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Oh, you're right. Clearly I'd forgotten the rule. `'a` is not inferred for `dyn Trait<'a>`, it is inferred for `&'a dyn Trait`[1]. [1]: https://github.com/rust

Oh, you're right. Clearly I'd forgotten the rule. `'a` is not inferred for `dyn Trait<'a>`, it is inferred for `&'a dyn Trait`[1]. [1]: https://github.com/rust-lang/rfcs/blob/master/text/0599-default-object-bound.md https://github.com/rust-lang/rfcs/blob/master/text/0599-defa...

IshKebab 6y ago
Aha! Thanks for the link. Why isn't this in the official documentation anywhere? Also I'm going to point people here next time someone tries to claim that Rust is simple.
- estebank 6y ago
  If you look at the date that RFC is from before my time. My guess would be that you don't need to know the rule: 95% of the time you don't need to use `+ '_` anywhere (these rules predate the ability to use `'_` to begin with, which is why the RFC talks about `+ 'a` only) because the behavior matches the user's expectation (not having to write `&'a Foo<'a> + 'a`), and, nowadays, when on the 5% of cases you do need 'a the compiler will tell you about it help: to declare that the trait object captures data from argument `self`, you can add an explicit `'_` lifetime bound | 14 | fn things(&self) -> Box<dyn Things<'_> + '_> { | ^^^^ Edit: having said that, the docs do explain these rules[1], reachable from the trait object documentation[2], while it seems that The Book doesn't. I'm also noticing that my confusion was likely due to `&'r Ref<'q, Trait>` being interpreted as `&'r Ref<'q, Trait+'q>`, which is behavior that was introduced in a follow up RFC[3]. [1]: https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes https://doc.rust-lang.org/reference/lifetime-elision.html#de... [2]: https://doc.rust-lang.org/reference/types/trait-object.html?highlight=trait,object#trait-object-lifetime-bounds https://doc.rust-lang.org/reference/types/trait-object.html?... [3]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md https://github.com/rust-lang/rfcs/blob/master/text/1156-adju...

IshKebab 6y ago

Aha! Thanks for the link. Why isn't this in the official documentation anywhere? Also I'm going to point people here next time someone tries to claim that Rust is simple.

estebank 6y ago
If you look at the date that RFC is from before my time. My guess would be that you don't need to know the rule: 95% of the time you don't need to use `+ '_` anywhere (these rules predate the ability to use `'_` to begin with, which is why the RFC talks about `+ 'a` only) because the behavior matches the user's expectation (not having to write `&'a Foo<'a> + 'a`), and, nowadays, when on the 5% of cases you do need 'a the compiler will tell you about it help: to declare that the trait object captures data from argument `self`, you can add an explicit `'_` lifetime bound | 14 | fn things(&self) -> Box<dyn Things<'_> + '_> { | ^^^^ Edit: having said that, the docs do explain these rules[1], reachable from the trait object documentation[2], while it seems that The Book doesn't. I'm also noticing that my confusion was likely due to `&'r Ref<'q, Trait>` being interpreted as `&'r Ref<'q, Trait+'q>`, which is behavior that was introduced in a follow up RFC[3]. [1]: https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes https://doc.rust-lang.org/reference/lifetime-elision.html#de... [2]: https://doc.rust-lang.org/reference/types/trait-object.html?highlight=trait,object#trait-object-lifetime-bounds https://doc.rust-lang.org/reference/types/trait-object.html?... [3]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md https://github.com/rust-lang/rfcs/blob/master/text/1156-adju...