Can not use argument impl Trait in type alias

[LHolten]

I tried this code:

type X = [(); {
    
    fn hello(x: impl Clone) {
        
    }
    
    3
}];

I expect this to compile, but instead i get this error.

error[E0658]: `impl Trait` in type aliases is unstable
 --> src/lib.rs:3:17
  |
3 |     fn hello(x: impl Clone) {
  |                 ^^^^^^^^^^
  |

Meta

Bug also reproduces on nightly.

rustc --version --verbose:

rustc 1.85.1 (4eb161250 2025-03-15)
binary: rustc
commit-hash: 4eb161250e340c8f48f66e2b929ef4a5bed7c181
commit-date: 2025-03-15
host: x86_64-unknown-linux-gnu
release: 1.85.1
LLVM version: 19.1.7

[fmease]

Yeep, that's the fragile visitors in ast_passes. In this case, we won't reset the context on AnonConst.

[fmease]

I've already reported the general issue here #119924 in the past incl. your specific issue #119924 (comment)

[fmease]

I guess you found this by fuzzing? If not, I can say I never thought sb. would stumble upon this for real ^^

[LHolten]

I actually stumbled into this while developing a macro that defines a new type wherever you are supposed to provide a type.
As you can see this is extremely useful and extremely cursed code :D

trait Sneak {
    type Out;
}

macro_rules! new_type {
    ($($fields:tt)*) => {
        <[(); {
            struct Bar {$($fields)*}
            impl Sneak for [(); line!() as usize] {
                type Out = Bar;
            }
            line!() as usize
        }] as Sneak>::Out
    };
}

type Test = new_type!(x: u32);
type Test2 = new_type!(x: String);

fn test(val: Test) -> Test2 {
    let Test { x } = val;
    Test2 { x: "hi".to_owned() }
}

P.S. I would appreciate anyone who uses this trick to credit me for the idea :P
(oh and you can expect to see this used to great effect in rust_query sometime soonish)

[fmease]

Nice! I'll send a fix shortly!