Error message when using non-derived constants from another crate in pattern matching is confusing

[seanmonstar]

The error message is perfectly fine when inside the crate that has the type, because changing or adding a derive(PartialEq, Eq) is something you can control.

error: to use a constant of type `mime::Mime` in a pattern, `mime::Mime` must be annotated with `#[derive(PartialEq, Eq)]`
   --> src/main.rs:187:39
    |
187 |                     Some(&ContentType(mime::TEXT_XML)) => (),
    |                                       ^^^^^^^^^^^^^^

However, when the type isn't in your crate, the message makes the user think the other crate simply "forgot" to implement PartialEq, and there is nothing for them to do. A suggestion would be to change the error message when the type is not a crate-local, and add a note of how to fix it.

error: a constant of type `mime::Mime` cannot be used in a pattern
   --> src/main.rs:187:39
    |
187 |                     Some(&ContentType(mime::TEXT_XML)) => (),
    |                                       ^^^^^^^^^^^^^^
    |
    = note: try pattern guards insteand: `Some(&ContentType(ref a)) if a == &mime::TEXT_XML => (),`

[chewi]

Having been confused by this, it would be helpful but is it still not true that the other crate forgot to implement PartialEq? Why wouldn't it? I hit this with http::uri::scheme::Scheme.

[hronro]

Same issue with http::uri::Schema.

However, I found http::uri::Schema actually has implemented Eq and PartialEq .

Does it means Eq and PartialEq must be implemented via derive macros? If so, why there is a such limitation in Rust compiler?

[chewi]

I just hit this again and found myself back here, having forgotten about the first time. 🤦‍♂️ @seanmonstar, http is your crate, can you please explain the situation there?

[sam0x17]

Just hit this implementing an assert_matches! macro for my sin crate:

#[macro_export]
macro_rules! assert_matches {
    ($expr:expr, Token::$variant:ident($sym:literal)) => {{
        const _: () = {
            const SYMBOL: $crate::Symbol = $crate::Symbol::from_static($sym);
            assert!(matches!($expr, $crate::Token::$variant(SYMBOL)));
        };
    }};
}

assert_matches!(tt![something], Token::CustomKeyword("something"));

error: to use a constant of type `Symbol` in a pattern, `Symbol` must be annotated with `#[derive(PartialEq, Eq)]`
  --> tests/tokens_tests.rs:71:5
   |
71 |     assert_matches!(tt![something], Token::CustomKeyword("something"));
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Confusingly, Symbol (which is from the symbol crate) does indeed have an implementation for PartialEq and a derive implementation for Eq. I'm actually not sure how to fix this so the message is definitely confusing.

Do I have to newtype Symbol and add derive implementations myself?

[shehroz-weoveri]

Fixed by adding
value-bag = "1.4.1"
to cargo.toml

[estebank]

Current output:

error: to use a constant of type `S` in a pattern, `S` must be annotated with `#[derive(PartialEq)]`
 --> src/main.rs:7:9
  |
7 |         C => {}
  |         ^
  |
  = note: the traits must be derived, manual `impl`s are not sufficient
  = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details