Confusing error message when matching on &str in const fn

[ilyvion]

Given the following code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=bc51fb79544b1be00dd8a4285103ff07

const fn foo(input: &'static str) {
    match input {
        "a" => (),
        _ => (),
    }
}

The current output is:

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 --> src/lib.rs:3:9
  |
3 |         "a" => (),
  |         ^^^

I feel like it's pretty self-explanatory, but this is obviously a very confusing error. A match arm for a string literal is not a "call" by any usual meaning of the word. In fact, I have no idea what actual objection rustc has to this code, if any.

As far as I can tell, this error is the same on all current versions of the compiler (stable, beta, nightly) per the submission of this issue.

[PatchMixolydic]

I'm not familiar with how matching on &str is implemented, so this is pure conjecture, but the "call" might be an implicit call to input.eq("a"). The == operator issues the same confusing diagnostic (playground):

const fn foo(input: &'static str) {
    input == "a";
}

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 --> src/lib.rs:2:5
  |
2 |     input == "a";
  |     ^^^^^^^^^^^^

@rustbot modify labels: +A-const-eval +D-confusing

[detly]

Note also that a byte string literal is fine:

const fn decode_bytes(b: &[u8]) -> Option<u8> {
    match b {
        b"0" => Some(0),
        _ => None,
    }
}

☝️ compiles on 1.64, but

const fn decode_string(s: &str) -> Option<u8> {
    match s {
        "0" => Some(0),
        _ => None,
    }
}

...fails with:

error[[E0015]](https://doc.rust-lang.org/stable/error-index.html#E0015): cannot call non-const operator in constant functions
 --> src/lib.rs:3:9
  |
3 |         "0" => Some(0),
  |         ^^^
  |
  = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

For more information about this error, try `rustc --explain E0015`.
error: could not compile `playground` due to previous error

[jnnnnn]

I ran into this as well. It's definitely calling <str as PartialEq>::eq as part of the match. The way to prove so is to look at the MIR (and remove const so the code compiles).

The const_str crate gets around this by converting to bytes for the comparison.

Here's a working example: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=286b897bb518dec6617d3da197cd318b

Note, this seems related to #103040 . <str as PartialEq>::eq probably can't be const until #67792 lands.