RFC: Allow inference of argument/return value in trait impls

[huonw]

From reddit.

The trait definition entirely determines the signatures of impl'd methods, and so the signature is known without examining the function body, e.g.

trait Foo<T> { fn bar(&self, x: T, y: Self, z: ~str) -> bool; }

impl Foo<int> for f64 {
     fn bar(&self, x: int, y: f64, z: ~str) -> bool { ... }
}
// could be
impl Foo<int> for f64 {
     fn bar(&self, x, y, z) { ... }
}

(Or something like fn bar(&self, x: _, y: _, z: _) -> _ { ... } to make it obvious that a inferred/blank type is being used.)

---

I'm personally ambivalent on this; it may result in

• some tooling being harder to write,
• confusing error messages,
• code that subtly changes behaviour if an upstream trait changes method definitions slightly, but the method body uses that argument in a way that is satisfied by both the old and new types. e.g. TreeMap<uint, T> becoming ~[T]: they both have a methods called insert and remove with the same signature, but they have different performance and semantics. (WIth explicit signatures, the downstream code would stop compiling, rather than changing behaviour.)

[glaebhoerl]

FWIW, another somewhat similar case is items declared inside fns. Theoretically if you have an fn or static declared inside the body of an fn, you could infer its type without breaking the principle of "no type inference at the API level". It probably doesn't make much difference from the perspective of complicating the type inference algorithm, though. (This is different from the case in the OP in that this is actually type inference, while that's something much simpler: just instantiating type parameters.)

My personal feeling w.r.t. both of these is that they're interesting possibilities but also tremendously unimportant, i.e.: I don't feel like we need to make a decision either way, we can just note the possibility (as we have) and come back to it (much) later when we don't have way more important things to think about.

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#309