Trait constraints should allow anonymous lifetimes for references.

[ciphergoth]

This compiles:

fn isum<'a, I: Iterator<&'a int>>(mut it: I) -> int { it.fold(0, |a, b| a + *b) }

This should be the same, but doesn't compile:

fn isum<I: Iterator<&int>>(mut it: I) -> int { it.fold(0, |a, b| a + *b)}

The compiler complains of a "missing lifetime specifier" at the &int.

[eddyb]

cc @nikomatsakis @pnkfelix

[steveklabnik]

/cc @rust-lang/lang , is this a bug? I would expect a lifetime specifier to be needed here. Updated code. Works:

fn isum<'a, I: Iterator<Item=&'a i32>>(it: I) -> i32 { it.fold(0, |a, b| a + *b)}

doesn't

fn isum<I: Iterator<Item=&i32>>(it: I) -> i32 { it.fold(0, |a, b| a + *b)}

[ciphergoth]

I had expected leaving out the lifetime to be the same as supplying an anonymous one, possibly in error.

[steveklabnik]

The only time you can leave out lifetimes is following the elision rules: https://doc.rust-lang.org/book/lifetimes.html#lifetime-elision and this signature isn't compatible with any of them.

[eddyb]

@steveklabnik In a way, this is a feature request.
Of course it doesn't work right now, this is what the issue is about.
Traits generic over lifetimes didn't work for the longest time, and their ergonomics were not improved once they were fixed to actually work.

[steveklabnik]

Of course it doesn't work right now, this is what the issue is about.

Well, adding more rules for elision would take an RFC these days. We don't keep feature requests in this repository. I've moved it over: rust-lang/rfcs#1352

[ciphergoth]

Makes sense. Thanks!

[steveklabnik]

No problem, thanks for bringing it up! Sorry I was confused about the bug/feature split.

[ciphergoth]

I think the confusion was mine, thanks for sorting it out!

[nikomatsakis]

(I personally would like this to work eventually, but we currently do not support it, so I think this is "not a bug".)