GAT Self: 'a bounds break some async blocks with impl Trait #92096
Description
As required by #87479, Self: 'a must be explicitly added for GATs with self lifetimes. But it seems such a bound breaks async blocks with impl Trait sometimes.
Given a trait with a GAT:
trait Client { type Connecting<'a>: Future + Send where Self: 'a; fn connect(&'_ self) -> Self::Connecting<'_>; }Awaiting on Client::connect() in an async block results in a confusing lifeime error on impl ...:
fn call_connect<C>(c: &'_ C) -> impl '_ + Future + Send // removing +Send will compile where C: Client + Send + Sync, { // c.connect().inspect(|_| println!("this compiles")) async move { println!("this does not compile"); c.connect().await } }error[E0311]: the parameter type `C` may not live long enough --> src/lib.rs:19:33 | 19 | fn call_connect<C>(c: &'_ C) -> impl '_ + Future + Send // removing +Send will compile | - ^^^^^^^^^^^^^^^^^^^^^^^ ...so that the type `C` will meet its required lifetime bounds | | | help: consider adding an explicit lifetime bound...: `C: 'a` It also breaks a generic impl on the trait with type_alias_impl_trait:
struct ClientWrapper<T>(T); impl<T> Client for ClientWrapper<T> where T: Client + Send + Sync, { type Connecting<'a> where T: 'a, = impl Future + Send; // removing +Send here & in the trait will compile fn connect(&'_ self) -> Self::Connecting<'_> { // self.0.connect() // <- this compiles // self.0.connect().inspect(|_| println!("this compiles")) async move { println!("this does not compile"); self.0.connect().await } } }error[E0311]: the parameter type `T` may not live long enough --> src/lib.rs:42:29 | 33 | impl<T> Client for ClientWrapper<T> | - help: consider adding an explicit lifetime bound...: `T: 'a` ... 42 | fn connect(&'_ self) -> Self::Connecting<'_> { | ^^^^^^^^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds Full code on the playground: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=15714304b9b5efab401efad8c2e84887
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rustc --version --verbose:
rustc 1.59.0-nightly (7abab1efb 2021-12-17) binary: rustc commit-hash: 7abab1efb21617ba6845fa86328dffa16cfcf1dc commit-date: 2021-12-17 host: x86_64-unknown-linux-gnu release: 1.59.0-nightly LLVM version: 13.0.0