Error when inferring a lifetime in return type using GATs #81487
Description
I tried this code:
#![feature(generic_associated_types)] struct IntRef<'a>(&'a mut i32); trait Trait { type Ref<'a>; } impl Trait for () { type Ref<'a> = IntRef<'a>; } struct RefWrapper<'a, T: Trait>(&'a mut <T as Trait>::Ref<'a>); fn wrap<'a, T: Trait>(the_ref: &'a mut T::Ref<'a>) -> RefWrapper<'a, T> { RefWrapper(the_ref) } fn main() { let mut x = 3; let mut int_ref = IntRef(&mut x); let wrapper = wrap::<()>(&mut int_ref); *wrapper.0.0 = 2; }I expected to see this happen: the code compiles
Instead, this happened: the code does not compile unless you replace:
RefWrapper(the_ref)with
RefWrapper::<'a, T>(the_ref)Meta
rustc --version --verbose:
rustc 1.51.0-nightly (a2f8f6281 2021-01-27) binary: rustc commit-hash: a2f8f6281817d430e20726128b739d3c6708561c commit-date: 2021-01-27 host: x86_64-unknown-linux-gnu release: 1.51.0-nightly LLVM version: 11.0.1 Backtrace
warning: the feature `generic_associated_types` is incomplete and may not be safe to use and/or cause compiler crashes --> x.rs:1:12 | 1 | #![feature(generic_associated_types)] | ^^^^^^^^^^^^^^^^^^^^^^^^ | = note: `#[warn(incomplete_features)]` on by default = note: see issue #44265 <https://github.com/rust-lang/rust/issues/44265> for more information error[E0309]: the associated type `<T as Trait>::Ref<'_>` may not live long enough --> x.rs:16:5 | 16 | RefWrapper(the_ref) | ^^^^^^^^^^ | = help: consider adding an explicit lifetime bound `<T as Trait>::Ref<'_>: 'a`... = note: ...so that the type `<T as Trait>::Ref<'_>` will meet its required lifetime bounds error[E0309]: the associated type `<T as Trait>::Ref<'_>` may not live long enough --> x.rs:16:5 | 16 | RefWrapper(the_ref) | ^^^^^^^^^^^^^^^^^^^ | = help: consider adding an explicit lifetime bound `<T as Trait>::Ref<'_>: 'a`... = note: ...so that the type `<T as Trait>::Ref<'_>` will meet its required lifetime bounds error: aborting due to 2 previous errors; 1 warning emitted For more information about this error, try `rustc --explain E0309`.