Add missing impl Fn for &mut F where F: Fn

[Kivooeo]

Currently, there are blanket implementations for &F and &mut F for FnMut and FnOnce, but not for Fn, as a result, &mut F implements fewer of the Fn traits than &F, which is inconsistent and this PR fills that coherence hole

Technically, this is a breaking change because Fn is a fundamental trait, however, practical breakage is expected to be minimal, as such overlapping impl patterns are extremely rare

pub trait Trait {}
impl<F: Fn()> Trait for F {}
impl Trait for &fn() {}     // not allowed
impl Trait for &mut fn() {} // allowed

Because of this, second impl for &mut fn() works fine but shouldn't

Fixes #147931

[rustbot]

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[Noratrieb]

@bors try

[Noratrieb]

r? libs-api

[rust-bors]

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

[theemathas]

Is this "since" line correct?

[Kivooeo]

This is going to be insta stable so I think so and it should be available on all versions

[theemathas]

What do you mean by "should be available on all versions"?

[Kivooeo]

Ah, you mean that if I make it since = 1.91 it will work not only in 1.91+? because in my opinion this should be available in all versions since 1.0

[Noratrieb]

This attribute is only describing when it was added for docs. We cannot add things to old versions.

[Kivooeo]

oh, ok then, I will change it after crater

[Kivooeo]

huh, seems like I can't?

error[E0711]: feature `async_closure` is declared stable since 1.93.0, but was previously declared stable since 1.85.0
   --> library/core/src/ops/async_function.rs:130:5
    |
130 |     #[stable(feature = "async_closure", since = "1.93.0")]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0711]: feature `rust1` is declared stable since 1.93.0, but was previously declared stable since 1.0.0
   --> library/core/src/ops/function.rs:314:5
    |
314 |     #[stable(feature = "rust1", since = "1.93.0")]
    |    

[Kivooeo]

it don't feel correct to change stable since for all impls

[saethlin]

@craterbot run mode=check-only

[craterbot]

👌 Experiment pr-148271 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

[craterbot]

🚧 Experiment pr-148271 is now running

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

[craterbot]

🎉 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

Footnotes

1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-148271/retry-regressed-list.txt ↩

[Noratrieb]

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

[craterbot]

👌 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

[craterbot]

🚧 Experiment pr-148271-1 is now running

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

[craterbot]

🎉 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

Footnotes

1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-148271-1/retry-regressed-list.txt ↩

[theemathas]

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...

[BoxyUwU]

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.

[programmerjake]

can you add AsyncFn for &mut impl AsyncFn too?

[Kivooeo]

@programmerjake, I have zero understanding of what is happening in async code, is this correct impl (I just copied from AsyncFn for &F and changed it to &mut self

    impl<'a, A: Tuple, F: ?Sized> AsyncFn<A> for &'a mut F
    where
        F: AsyncFn<A>,
    {
        extern "rust-call" fn async_call(&self, args: A) -> Self::CallRefFuture<'_> {
            F::async_call(*self, args)
        }
    }

For context: https://doc.rust-lang.org/src/core/ops/async_function.rs.html#60

[programmerjake]

@programmerjake, I have zero understanding of what is happening in async code, is this correct impl (I just copied from AsyncFn for &F and changed it to &mut self

it looks correct to me