List the available field names of a struct instead of reporting "field not found"

[P-E-Meunier]

Rust errors have improved a lot in this direction over the past year, but this would make it even easier to code without having to look for struct field names in the documentation:

#[derive(Default)]
struct A { a: (), b: (), c:(), d: (), e: () }

fn main() {
   let a = A::default();
   let b = a.z;
}

At the moment, rustc reports "did you mean a?". It would be more convenient to report "no such field. Available fields are: a, b, c, d, e".

In a codebase in a "prototyping stage", it can be the case that several types have similar field names. Listing the fields would even make it easier to refactor, by highlighting similarities.

[zackmdavis]

I should have a PR for this ready within a couple days. (Although to reduce clutter, I don't think the available-fields note should show in all cases; the current "did you mean" suggestions are overzealous; the PR will explain.)