Incorrect 'static lifetime bound error with associated type defaults #150081
Description
Code
#![feature(associated_type_defaults)] struct Bar<A>(A); trait Foo<A> { type Inner<'a> = &'a mut Bar<A> where A: 'a; fn expects_inner(&self, inner: Self::Inner<'_>); } impl<A> Bar<A> { fn something<F>(&mut self, foo: F) where for<'a> F: Foo<A, Inner<'a> = &'a mut Bar<A>>, { foo.expects_inner(self); let _ = Box::new(|| {}); } }Current output
error: `A` does not live long enough --> src/lib.rs:19:17 | 19 | let _ = Box::new(|| {}); | ^^^^^^^^ | note: due to a current limitation of the type system, this implies a `'static` lifetime --> src/lib.rs:14:20 | 14 | for<'a> F: Foo<A, Inner<'a> = &'a mut Bar<A>>, | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider restricting the type parameter to the `'static` lifetime | 14 | for<'a> F: Foo<A, Inner<'a> = &'a mut Bar<A>> + 'static, | +++++++++ error: `A` does not live long enough --> src/lib.rs:19:17 | 19 | let _ = Box::new(|| {}); | ^^^^^^^^^^^^^^^ | note: due to a current limitation of the type system, this implies a `'static` lifetime --> src/lib.rs:14:20 | 14 | for<'a> F: Foo<A, Inner<'a> = &'a mut Bar<A>>, | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider restricting the type parameter to the `'static` lifetime | 14 | for<'a> F: Foo<A, Inner<'a> = &'a mut Bar<A>> + 'static, | +++++++++ error: `A` does not live long enough --> src/lib.rs:19:26 | 19 | let _ = Box::new(|| {}); | ^^^^^ | note: due to a current limitation of the type system, this implies a `'static` lifetime --> src/lib.rs:14:20 | 14 | for<'a> F: Foo<A, Inner<'a> = &'a mut Bar<A>>, | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider restricting the type parameter to the `'static` lifetime | 14 | for<'a> F: Foo<A, Inner<'a> = &'a mut Bar<A>> + 'static, | +++++++++Desired output
error: `A` does not live long enough --> src/lib.rs:19:17 | 19 | let _ = Box::new(|| {}); | ^^^^^^^^ | note: due to a current limitation of the type system, this implies a `'static` lifetime help: consider restricting the type parameter to the `'static` lifetime | 14 | A: 'static, | +++++++++++ error: `A` does not live long enough --> src/lib.rs:19:17 | 19 | let _ = Box::new(|| {}); | ^^^^^^^^^^^^^^^ | note: due to a current limitation of the type system, this implies a `'static` lifetime help: consider restricting the type parameter to the `'static` lifetime | 14 | A: 'static, | +++++++++++ error: `A` does not live long enough --> src/lib.rs:19:26 | 19 | let _ = Box::new(|| {}); | ^^^^^ | note: due to a current limitation of the type system, this implies a `'static` lifetime help: consider restricting the type parameter to the `'static` lifetime | 14 | A: 'static, | +++++++++++ (or whatever is the correct error format for requiring a new bound)Rationale and extra context
If I understand correctly, the limitation of the trait system is referring to the fact that closure will capture all the generics from the outer environment. In the example code, it will capture both A and F, but because of the Box, it requires that A must be 'static. This is stated in the error message:
error: `A` does not live long enough However, the diagnostic help text incorrectly says to apply the 'static bound to F, which is wrong. Even doing so, it will just continue to ask for 'static to be added to F:
| 14 | for<'a> F: Foo<A, Inner<'a> = &'a mut Bar<A>> + 'static + 'static, | +++++++++ Instead, adding A: 'staticwill fix the error.
Other cases
Rust Version
rustc 1.93.0-nightly (b84478a1c 2025-11-30) binary: rustc commit-hash: b84478a1c477756cd3e1974eda867a6bb31e8902 commit-date: 2025-11-30 host: x86_64-pc-windows-msvc release: 1.93.0-nightly LLVM version: 21.1.5Anything else?
No response