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

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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.
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bors 2021-09-25 11:01:13 +00:00
commit e9f29a8519
31 changed files with 478 additions and 237 deletions
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5
compiler/rustc_middle/src/mir/visit.rs
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@ -753,6 +753,11 @@ macro_rules! make_mir_visitor {
self.visit_operand(operand, location);
}
}
Rvalue::ShallowInitBox(operand, ty) => {
self.visit_operand(operand, location);
self.visit_ty(ty, TyContext::Location(location));
}
}
}