E0045 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0045.rs

Error E0045 needs a span_label, updating it from:

error[E0045]: variadic function must have C calling convention
  --> src/test/compile-fail/E0045.rs:11:17
   |
11 | extern "Rust" { fn foo(x: u8, ...); } //~ ERROR E0045
   |                 ^^^^^^^^^^^^^^^^^^^

To:

error[E0045]: variadic function must have C calling convention
  --> src/test/compile-fail/E0045.rs:11:17
   |
11 | extern "Rust" { fn foo(x: u8, ...); } //~ ERROR E0045
   |                 ^^^^^^^^^^^^^^^^^^^ variadics require C calling conventions

Bonus: Underline only the variadic part of the call.

error[E0045]: variadic function must have C calling convention
  --> src/test/compile-fail/E0045.rs:11:17
   |
11 | extern "Rust" { fn foo(x: u8, ...); } //~ ERROR E0045
   |                               ^^^ variadics require C calling conventions

[mikhail-m1]

I'd like to discus bonus part fix,

I've find out than function (FnDecl) with ellipsis doesn't contain span information about ellipsis.
I see three variants to solve it but each has some minus:

• evaluate span position from FnDecl items, start take from last argument, end from result or decl span. minus - not exact match, may be some bugs in multiline cases.
• add ellipsis span information in to FnDecl. Minus - too big change for the issue
• add ellipsis as fn arg, need to fix another parts of compiler

May be you could provide better solution or help me choose one.

[sophiajt]

After playing with it, I think you're right. This seems like it would take updating how FnDecl handles variadic fns in a more robust way. This doesn't seem worth it, since this isn't really a fully supported feature (I think it's only for C interop?)

Fun to poke around and see what was available, though.

[sophiajt]

Yeah, let's mark this bonus as "not worth it, for now". We might come back to it later if more span information gets put in.