
+#[unstable(feature = "transmutability", issue = "99571")]
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct Assume {
+    /// When `true`, the compiler assumes that *you* are ensuring (either dynamically or statically) that
+    /// destination referents do not have stricter alignment requirements than source referents.
+    pub alignment: bool,
+
+    /// When `true`, the compiler assume that *you* are ensuring that lifetimes are not extended in a manner
+    /// that violates Rust's memory model.
+    pub lifetimes: bool,
+
+    /// When `true`, the compiler assumes that *you* are ensuring that the source type is actually a valid
+    /// instance of the destination type.
+    pub validity: bool,
+
+    /// When `true`, the compiler assumes that *you* have ensured that it is safe for you to violate the
+    /// type and field privacy of the destination type (and sometimes of the source type, too).
+    pub visibility: bool,
+}