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Description
openedon Dec 19, 2021
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
Meta
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
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