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bors e9f29a8519 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.
2021-09-25 11:01:13 +00:00
..
src Auto merge of #89030 - nbdd0121:box2, r=jonas-schievink 2021-09-25 11:01:13 +00:00
Cargo.toml Migrate to 2021 2021-09-20 22:21:42 -04:00
README.md mv compiler to compiler/ 2020-08-30 18:45:07 +03:00

README.md

Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.