0000-unreachable-patterns-error.md

• Feature Name: Make unreachable patterns error, not warning
• Start Date: 2017-07-06
• RFC PR: (leave this empty)
• Rust Issue: (leave this empty)

Summary

At now compiler accept such nonsense:

let x = 1;
match x {
  y => {}
  z => {}
  _ => {}
}

rustc generate warnings, not errors about the same patterns, but this is 100% error of developer, so let's show this as error.

Motivation

Novice in rustc can try to use match with variables, not literals, like this:

let x = calculate();
let y = 2;
let z = 3;
match x {
  y => {}
  z => {}
  _ => {}
}

this code compiles and run and sometimes warning about unused variables (y, z) comes first before warning about unreachable pattern, what makes unclear what is going on.

So to simplify for novice match usage, let's make unreachable pattern is error, not warning.

Also not novice may made typo, for example miss z if x == z in some of "match arms", so error clear show typo.

How We Teach This

I suppose that this behaviour expected from match. Compiler/language is strict enough to not allow miss some branch, so obviously expected that it not allow duplicate branches.

Drawbacks

Some buggy code will be not accepted by compiler.