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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
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
Reacted by Gustav Trede, Ali MJ Al-Nasrawy, Yifei, Ireina, Michael Pfaff and Rene Leonhardt
Activity
jackh726 commentedon Dec 19, 2021
It's quite concerning to me that just adding the extra Send bound causes a compilation error (particularly because Send doesn't have lifetimes on it). Will have to look into it.
jackh726 commentedon Jan 10, 2022
So, I think this is ending up into the same problem that causes us trouble with iterator adapters with GATs: we have late bound vars, which get turned into placeholders, and we end up required that
for<'r> C: 'r. I don't think we can solve this easily.For a bit more detail on this issue: Specifically here, we end up with a generator witness with a type
for<'r, 's> { .., <C as Client>::Connecting<ReLateBound(DebruijnIndex(0), BoundRegion { var: 1, kind: BrNamed('s) })> }, that we have to prove isSend. So, we have to prove that that projection isSend. We use thetype Connecting<'a>: Future + Send where Self: 'adefinition, but this requires us to thatfor<'s> C: 's.jackh726 commentedon Feb 7, 2022
GATs issue triage: not blocking. This isn't a design concern for the
Self: 'alint; instead, it's a borrow checker limitation. Fixing it won't be backwards-incompatible.nikomatsakis commentedon Apr 13, 2022
Is this a dup of #71723 ? I guess the error looks different.
jackh726 commentedon Apr 13, 2022
I think this is different. This is caused (once again) by normalizing under a binder, creating an obligation with a placeholder region. That other issue looks like to be caused by a simple type mismatch.
Auto merge of rust-lang#96709 - jackh726:gats-stabilization, r=compil…
Auto merge of rust-lang#96709 - jackh726:gats-stabilization, r=compil…