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r? libs-api

☀️ Try build successful (CI)
Build commit: ddcf048 (ddcf048784667508cd676fef4667f88ae15628a3, parent: 292be5c7c05138d753bbd4b30db7a3f1a5c914f7)

@craterbot run mode=check-only

👌 Experiment pr-148271 created and queued.
🤖 Automatically detected try build ddcf048
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🚧 Experiment pr-148271 is now running

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🎉 Experiment pr-148271 is completed!
📊 6 regressed and 4 fixed (726473 total)
📊 1786 spurious results on the retry-regessed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

@craterbot run mode=check-only crates=https://crater-reports.s3.amazonaws.com/pr-148271/retry-regressed-list.txt p=1

👌 Experiment pr-148271-1 created and queued.
🤖 Automatically detected try build ddcf048
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

🚧 Experiment pr-148271-1 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

🎉 Experiment pr-148271-1 is completed!
📊 1 regressed and 0 fixed (1777 total)
📊 102 spurious results on the retry-regessed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

The only non-spurious failure was from the currying crate. I've minimized the code from that crate to below:

#![feature(unboxed_closures)]
#![feature(fn_traits)]

#[allow(dead_code)]
pub struct Curried<F> {
    func: F,
}

impl<F: FnOnce()> FnOnce<()> for Curried<F> {
    type Output = ();
    extern "rust-call" fn call_once(self, _: ()) {}
}
impl<F: FnMut()> FnMut<()> for Curried<F> {
    extern "rust-call" fn call_mut(&mut self, _: ()) {}
}
impl<F: Fn()> Fn<()> for Curried<F> {
    extern "rust-call" fn call(&self, _: ()) {}
}

pub fn test_mut() {
    let mut i = 0;
    let mut f = || {
        i += 1;
    };
    let mut fj = Curried { func: &mut f };
    fj();
}

Error after this PR:

error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnMut`
  --> src\lib.rs:22:17
   |
22 |     let mut f = || {
   |                 ^^ this closure implements `FnMut`, not `Fn`
23 |         i += 1;
   |         - closure is `FnMut` because it mutates the variable `i` here
...
26 |     fj();
   |     ---- the requirement to implement `Fn` derives from here
   |
   = note: required for `&mut {closure@src\lib.rs:22:17: 22:19}` to implement `Fn()`
   = note: 1 redundant requirement hidden
   = note: required for `Curried<&mut {closure@src\lib.rs:22:17: 22:19}>` to implement `Fn()`

For more information about this error, try `rustc --explain E0525`.
error: could not compile `currying` (lib) due to 1 previous error

I don't understand why it's erroring though...

gonna nominate this for T-types. I am not sure to what extent we actually depend on the exact set of Fn* impls in core/std from within the compiler. It would seem plausible to me that method lookup or the trait solver could be making assumptions about the kind of impls that exist.

can you add AsyncFn for &mut impl AsyncFn too?