Attribute(?) to selectively disable "unreachable pattern" errors

[rntz]

When writing macros that expand to "match" expressions, it can be hard to guarantee that all cases of the match will be necessary. Consider the following macro:

macro_rules! case(
  ($e:expr, $p:pat where $cond:expr => $thn:expr, $els:expr) => (
    match $e {
      $p => if $cond { $thn } else { $els },
      _ => $els
    }
  )
)

An example use of this macro would be case!(some_expr, x where x > 3 => x, 3), which is equivalent to max(3, some_expr) (a silly example, but still).

Unfortunately, this example fails to compile because it expands to

match some_expr {
  x => if x > 3 { x } else { 3 },
  _ => 3
}

The last case of the match is redundant. However, there are also uses of the macro where the last case will not be redundant, eg. case!(some_expr, [email protected] where x % 2 == 0 => true, false).

It would be nice if there were some way to mark that a particular match expression is allowed to have unreachable cases, or (better yet) that a particular case was allowed to be unreachable.

[catamorphism]

Nominating for milestone 3, feature-complete

[graydon]

this would just be a scoped lint, right? #[allow(unreachable_pattern)] like?

[catamorphism]

I think this was never actually discussed, so, re-labelling it as "nominated".

[emberian]

Still relevant.

[graydon]

accepted for feature-complete milestone

[pnkfelix]

Not a 1.0 blocker, P-low.

[huonw]

Triage, no change.

[steveklabnik]

Triage: no change

[jonas-schievink]

To anyone needing this now:

match 0 {
    1 ... 3 => {}
    x if true => {}
    _ => {}
}

[tbu-]

There's an easy work-around, don't think an attribute is necessary here.

[steveklabnik]

Since new lints have a big impact on users of rustc, the policy is that they should go through the RFC process like other user-facing changes. As such, I'm going to give this one a close, but if anyone comes across this ticket and wants this lint, consider adding it to clippy and/or writing up an RFC. Thanks!