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MIR MutVisitor::visit_local cannot mutate all Locals #71008

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jonas-schievink opened this issue Apr 10, 2020 · 1 comment · Fixed by #71013
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MIR MutVisitor::visit_local cannot mutate all Locals #71008

jonas-schievink opened this issue Apr 10, 2020 · 1 comment · Fixed by #71013
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A-MIR Area: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.html A-mir-opt Area: MIR optimizations C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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jonas-schievink commented Apr 10, 2020 •
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Specifically, when a ProjectionElem::Index is encountered, the contained Local is not passed to visit_local, requiring the MutVisitor to also implement process_projection_elem.

This is a footgun that may cause unsound MIR transforms, which is what happened to me while working on #71003. The CopyProp pass also has to explicitly implement process_projection_elem to be sound.
@jonas-schievink jonas-schievink added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. A-MIR Area: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.html C-bug Category: This is a bug. A-mir-opt Area: MIR optimizations labels Apr 10, 2020
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cc @rust-lang/wg-mir-opt

@jonas-schievink jonas-schievink self-assigned this Apr 10, 2020
@jonas-schievink jonas-schievink mentioned this issue Apr 10, 2020
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@jonas-schievink jonas-schievink changed the title MIR MutVisitor cannot mutate all Locals MIR MutVisitor::visit_local cannot mutate all Locals Apr 11, 2020
Dylan-DPC-zz pushed a commit to Dylan-DPC-zz/rust that referenced this issue Apr 12, 2020
@Dylan-DPC
Rollup merge of rust-lang#71013 - jonas-schievink:visit-projection, r…
6d36a4a 
…=eddyb

Pass the `PlaceElem::Index` local to `visit_local`

Fixes rust-lang#71008

cc @rust-lang/wg-mir-opt

r? @spastorino
@bors bors closed this as completed in c076da0 Apr 13, 2020
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A-MIR Area: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.html A-mir-opt Area: MIR optimizations C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Pass the PlaceElem::Index local to visit_local
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