Rustc does not to recognize array as a locally defined type.

[georgschoelly]

I stumbled upon strange behaviour of rustc (1.0.0-nightly 2015-01-06). It seems to treat [uint; 5] differently from [SomeStruct; 5].

the type [uint; 1] does not reference any types defined in this crate; only traits defined in the current crate can be implemented for arbitrary types [E0117]

Is this expected behaviour or a bug in the compiler?

Code 1 (does not compile):

use std::default::Default;

type AnArray = [uint; 1];

impl Default for AnArray {
    fn default() -> AnArray {
        [0; 1]
    }
}

fn main() {}

Code 2 (compiles):

use std::default::Default;

struct UnitStruct;

type AnArray = [UnitStruct; 1];

impl Default for AnArray {
    fn default() -> AnArray {
        [UnitStruct; 1]
    }
}

fn main() {}

[kmcallister]

The type synonym is just a transparent alias, so it has no bearing on this.

It's expected behavior that you can't implement Default for [uint; 1], when both are from the standard library. This is part of the "coherence" rules on traits. One reason for the restriction is that another crate might have a conflicting implementation, and this would cause problems when those crates are used together.