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Emit warning when bound is trivially false #100090

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compiler-errors
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@compiler-errors compiler-errors commented Aug 3, 2022

We really should be rejecting bounds that are global, even if they have higher-ranked lifetimes. We have the ability to prove whether for<'a> &'a mut String: Clone is true or not, so there's no reason to silently allow this to typecheck, especially because it's inconsistent with a non-HRTB like &'static mut String: Clone and is the source of ICEs.

For now, though, just add a warning, so that when an ICE does occur (because sometimes it will), then the user knows what might've caused it.

This is a revival of #95611.

@rustbot rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Aug 3, 2022
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r? @petrochenkov

(rust-highfive has picked a reviewer for you, use r? to override)

@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 3, 2022
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r? types

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jackh726 commented Aug 7, 2022

Let's specifically r? @nikomatsakis, since he has some specific thoughts on this :)

(Though I personally do think a warning is good)

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Warnings here...aren't great.

@@ -0,0 +1,14 @@
warning: the where-clause bound `<T as Broken>::Assoc: Foo` is impossible to satisfy
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It's a bit unfortunate that the bound we say is not the bound that's written.

@@ -0,0 +1,26 @@
warning: the where-clause bound `[for<'r> fn(&'r ())]: Copy` is impossible to satisfy
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This is certainly not great.

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bors commented Aug 22, 2022

☔ The latest upstream changes (presumably #100676) made this pull request unmergeable. Please resolve the merge conflicts.

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We really should be rejecting bounds that are global, even if they have higher-ranked lifetimes.

As per RFC 2056, they should not be rejected. The HRTB's this targets can currently be used to achieve trivial bounds without #[feature(trivial_bounds)]. The motivating example in that comment is the inability to omit unsatisfiable trait members, i.e. it works around a practical problem. Until true trivial bounds are allowed on stable, it's arguably a more sensible workaround than leaving the bound off entirely -- especially once such a refined implementation can in other cases be called / relied upon.

Warning makes sense though.

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I agree with warning, but I feel strongly that we should not be rejecting where-clauses that are known to be false. As @QuineDot noted, RFC 2056 decided against it, and I think that is well motivated.

@compiler-errors compiler-errors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 13, 2022
@compiler-errors compiler-errors deleted the trivially-false-bound branch August 11, 2023 20:21
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8 participants