rustc error for missing struct construction args could be improved

[pnkfelix]

Code:

struct S { now_req: bool }
trait T { }
impl T for S {}
fn main() {
    let x = @S as @T;
    println("Hello World");
}

rustc output:

/tmp/mt.rs:5:13: 5:14 error: unresolved name `S`.
/tmp/mt.rs:5     let x = @S as @T;
                          ^
error: aborting due to previous error

We can do better (in terms of looking S up in the struct namespace and informing the user that they need to provide the { ... } portion).

[sanxiyn]

Now you get (after #6702 is fixed):

error: `S` is a structure name, but this expression uses it like a function name
note: Did you mean to write: `S { /* fields */ }`?

This looks great, except "uses it like a function name" is confusing, since there is no call here. I think we just need to reword the message.

[alexcrichton]

Closing, I believe this can be satisfactorily considered solved since #6702 landed.