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opaque type definition: strict lifetime equality vs equal-by-inference #113971
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Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch.
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…2, r=<try> rework opaque type region inference fixes rust-lang#113971 Pass -> Error fixes rust-lang#111906 ICE -> Pass fixes rust-lang#110623 == fixes rust-lang#109059 == fixes rust-lang#112841 Pass -> Error fixes rust-lang#110726 ICE->Error r? `@ghost`
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…2, r=<try> rework opaque type region inference fixes rust-lang#113971 Pass -> Error fixes rust-lang#111906 ICE -> Pass fixes rust-lang#110623 == fixes rust-lang#109059 == fixes rust-lang#112841 Pass -> Error fixes rust-lang#110726 ICE->Error r? `@ghost`
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…2, r=<try> rework opaque type region inference fixes rust-lang#113971 Pass -> Error fixes rust-lang#111906 ICE -> Pass fixes rust-lang#110623 == fixes rust-lang#109059 == fixes rust-lang#112841 Pass -> Error fixes rust-lang#110726 ICE->Error fixes rust-lang#111935 Pass -> Error fixes rust-lang#113916 == r? `@ghost`
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Labels
A-impl-trait
Area: impl Trait. Universally / existentially quantified anonymous types with static dispatch.
C-enhancement
Category: An issue proposing an enhancement or a PR with one.
F-type_alias_impl_trait
`#[feature(type_alias_impl_trait)]`
T-types
Relevant to the types team, which will review and decide on the PR/issue.
The following code compiles but I don't think it should:
Here we have a defining use of an opaque type
Opaque<'_> == u8
with a questionable legality. Let's call the inferred lifetime'?1
.At each defining use of an opaque type, we require its lifetime arguments (
['?1]
) to be equal to one of the lifetime parameters in scope (in this case there is only'x
). This defining use passes this check becauseensure_outlives
registers a region constraint'?1: 'x
, and, because we infer the least possible value for lifetime variables, we end up inferring'?1 == 'x
.The current situation is fine in regard to borrowck soundness (which is the reason we have such check in the first place) but it requires a subtle reasoning of lifetimes and makes the code more fragile (more on that later). For these reason, I believe we should enforce a stricter form of equality that requires both constraints
['x: '?1, '?1: 'x]
in order to consider the lifetimes equal. It would be backward-compatible to change that later.To demonstrate how fragile the current behavior can be, here is a couple of trivial changes that break the original example.
'?1: 'x
constraint is no longer generated atensure_equal
:cc @rust-lang/nitiative-impl-trait
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