WIP: [PAC] Add intrinsic based support for transmute_copy

[jchlanda]

Lower the intrinsic to a dedicated TransmuteCopy cast and, when available, insert the library precondition check requiring the source to contain at least size_of::<Dst>() bytes.

Handle TransmuteCopy as a direct read of Dst from the source address, allowing shrinking transmutes and unsized sources. Use the source's known alignment together with the destination alignment to avoid emitting loads with stronger alignment requirements than the source can guarantee.

Statically provable size violations are treated as unreachable during codegen, while dynamically-sized sources retain the runtime precondition check. For slices and str, known length metadata is used to prove that the source is too short; for dyn Trait no such support is provided (to avoid reading v-tables).

Intrinsic-based implementation makes it possible for the codegen to provide support for function pointer type discrimination, in particular the need to resign function pointers when a transmute changes their discriminator.

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] syn test:false 4.831
   Compiling zerofrom-derive v0.1.6
[RUSTC-TIMING] gimli test:false 1.800
   Compiling yoke-derive v0.8.2
error[E0004]: non-exhaustive patterns: `&rustc_middle::mir::Rvalue::Cast(rustc_middle::mir::CastKind::TransmuteCopy, _, _)` not covered
    --> src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs:138:11
     |
 138 |     match rvalue {
     |           ^^^^^^ pattern `&rustc_middle::mir::Rvalue::Cast(rustc_middle::mir::CastKind::TransmuteCopy, _, _)` not covered
     |
note: `rustc_middle::mir::Rvalue<'_>` defined here
    --> compiler/rustc_middle/src/mir/syntax.rs:1362:1
     |
1362 | pub enum Rvalue<'tcx> {
     | ^^^^^^^^^^^^^^^^^^^^^
...
1407 |     Cast(CastKind, Operand<'tcx>, Ty<'tcx>),
     |     ---- not covered
     = note: the matched value is of type `&rustc_middle::mir::Rvalue<'_>`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
     |
 226 ~         },
 227 ~         &rustc_middle::mir::Rvalue::Cast(rustc_middle::mir::CastKind::TransmuteCopy, _, _) => todo!(),
     |

For more information about this error, try `rustc --explain E0004`.
[RUSTC-TIMING] clippy_utils test:false 1.861
error: could not compile `clippy_utils` (lib) due to 1 previous error

[mejrs]

Suggested change

#[rustc_allowed_through_unstable_modules = "import this function via std::mem instead"]

Please do not add more uses of this attribute. We are trying very hard to get rid of it entirely.

View changes since the review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.