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impl Trait in return position implicitly captures non-'static type parameter despite + 'static #132364
Description
Code
trait MyTrait {}
struct MyStruct;
impl MyTrait for MyStruct {}
trait DeserTrait<'de> {}
struct DeserStruct;
impl<'de> DeserTrait<'de> for &'de DeserStruct {}
pub fn testfn<'de, D: DeserTrait<'de>>(_deserializer: D) -> impl MyTrait + 'static {
MyStruct
}
fn test() -> impl Send {
let deserializer = DeserStruct;
let graph = testfn(&deserializer);
graph
}Current output
error[E0597]: `deserializer` does not live long enough
--> src/lib.rs:15:24
|
14 | let deserializer = DeserStruct;
| ------------ binding `deserializer` declared here
15 | let graph = testfn(&deserializer);
| -------^^^^^^^^^^^^^-
| | |
| | borrowed value does not live long enough
| argument requires that `deserializer` is borrowed for `'static`
16 | graph
17 | }
| - `deserializer` dropped here while still borrowed
Desired output
I'm not sure, but adding + use<> to the function's return type explains what the issue is actually that impl Trait implicitly captured the trait bound:
error: `impl Trait` must mention all type parameters in scope in `use<...>`
--> src/lib.rs:9:61
|
9 | pub fn testfn<'de, D: DeserTrait<'de>>(_deserializer: D) -> impl MyTrait + use<> + 'static {
| - ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| type parameter is implicitly captured by this `impl Trait`
|
= note: currently, all type parameters are required to be mentioned in the precise captures list
Rationale and extra context
It's impossible to debug (or google) otherwise, and I only figured it out because I remembered + use<> from the stabilization blog post
Other cases
No response
Rust Version
rustc 1.82.0
Anything else?
No response
Activity
compiler-errors commentedon Oct 30, 2024
This is a known limitation, see #116733 Limitation: Opaque hidden types.
compiler-errors commentedon Oct 30, 2024
I'd recommend usingedit: Actually, not totally sure if that will work for your usecase.+ use<>syntax, which doesn't have this problem.progval commentedon Oct 30, 2024
I understand, but I wrote issue about the diagnostic that is unclear, not about limitations of the type system