E0407 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0407.rs

E0407 needs a span_label, updating it from:

error[E0407]: method `b` is not a member of trait `Foo`
  --> src/test/compile-fail/E0407.rs:19:5
   |
19 |     fn b() {} //~ ERROR E0407
   |     ^^^^^^^^^

To:

error[E0407]: method `b` is not a member of trait `Foo`
  --> src/test/compile-fail/E0407.rs:19:5
   |
19 |     fn b() {} //~ ERROR E0407
   |     ^^^^^^^^^ not a member of `Foo`

Bonus: tighten the span to focus on b:

error[E0407]: method `b` is not a member of trait `Foo`
  --> src/test/compile-fail/E0407.rs:19:5
   |
19 |     fn b() {} //~ ERROR E0407
   |        ^ not a member of `Foo`

[tvladyslav]

@jonathandturner give me a hint how to implement bonus part, please.
I see, that my span is transferred into error handler from here:
rust/src/librustc_resolve/lib.rs:1952

ImplItemKind::Method(ref sig, _) => {
    // If this is a trait impl, ensure the method
    // exists in trait
    this.check_trait_item(impl_item.ident.name,
        impl_item.span,
        |n, s| ResolutionError::MethodNotMemberOfTrait(n, s));

ImplItem does not contain useful info. impl_item.span corresponds to whole function. The only stupid idea, that I tried - hardcode, like this:

        ResolutionError::MethodNotMemberOfTrait(method, trait_) => {
            use std::ops::{Add, Sub};
            use syntax_pos::Pos;
            use syntax::codemap::BytePos;
            let func_span = Span {
                lo: span.lo.add(BytePos::from_usize(3)),
                hi: span.hi.sub(BytePos::from_usize(5)),
                expn_id: span.expn_id
            };
            let mut err = struct_span_err!(resolver.session,
                                           func_span,
                                           E0407,
                                           "method `{}` is not a member of trait `{}`",
                                           method,
                                           trait_);
            err.span_label(func_span, &format!("not a member of `{}`", trait_));
            err

Hardcoded shift for lower bound is more or less acceptable, because usually function starts from "fn ", upper bound is stupid 100%.
Another idea is to use span_to_string from CodeMap, parse string, get function name index and length and then create a Span. I don't know if it worth to implement.

[tvladyslav]

Right now without bonus. But I work on it!

[sophiajt]

I took a look at the bonus last night. I had a few ideas, but for what time I spent with it, it seemed like it wasn't going to be one of the easier bonuses :)

In general, it's much better to thread the Span information you need through or to look it up with a span_if_local. We're trying to avoid actually changing the spans (though there are a couple of rare exceptions).

Pinging @KiChjang who is also working on some of the bonuses and might have some ideas.

[KiChjang]

My ideas are all bad in this case, because there is no span information stored by the parser :(. Both span_if_local and span_to_string would still not work in this case since a NodeId wouldn't map to an Ident while the latter expects a span parameter. We might have to break the parser API again by adding a span field to the Ident struct...

[sophiajt]

@KiChjang - possibly, though I'd like to underline other names as well, like in the case of an unused function we could underline the name instead of the whole function.

@eddyb - any thought on adding spans? I know @nikomatsakis mentioned you were thinking about possibly adding some spans in the future.

[tvladyslav]

@KiChjang So shell I wait for changes in parser API?

[KiChjang]

You can add span information for the Ident struct yourself. Look in parser.rs.

[sophiajt]

Closing this issue, but feel free to open one about the bonus.