Skip filled in fields when suggesting field names

[gilescope]

Code

struct S {
    ant: u32,
    bat: u32,
    cat: u32,
    dog: u32,
    elephant: u32,
    fish: u32,
    giraffe: u32
}


fn main() {
    let s = S {
        ant: 1,
        bat: 2,
        cat: 3,
        dog: 4,
        elephant: 5,
        fish: 6,
        zebra: 7
    };
}

Current output

error[E0560]: struct `S` has no field named `zebra`
  --> src/main.rs:20:9
   |
20 |         zebra: 7
   |         ^^^^^ `S` does not have this field
   |
   = note: available fields are: `ant`, `bat`, `cat`, `dog`, `elephant` ... and 2 others

Desired output

error[E0560]: struct `S` has no field named `zebra`
  --> src/main.rs:20:9
   |
20 |         zebra: 7
   |         ^^^^^ `S` does not have this field
   |
   = note: unassigned fields are: `giraffe`

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=b2d383789fe03fc2ca056487e3e7eb10

Rationale and extra context

We can't set a field on a struct more than once, so we can discount those already (likely correctly) set fields from the set of available fields to choose from. This is especially useful if you're only following the compiler error messages as the current error output does not mention giraffe in the error message. If I'm coding blind I get prompted correctly what the first few fields of the struct are named, but once I've filled in the first 5 fields correctly there's no more hints on the other fields. If we exclude the fields we've already filled in then rustc should be able to hint me to get to the end of a struct no matter how many fields it has (after a few compiles as I fill in what I need to).

Other cases

This would be especially useful if I am filling out a struct that has been autogenerated by some macro and I can't easily view the source of it.

Anything else?

This just seems like a cute quality of life improvement in the diagnostics.
(I'm hoping it's not too difficult to implement)

[ilyvion]

We might also want to consider what it should say when there are no "unassigned fields" and you've just attempted to add an extra one. Should it fall back to listing all of them, or have a custom message for that case?

[gilescope]

Yes it would be very helpful to be told that all fields in that struct have been provided and that I've just got extra fields that are unrequited.

[gilescope]

Sweet - thank you very much!

…

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