Missing "unreachable pattern" error with range

[eefriedman]

fn main() {
    match 'a' {
        'a' => {}
        'b' => {}
        'a'...'b' => {}
        _ => {}
    }
}

Currently compiles, but the 'a'...'b' pattern is clearly unreachable.

[Aatch]

I looked into this, and I'm not sure it's possible to fix without a massive amount of work. Basically, we check for unreachable patterns by seeing if a pattern is still "useful" by the time we get to it. We determine whether or not it's useful by specialising as we go down. The way the code is written right now means you can't create new patterns or modify existing ones, so we can't modify or split the range pattern during specialisation.

Ideally we should be able to do something like this (modified to be a bit more explanatory):

match 'a' {
  'a' => { }
  'c' => { }
  'a'...'e' => { }
  _ => { }
}
// Specialize for the first arm
if 'a' == 'a' { } else {
  match 'a' {
    'c' => { }
    'b'...'e' => { } // Modified range to account for != 'a'
    _ => { }
  }
}
// specialize for the next arm
if 'a' == 'a' { } 
else if 'a' == 'c' { }
else {
    match 'a' {
    'b'...'b' | 'd'...'e' => { } // split range to account for !='c'
    _ => { }
  }
}
// And so on...

However, doing so will require a signfiicant amount of work in the relevant part of the compiler.

[brson]

@nikomatsakis says not a bug.