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openedon Feb 18, 2023
Consider following code:
#[derive(Debug, PartialEq, Eq)]
struct Foo([u32; 4]);
const fn bar() {
const ONE: Foo = Foo([1, 0, 0, 0]);
match Foo([0, 0, 0, 0]) {
ONE => unreachable!(),
_ => {},
}
if ONE == Foo([0, 0, 0, 0]) {
unreachable!()
}
}https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=f2b2c2421583fbb47472f8e8746b394f
It fails with "the trait PartialEq<_> is implemented for Foo, but that implementation is not const".
But if we do add a const implementation:
#![feature(const_trait_impl)]
#[derive(Debug, Eq)]
struct Foo([u32; 4]);
impl const PartialEq for Foo {
fn eq(&self, other: &Self) -> bool {
self.0[0] == other.0[0]
&& self.0[1] == other.0[1]
&& self.0[2] == other.0[2]
&& self.0[3] == other.0[3]
}
}
const fn bar() {
const ONE: Foo = Foo([1, 0, 0, 0]);
match Foo([0, 0, 0, 0]) {
ONE => unreachable!(),
_ => {},
}
if ONE == Foo([0, 0, 0, 0]) {
unreachable!()
}
}https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=a980e3ab7d72b9545886de5547cf4963
then it fails with "to use a constant of type Foo in a pattern, Foo must be annotated with `#[derive]"
So we found ourselves between rock and hard place where we have to choose between using pattern matching and equality.
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Area: const fn foo(..) {..}. Pure functions which can be applied at compile time.Area: const fn foo(..) {..}. Pure functions which can be applied at compile time.Relating to patterns and pattern matchingRelating to patterns and pattern matching