PartialEq and matching in const contexts #13
Description
Currently we allow matches to emit PartialEq::partial_eq calls, but don't enforce anything about those calls' constness. AFAICT the only PartialEq::partial_eq call that seems to be emitted is for &str, since ADTs seem to be decomposed into matches on their fields.
The problem here is that I thought the whole point of matches requiring PartialEquality of scrutinees is that we can conceptually think of const arms as PartialEq calls; the fact that we decompose them seems like an impl detail.
When a struct contains a &str, this ends up getting caught by the MIR constness checking, but this seems sub-optimal. OTOH, we can't just require everyone to derive_const(PartialEq, Eq) on their match scrutinees, since they may be relying on this behavior...
What should we do 🤔
#[derive(Eq, PartialEq)]
struct Struct {
f: &'static str,
g: i32,
}
const FOO: Struct = Struct { f: "", g: 1 };
const BAR: Struct = Struct { f: "", g: 2 };
const fn test() {
match BAR {
FOO => {}
_ => {}
}
}error[E0015]: cannot match on `str` in constant functions
--> src/lib.rs:12:9
|
12 | FOO => {}
| ^^^
|
= note: `str` cannot be compared in compile-time, and therefore cannot be used in `match`es
= note: calls in constant functions are limited to constant functions, tuple structs and tuple variants