Incorrect lifetime error even with explicit re-borrowing

[Diggsey]

See this playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=4e5e2a45ede09db3cb4d6959adc7b3c6

I expect this to compile, but it fails with a borrow check error.

[compiler-errors]

The problem is that although you're reborrowing the mutable pointer, the compiler isn't inserting a coercion from &'a mut (dyn Debug + 'a) to &'shorter mut (dyn Debug + 'shorter). That means you end up having a pointer of &'shorter (dyn Debug + 'a). Then because the implementation is for &'a mut dyn Debug (which is &'a mut (dyn Debug + 'a), that requires the lifetime of the reborrow 'shorter to be equal to the lifetime of the original mut ref.

Writing as &mut dyn Debug instead of a reborrow would fix it. Not sure if the compiler is smart enough to insert a reborrow here in general.

[Diggsey]

Ah yes, changing the implementation to be generic over the inner lifetime solves the problem:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=3600b136d27db37ac1a9c33efd16ad0a

It's easy to forget there's a hidden lifetime on implementations on dyn. It's somewhat surprising that in impl<'a> Test for &'a mut dyn Debug { ... } the inferred lifetime of dyn Debug is 'a rather than an arbitrary lifetime bounded by 'a.