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

Introduce `Rvalue::ShallowInitBox`

Polished version of #88700.

Implements MCP rust-lang/compiler-team#460, and should allow #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 #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.

compiler/rustc_mir_transform/src/const_prop.rs

@@ -723,6 +723,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             | Rvalue::Repeat(..)
             | Rvalue::Len(..)
             | Rvalue::Cast(..)
+            | Rvalue::ShallowInitBox(..)
             | Rvalue::Discriminant(..)
             | Rvalue::NullaryOp(..) => {}
         }

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -967,6 +967,7 @@ impl<'tcx> Visitor<'tcx> for BorrowCollector {
             }
 
             Rvalue::Cast(..)
+            | Rvalue::ShallowInitBox(..)
             | Rvalue::Use(..)
             | Rvalue::Repeat(..)
             | Rvalue::Len(..)

compiler/rustc_mir_transform/src/separate_const_switch.rs

@@ -204,6 +204,7 @@ fn is_likely_const<'tcx>(mut tracked_place: Place<'tcx>, block: &BasicBlockData<
                         | Rvalue::AddressOf(_, _)
                         | Rvalue::Cast(_, Operand::Constant(_), _)
                         | Rvalue::NullaryOp(_, _)
+                        | Rvalue::ShallowInitBox(_, _)
                         | Rvalue::UnaryOp(_, Operand::Constant(_)) => return true,
 
                         // These rvalues make things ambiguous
@@ -301,6 +302,7 @@ fn find_determining_place<'tcx>(
                     | Rvalue::ThreadLocalRef(_)
                     | Rvalue::AddressOf(_, _)
                     | Rvalue::NullaryOp(_, _)
+                    | Rvalue::ShallowInitBox(_, _)
                     | Rvalue::UnaryOp(_, Operand::Constant(_))
                     | Rvalue::Cast(_, Operand::Constant(_), _)
                     => return None,