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NRVO: Allow occurrences of the return place in var debug info

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The non-use occurrence of the return place in var debug info does not
currently inhibit NRVO optimization, but it will fail assertion in
`visit_place` when optimization is performed.

Relax assertion check to allow the return place in var debug info.

This case might be impossible to hit in optimization pipelines as of
now, but can be encountered in customized mir-opt-level=2 pipeline with
copy propagation disabled. For example in:

```
pub fn b(s: String) -> String {
    a(s)
}

#[inline]
pub fn a(s: String) -> String {
    let x = s;
    let y = x;
    y
}
```
This commit is contained in:
Tomasz Miąsko 2020-09-03 00:00:00 +00:00
parent 0d0f6b1130
commit 01510612ee
1 changed files with 5 additions and 4 deletions
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9
compiler/rustc_mir/src/transform/nrvo.rs
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@ -1,6 +1,6 @@
use rustc_hir::Mutability;
use rustc_index::bit_set::HybridBitSet;
use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
use rustc_middle::mir::visit::{MutVisitor, NonUseContext, PlaceContext, Visitor};
use rustc_middle::mir::{self, BasicBlock, Local, Location};
use rustc_middle::ty::TyCtxt;
@ -196,9 +196,10 @@ impl MutVisitor<'tcx> for RenameToReturnPlace<'tcx> {
self.super_terminator(terminator, loc);
}
fn visit_local(&mut self, l: &mut Local, _: PlaceContext, _: Location) {
assert_ne!(*l, mir::RETURN_PLACE);
if *l == self.to_rename {
fn visit_local(&mut self, l: &mut Local, ctxt: PlaceContext, _: Location) {
if *l == mir::RETURN_PLACE {
assert_eq!(ctxt, PlaceContext::NonUse(NonUseContext::VarDebugInfo));
} else if *l == self.to_rename {
*l = mir::RETURN_PLACE;
}
}