`&mut impl Fn()` does not implement `Fn()`

There are the following blanket implementations of the Fn traits on reference types (abbreviated to omit the argument type):

• impl Fn for &F where F: Fn
• impl FnMut for &F where F: Fn
• impl FnOnce for &F where F: Fn
• impl FnMut for &mut F where F: FnMut
• impl FnOnce for &mut F where F: FnMut

The following blanket implementation is possible to implement, but is missing from the standard library:

• impl Fn for &mut F where F: Fn

As far as I can tell, this blanket impl was overlooked in the initial implementation.

This leads to a weird situation where &mut F implements fewer of the Fn traits than &F. This is somewhat strange.

I'm not sure if this needs to be fixed though, since I can't come up with a realistic use case that would run into this.

I am unsure if this is a breaking change or not, as this is a new implementation of a fundamental trait on a fundamental type. I am unable to find code that compiles in stable rust that would break from this change though.

Meta

Affects stable rust version 1.90.0.