rewrite `...` to `..=` as an idiom lint for Rust 2018 edition

[nikomatsakis]

Current status

Implemented in #51149 but currently not marked as automatically applicable due to this issue which is explained below

[nikomatsakis]

Do we have an existing lint for this?

[zackmdavis]

If we hope to someday get rid of ... in patterns (the current description of #28237 describes it as "(silently) deprecated"), wouldn't we want a future-incompatible lint rather than an idiom lint? (... in expressions is already illegal.)

[kennytm]

One remaining issue of a..=b pattern is #48501. When stabilizing a..=b, to avoid the confusing interpretation of &a...b, in the following example:

fn main() {
    match &5 {
        &3...6 => {}
        _ => {}
    }
}

if we just s/.../..=/g the code will error with

error: the range pattern here has ambiguous interpretation
 --> src/main.rs:3:10
  |
3 |         &3..=6 => {}
  |          ^^^^^ help: add parentheses to clarify the precedence: `(3 ..=6)`

Note that pattern_parentheses (#48500) is still unstable 🙃. However, given that default binding modes (#42640) is already stable, an even more idiomatic fix would probably be

fn main() {
    match &5 {
        3..=6 => {}   // no &, no (), no whatever
        _ => {}
    }
}

[nikomatsakis]

@zackmdavis

If we hope to someday get rid of ... in patterns (the current description of #28237 describes it as "(silently) deprecated"), wouldn't we want a future-incompatible lint rather than an idiom lint? (... in expressions is already illegal.)

I classified it as an idiom lint because the code will not cause a hard error in the new edition, even if it is frowned upon.