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.

1 files changed, 7 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_cranelift/src/base.rs b/compiler/rustc_codegen_cranelift/src/base.rs
index d8fa2c76904..1b30edd2938 100644
--- a/compiler/rustc_codegen_cranelift/src/base.rs
+++ b/compiler/rustc_codegen_cranelift/src/base.rs
@@ -701,6 +701,13 @@ fn codegen_stmt<'tcx>(
                     let len = codegen_array_len(fx, place);
                     lval.write_cvalue(fx, CValue::by_val(len, usize_layout));
                 }
+                Rvalue::ShallowInitBox(ref operand, content_ty) => {
+                    let content_ty = fx.monomorphize(content_ty);
+                    let box_layout = fx.layout_of(fx.tcx.mk_box(content_ty));
+                    let operand = codegen_operand(fx, operand);
+                    let operand = operand.load_scalar(fx);
+                    lval.write_cvalue(fx, CValue::by_val(operand, box_layout));
+                }
                 Rvalue::NullaryOp(NullOp::Box, content_ty) => {
                     let usize_type = fx.clif_type(fx.tcx.types.usize).unwrap();
                     let content_ty = fx.monomorphize(content_ty);