Incorrect "unreachable pattern" warning on nightly

[alex]

I tried this code:

pub fn parse_data(data: &[u8]) -> u32 {
    match data {
        b"" => 1,
        _ => 2,
    }
}

fn main() {
    println!("{}", parse_data(b"asd"))
}

It produces the following, incorrect, warning:

warning: unreachable pattern
 --> src/main.rs:4:9
  |
4 |         _ => 2,
  |         ^
  |
  = note: `#[warn(unreachable_patterns)]` on by default

however, the output is 2 so it's clearly incorrect

Meta

rustc --version:

1.50.0-nightly (2020-11-13 74f7e32f43b5fb0f8389)

[jonas-schievink]

Segfault:

fn parse_data(data: &[u8]) -> u32 {
    match data {
        b"" => 1,
    }
}

fn main() {
    let val = parse_data(b"oops");
    
    println!("{}", val);
}

[jonas-schievink]

cc @Nadrieril

[sfackler]

Fails to compile on both stable and beta:

error[E0004]: non-exhaustive patterns: `&[_, ..]` not covered
 --> src/main.rs:2:11
  |
2 |     match data {
  |           ^^^^ pattern `&[_, ..]` not covered
  |
  = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
  = note: the matched value is of type `&[u8]`

[alex]

I'm bisecting now.

[alex]

searched nightlies: from nightly-2020-10-10 to nightly-2020-11-13
regressed nightly: nightly-2020-10-26
searched commits: from ffa2e7a to 4760b8f
regressed commit: f58ffc9

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --end=2020-11-13 --test-dir=foo --prompt 

[jyn514]

Might be #78072 ?

[alex]

I'm far from a rustc expert, but based on the PR title and the fact that the changes to compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs specifically impact a special-case for bytes literals, I think that's a very likely candidate.

[estebank]

Purely based of the description, #78072 seems like a likely culprit. CC @Nadrieril, would you be able to take a look?

Edit: as mentioned by @alex, this change looks likely to be the cause https://github.com/rust-lang/rust/pull/78072/files#diff-6d8d99538aca600d633270051580c7a9e40b35824ea2863d9dda2c85a733b5d9L390