Mistyping `<>` in return value for `()` could have better error message

[mdinger]

This errors but doesn't point out the problem very well:

fn fun() -> Option<int> {
    Some<7>
}
fn main(){ 
    fun();
}

The error:

<anon>:4:1: 4:2 error: unexpected token: `}`
<anon>:4 }
         ^
playpen: application terminated with error code 101

Better would be:

fn fun() -> Option<int> {
    Some<7>
    //  ^~~~~~~ These should be `()` and not`<>`
}

[nodakai]

I guess rustc thought Some<7> was two comparison expressions (the second one lacks the right hand side). The parser didn't know Some could resolve to an enum constructor with arity 1. Just a speculation, though.

[steveklabnik]

This error is much better now:

hello.rs:2:13: 3:2 error: chained comparison operators require parentheses
hello.rs:2         Some<7>
hello.rs:3 }
hello.rs:2:13: 3:2 help: use `::<...>` instead of `<...>` if you meant to specify type arguments
hello.rs:3:1: 3:2 error: unexpected token: `}`
hello.rs:3 }
           ^~

but still not as helpful as it could be.

[estebank]

Full current output:

error: chained comparison operators require parentheses
 --> src/main.rs:2:9
  |
2 |       Some<7>
  |  _________^
3 | | }
  | |_^
  |
  = help: use `::<...>` instead of `<...>` if you meant to specify type arguments

error: expected expression, found `}`
 --> src/main.rs:3:1
  |
3 | }
  | ^

error[E0308]: mismatched types
 --> src/main.rs:1:27
  |
1 |   fn fun() -> Option<usize> {
  |  ___________________________^
2 | |     Some<7>
3 | | }
  | |_^ expected enum `std::option::Option`, found ()
  |
  = note: expected type `std::option::Option<usize>`
             found type `()`

---

I believe that this ticket is asking for a generic diagnostic check to look code that seems to be setting a type argument on a fn, fn struct or a fn enum variant, but in libsyntax/parse/parser.rs you only have access to the Ident, not the type information in order to limit this suggestion only to fn-like types. A simple solution can be to expand the "type arguments" note to state

or use `(...)` if you meant to specify fn arguments

[jean-lourenco]

@estebank I'd like to try and add that extra compile output that you mentioned.
I'll just need a little bit of time to figure out how to test it, as I'm fairly new to Rust and this project. I've made the change and builded the solution, but the output is still the same. (Probably using the wrong binaries to test or something)

[estebank]

It's great @jean-lourenco! Make sure you build using ./x.py build --stage 1, and you'll find the binary in build/x86_64-apple-darwin/stage1/bin/rustc. You can also use ./x.py test --stage 1 src/test, go get coffee and see if any test actually fails after your modification.