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Description
Given the following code (playground 1):
trait Foo {
type Bar<T>;
}
fn bar(x: &dyn Foo) {}The current output is:
error[E0191]: the value of the associated type `Bar` (from trait `Foo`) must be specified
--> src/lib.rs:5:16
|
2 | type Bar<T>;
| ----------- `Bar` defined here
...
5 | fn bar(x: &dyn Foo) {}
| ^^^ help: specify the associated type: `Foo<Bar = Type>`
Following this suggestion (playground 2)...
trait Foo {
type Bar<T>;
}
fn bar(x: &dyn Foo<Bar = i32>) {}... produces another diagnostic:
error[E0107]: missing generics for associated type `Foo::Bar`
--> src/lib.rs:5:20
|
5 | fn bar(x: &dyn Foo<Bar = i32>) {}
| ^^^ expected 1 generic argument
|
note: associated type defined here, with 1 generic parameter: `T`
--> src/lib.rs:2:10
|
2 | type Bar<T>;
| ^^^ -
help: add missing generic argument
|
5 | fn bar(x: &dyn Foo<Bar<T> = i32>) {}
| ~~~~~~
Finally, specifying the generic argument for Bar (playground 3)...
trait Foo {
type Bar<T>;
}
fn bar(x: &dyn Foo<Bar<i32> = i32>) {}... reveals that our struggle was all for naught:
error[E0038]: the trait `Foo` cannot be made into an object
--> src/lib.rs:5:12
|
5 | fn bar(x: &dyn Foo<Bar<i32> = i32>) {}
| ^^^^^^^^^^^^^^^^^^^^^^^ `Foo` cannot be made into an object
|
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
--> src/lib.rs:2:10
|
1 | trait Foo {
| --- this trait cannot be made into an object...
2 | type Bar<T>;
| ^^^ ...because it contains the generic associated type `Bar`
= help: consider moving `Bar` to another trait
Ideally, the first snippet should produce an object safety error right away:
error[E0038]: the trait `Foo` cannot be made into an object
--> src/lib.rs:5:12
|
5 | fn bar(x: &dyn Foo) {}
| ^^^^^^^ `Foo` cannot be made into an object
|
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
--> src/lib.rs:2:10
|
1 | trait Foo {
| --- this trait cannot be made into an object...
2 | type Bar<T>;
| ^^^ ...because it contains the generic associated type `Bar`
= help: consider moving `Bar` to another trait
In case #![feature(generic_associated_types_extended)] is enabled, E0191 should still be emitted, but the suggestion should include the generic argument for Bar:
error[E0191]: the value of the associated type `Bar` (from trait `Foo`) must be specified
--> src/lib.rs:5:16
|
2 | type Bar<T>;
| ----------- `Bar` defined here
...
5 | fn bar(x: &dyn Foo) {}
| ^^^ help: specify the associated type: `Foo<Bar<T> = Type>`
@rustbot modify labels +D-terse +F-generic_associated_types
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Area: Generic associated types (GATs)Area: Messages for errors, warnings, and lintsDiagnostics: An error or lint that doesn't give enough information about the problem at hand.`#![feature(generic_associated_types)]` a.k.a. GATsRelevant to the compiler team, which will review and decide on the PR/issue.