New lint: collapsible_guards

[camsteffen]

What it does

Detects match guards that can be collapsed into the pattern.

Categories (optional)

• Kind: complexity

What is the advantage of the recommended code over the original code

It is simpler.

Drawbacks

None.

Example

match x {
    Some(x) if matches!(x, Some(1)) => .., // if matches!
    Some(x) if x == Some(2) => .., // if ==
    x => matches!(x, Some(x) if x == 3), // guard inside matches!
}

Could be written as:

match x {
    Some(Some(1)) => ..,
    Some(Some(2)) => ..,
    x => matches!(x, Some(3)),
}

This lint can share some logic with equatable_if_let. @HKalbasi maybe you'd like to take this?

[isomer]

I came here to suggest a similar lint. While I was learning rust I was using guards where I should instead be using ranges eg:

match x {
  n if n < 10 => { ... },
  n if n >= 10 && n <= 20 => { ... },
  n if n > 20 => { ... },
  _ => unreachable!(),
}

and was frustrated that the compiler was requiring the unreachable clause. After looking at this a bit I discovered I really should have been writing this as:

match x {
 0..10 @ n => { ... },
 10..=20 @ n => { ... },
 20.. @ n => { ... },
}

The advantages here are both simplicity, teaching new rustaceans how to use pattern matching better, as well as additional safety guarantees. This also no longer needs the unreachable clause at the end.