Inference weirdness with a closure

[nrc]

Sorry this is a vague bug report and a crap example, it comes from trans_struct in expr.rs, the following fails:

let mut need_base: Vec<_> = repeat(true).take(field_tys.len()).collect();

        let numbered_fields = fields.iter().map(|field| {
            let opt_pos =
                field_tys.iter().position(|field_ty|
                                          field_ty.name == field.ident.node.name);
            let result = match opt_pos {
                Some(i) => {
                    need_base[i] = false;
                    (i, &*field.expr)
                }
                None => {
                    tcx.sess.span_bug(field.span,
                                      "Couldn't find field in struct type")
                }
            };
            result
        }).collect::<Vec<_>>();

Changing Vec<_> to Vec<bool> makes it compile.

The error message is unable to infer enough type information about "closure[/home/ncameron/rust3/src/librustc_trans/trans/expr.rs:1390:74: 1405:10]; type annotations required".

The reason I think this is a bug and not just annoying is that it is so obvious that the parameter is bool.

cc @nikomatsakis

[lifthrasiir]

Maybe related:

fn foo<F: FnMut(&[u8])>(mut f: F) {
    f(b"hello ");
    f(b"world!");
}

fn main() {
    let mut v = Vec::new();
    // error: unable to infer enough type information about `closure[...]`; type annotations required
    foo(|buf| v.extend(buf.iter().cloned()));
    println!("{:?}", v);
}

At least we need a good error message here. It says "type annotations required", but doesn't say which ones (and confusingly enough, variables above that closure have to be annotated).