Miss Dada  🏳️‍⚧️

When doing orphan checks rust basically ignores what it knows from the bounds: impl Iterator for &mut I where I: Iterator is equivalent to Impl iterator for &mut I from the POV of the orphan rules. They're conservative.

Actually this particular case isn't one of it being conservative: the stdlib is totally *allowed* to add `impl Iterator for i64`. The orphan rules protect against future impls that could be added as well

right; I was wondering where I hadn't already tripped over this with this impl: impl<T, F> Input<T> for F where F: FnMut() -> T

huh no idea

I guess Fn{,Mut,Once} are special and only implemented for closures, not traits that any other type can implement

Nope, you can implement them. It's behind a feature flag for ABI reasons but you can implement them.

Oh, wait! I misunderstood. You meant that the &mut i64 and FnOnce impl work fine. That's because of #[fundamental]

#[fundamental] is what lets you write impls for &T, &mut T, and Box<T> even though they're not from your crate. These three types are fundamental, none others iirc