-
Notifications
You must be signed in to change notification settings - Fork 13.6k
Open
Labels
A-impl-traitArea: `impl Trait`. Universally / existentially quantified anonymous types with static dispatch.Area: `impl Trait`. Universally / existentially quantified anonymous types with static dispatch.A-lifetimesArea: Lifetimes / regionsArea: Lifetimes / regionsA-type-systemArea: Type systemArea: Type systemT-langRelevant to the language teamRelevant to the language teamT-typesRelevant to the types team, which will review and decide on the PR/issue.Relevant to the types team, which will review and decide on the PR/issue.
Description
According to RFC 1951, impl Trait
in return position should capture the lifetime of any associated generic parameters. However, in this code, that logic appears not to work (playground):
use std::future::Future;
use std::pin::Pin;
struct A;
impl A {
fn a<T>(&self, t: T) -> impl Future<Output = ()> {
let _ = t;
async {}
}
}
struct B(A);
impl B {
fn works<'a, T: 'a>(
self,
t: T,
) -> impl Future<Output = (Self, impl Future<Output = ()> + 'a)> + 'a {
async move {
let f = Pin::from(Box::new(self.0.a(t)) as Box<dyn Future<Output = _>>);
(self, f)
}
}
fn doesnt_work<T>(self, t: T) -> impl Future<Output = (Self, impl Future<Output = ()>)> {
async move {
let f = Pin::from(Box::new(self.0.a(t)) as Box<dyn Future<Output = _>>);
(self, f)
}
}
}
Specifically, the compiler gives two almost identical errors for the doesnt_work
method:
error[E0310]: the parameter type `T` may not live long enough
--> src/lib.rs:28:31
|
26 | fn doesnt_work<T>(self, t: T) -> impl Future<Output = (Self, impl Future<Output = ()>)> {
| - help: consider adding an explicit lifetime bound `T: 'static`...
27 | async move {
28 | let f = Pin::from(Box::new(self.0.a(t)) as Box<dyn Future<Output = _>>);
| ^^^^^^^^^^^^^^^^^^^^^
|
note: ...so that the type `impl std::future::Future` will meet its required lifetime bounds
--> src/lib.rs:28:31
|
28 | let f = Pin::from(Box::new(self.0.a(t)) as Box<dyn Future<Output = _>>);
| ^^^^^^^^^^^^^^^^^^^^^
and
error[E0310]: the parameter type `T` may not live long enough
--> src/lib.rs:28:31
|
26 | fn doesnt_work<T>(self, t: T) -> impl Future<Output = (Self, impl Future<Output = ()>)> {
| - help: consider adding an explicit lifetime bound `T: 'static`...
27 | async move {
28 | let f = Pin::from(Box::new(self.0.a(t)) as Box<dyn Future<Output = _>>);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
note: ...so that it can be closed over into an object
--> src/lib.rs:28:31
|
28 | let f = Pin::from(Box::new(self.0.a(t)) as Box<dyn Future<Output = _>>);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I was (perhaps mistakenly) under the impression that the semantics of impl Trait
in return position should be as the explicit lifetimes on the works
method indicate, but that doesn't seem to be the case? When the lifetimes are explicitly annotated, as in the works
method, the method compiles just fine.
towry
Metadata
Metadata
Assignees
Labels
A-impl-traitArea: `impl Trait`. Universally / existentially quantified anonymous types with static dispatch.Area: `impl Trait`. Universally / existentially quantified anonymous types with static dispatch.A-lifetimesArea: Lifetimes / regionsArea: Lifetimes / regionsA-type-systemArea: Type systemArea: Type systemT-langRelevant to the language teamRelevant to the language teamT-typesRelevant to the types team, which will review and decide on the PR/issue.Relevant to the types team, which will review and decide on the PR/issue.