Auto merge of rust-lang#89030 - nbdd0121:box2, r=jonas-schievink

Introduce `Rvalue::ShallowInitBox`

Polished version of rust-lang#88700.

Implements MCP rust-lang/compiler-team#460, and should allow rust-lang#43596 to go forward.

In short, creating an empty box is split from a nullary-op `NullOp::Box` into two steps, first a call to `exchange_malloc`, then a `Rvalue::ShallowInitBox` which transmutes `*mut u8` to a shallow-initialized `Box<T>`. This allows the `exchange_malloc` call to unwind. Details can be found in the MCP.

`NullOp::Box` is not yet removed, purely to make reverting easier in case anything goes wrong as the result of this PR. If revert is needed a reversion of "Use Rvalue::ShallowInitBox for box expression" commit followed by a test bless should be sufficient.

Experiments in rust-lang#88700 showed a very slight compile-time perf regression due to (supposedly) slightly more time spent in LLVM. We could omit unwind edge generation (in non-`oom=panic` case) in box expression MIR construction to restore perf; but I don't think it's necessary since runtime perf isn't affected and perf difference is rather small.

Author: bors

alloc/src/alloc.rs

@@ -307,7 +307,6 @@ unsafe impl Allocator for Global {
 }
 
 /// The allocator for unique pointers.
-// This function must not unwind. If it does, MIR codegen will fail.
 #[cfg(all(not(no_global_oom_handling), not(test)))]
 #[lang = "exchange_malloc"]
 #[inline]