Make "missing field" error message more natural

[wooster0]

struct A {
    x: i32,
    y: i32,
    z: i32,
}

fn main() {
    A { };
}

error[E0063]: missing fields `x`, `y`, `z` in initializer of `A`
 --> src/main.rs:8:5
  |
8 |     A { };
  |     ^ missing `x`, `y`, `z`

This error is now:

error[E0063]: missing fields `x`, `y` and `z` in initializer of `A`
 --> src/main.rs:8:5
  |
8 |     A { };
  |     ^ missing `x`, `y` and `z`

I thought it looked nicer and more natural this way. Also, if there is >3 fields missing, there is an "and" as well ("missing `x`, `y`, `z` and 1 other field"), but for <=3 there is not. As such it improves consistency too.

As for the implementation, originally I ended up with a chunky push_str algorithm but then I figured I could just do the formatting manually since it's just 3 field names at maximum. It is comparatively readable.

As a sidenote, one thing I was wondering about is, isn't there more cases where you have a list of things like field names? Maybe this whole thing can at some point later be made into a more general function to be used in multiple areas.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @petrochenkov (or someone else) soon.

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[LingMan]

Looks like there's currently no test for the case with >3 missing fields. Would be good to add one.

[wooster0]

Looks like there's currently no test for the case with >3 missing fields. Would be good to add one.

There is such tests, like in src/test/ui/error-codes/E0063.rs and src/test/ui/error-codes/E0063.stderr. They didn't show up because I didn't need to change them.

[LingMan]

They didn't show up because I didn't need to change them.

Right, forget my comment.

[petrochenkov]

Thanks!
r=me after squashing commits.

[wooster0]

@bors r=petrochenkov

[bors]

@r00ster91: 🔑 Insufficient privileges: Not in reviewers

[wooster0]

@bors r=me

[Mark-Simulacrum]

@bors r=petrochenkov

[bors]

📌 Commit 447ce27 has been approved by petrochenkov