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fix the implied_bounds_entailment future compatibility lint. #5

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lcnr
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@lcnr lcnr commented Jan 12, 2023

see rust-lang/rust#105572. Your crate unintentionally relied on a bug in the compiler of Rust itself. This bug is now getting fixed and will be a hard error in a future release.

The issue is that the signature of the impl method differs from the signature of the trait method in a way where the impl would actually have stronger requirements for the caller. This could be use to trigger undefined behavior. https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=71fd0f91713510e75174a5cda4da1edc

The impl requires T: 'static because 'a has to outlive 'static for Box<dyn Any + 'a> as Any requires Self: 'static.

This PR fixes this by adding the Self: 'static bound to the impl itself. This bound is required to call downcast in coalesce_any.

Please ask if there are any questions.

@lcnr lcnr changed the title fix the implied_bounds_entailment lint. fix the implied_bounds_entailment future compatibility lint. Jan 12, 2023
`Any` already requires `'static`
@QuartzLibrary
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Hi @lcnr,

Thank you for the fix and the clear example of how this could lead to UB!

It seems that by not understanding at the time that Any requires 'static anyway I stumbled through adding a spurious lifetime and then the bug. In any case, the 'a lifetime can be removed entirely.

Really fantastic to see things like this being systematically addressed. Thanks again.

@QuartzLibrary QuartzLibrary merged commit fffa802 into QuartzLibrary:master Jan 13, 2023
@lcnr lcnr deleted the fix-implied_bounds_entailment-lint branch January 14, 2023 00:22
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