Message reporting missing patterns for inexhaustive match is misleading

[vandenheuvel]

A small demo:

#[derive(PartialEq, Eq, Clone, Copy)]
enum X { A, B, C, }

fn main() {
  match (X::A, Some(X::A)) {
    (_, None) => false,
    (v, Some(w)) if v == w => true,
    (X::B, Some(X::C)) => false,
    (X::B, Some(X::A)) => false,
    (X::A, Some(X::C)) | (X::C, Some(X::A)) => false,
  };
}

Playground.

I expected to see this happen
The usual error message indicating that patterns (A, Some(B)) and (C, Some(B)) are not covered.

Instead, this happened
Patterns (A, Some(A)) and (A, Some(B)) are detected as not covered.

rustc --version --verbose:

rustc 1.45.0-nightly (a74d1862d 2020-05-14)
binary: rustc
commit-hash: a74d1862d4d87a56244958416fd05976c58ca1a8
commit-date: 2020-05-14
host: x86_64-unknown-linux-gnu
release: 1.45.0-nightly
LLVM version: 9.0

[jonas-schievink]

If you add the suggested arms, it will in fact detect the other ones as missing. It is expected that (A, Some(A)) is detected as not covered since the match guard is not taken into account when checking exhaustiveness (and couldn't be, since it's an arbitrary expression).

[jonas-schievink]

Closing as expected behavior.

[vandenheuvel]

It is expected that (A, Some(A)) is detected as not covered since the match guard is not taken into account when checking exhaustiveness

Wouldn't you then expect that it mentions (A, Some(A)), (B, Some(B)), (C, Some(C)), (A, Some(B)) and (C, Some(B)) as not covered? Or is the number of outputs always restricted to two? In that case, I think it would be useful to indicate that some patterns might be missing, e.g.:

error[E0004]: non-exhaustive patterns: at least `(A, Some(A))` and `(A, Some(B))` not covered

[jonas-schievink]

Yeah that's true