weird-ass rust problem

busted.rs

enum IResult<I, O, E = u32> {
    Done(I, O),
    Error(E),
}

fn foo(input: &[u8]) {
    if let IResult::Done(i, o) = IResult::Done(input, input) {}
}

invocation.sh

$ rustc --crate-type lib lib-minimal.rs -Ltarget/debug/deps
error[E0282]: unable to infer enough type information about `E`
 --> lib-minimal.rs:7:34
  |
7 |     if let IResult::Done(i, o) = IResult::Done(input, input) {}
  |                                  ^^^^^^^^^^^^^ cannot infer type for `E`
  |
  = note: type annotations or generic parameter binding required

error: aborting due to previous error

$ rustc --version
rustc 1.15.1