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Matching on an enum should read the entire enum #56797

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matthewjasper opened this issue Dec 13, 2018 · 2 comments
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Matching on an enum should read the entire enum #56797

matthewjasper opened this issue Dec 13, 2018 · 2 comments
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A-borrow-checker Area: The borrow checker A-NLL Area: Non-lexical lifetimes (NLL) I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness NLL-fixed-by-NLL Bugs fixed, but only when NLL is enabled. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@matthewjasper
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matthewjasper commented Dec 13, 2018 •
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Enum layout optimizations mean that the discriminant of an enum may not be stored in a tag disjoint from the rest of the fields of the enum. Run this example with MIRI to see this is a problem. https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=4ea0e31adbb3fe1893be528277060248

This is fixed by #56790, but this issue is being raised for tracking any backports.

cc @nikomatsakis
@matthewjasper matthewjasper added A-borrow-checker Area: The borrow checker I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness labels Dec 13, 2018
@matthewjasper matthewjasper mentioned this issue Dec 14, 2018
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@nikomatsakis nikomatsakis added the A-NLL Area: Non-lexical lifetimes (NLL) label Dec 14, 2018
@goffrie
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goffrie commented Dec 14, 2018

err... maybe I'm missing something, but doesn't the linked code sample already get rejected by NLL?

@matthewjasper
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Yes, the example has been corrected now.

Centril added a commit to Centril/rust that referenced this issue Dec 16, 2018
@Centril
Rollup merge of rust-lang#56790 - rust-lang:borrowck-niche-discrimina…
d91032a 
…nts, r=nikomatsakis

Make RValue::Discriminant a normal Shallow read

Enum layout optimizations mean that the discriminant of an enum may not be stored in a tag disjoint from the rest of the fields of the enum. Stop borrow checking as though they are.

Run with MIRI to see why this is needed: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=09a3236685a06b6096e2e2e3968b852c.

This issue exists with the lexical borrow checker as well (see rust-lang#45045) so migrate mode should prevent this from being immediately breaking.

r? @nikomatsakis

Fixes rust-lang#56797
@pnkfelix pnkfelix added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. NLL-fixed-by-NLL Bugs fixed, but only when NLL is enabled. labels Dec 20, 2018
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A-borrow-checker Area: The borrow checker A-NLL Area: Non-lexical lifetimes (NLL) I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness NLL-fixed-by-NLL Bugs fixed, but only when NLL is enabled. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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