Derive `PartialOrd` via `Ord` when deriving both on a concrete type

[scottmcm]

Today, if you do https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=1ba35655ac2a5a415dc94df6a2b04988

#[derive(Copy, Clone)]
struct Foo(i32, u32);

it expands to

#[automatically_derived]
impl ::core::marker::Copy for Foo { }
#[automatically_derived]
impl ::core::clone::Clone for Foo {
    #[inline]
    fn clone(&self) -> Foo {
        let _: ::core::clone::AssertParamIsClone<i32>;
        let _: ::core::clone::AssertParamIsClone<u32>;
        *self
    }
}

That's wonderful -- it's simpler than a bunch of clone calls, and likely codegens better too.

However, if you try

#[derive(Ord, PartialOrd)]
struct Foo(i32, u32);

you just get

#[automatically_derived]
impl ::core::cmp::Ord for Foo {
    #[inline]
    fn cmp(&self, other: &Foo) -> ::core::cmp::Ordering {
        match ::core::cmp::Ord::cmp(&self.0, &other.0) {
            ::core::cmp::Ordering::Equal =>
                ::core::cmp::Ord::cmp(&self.1, &other.1),
            cmp => cmp,
        }
    }
}
#[automatically_derived]
impl ::core::cmp::PartialOrd for Foo {
    #[inline]
    fn partial_cmp(&self, other: &Foo)
        -> ::core::option::Option<::core::cmp::Ordering> {
        match ::core::cmp::PartialOrd::partial_cmp(&self.0, &other.0) {
            ::core::option::Option::Some(::core::cmp::Ordering::Equal) =>
                ::core::cmp::PartialOrd::partial_cmp(&self.1, &other.1),
            cmp => cmp,
        }
    }
}

which is unfortunate, because all those options take more time to compile and are harder for backends to optimize away.

It would be nice if, instead, it expanded to something like

#[automatically_derived]
impl ::core::cmp::Ord for Foo {
    #[inline]
    fn cmp(&self, other: &Foo) -> ::core::cmp::Ordering {
        match ::core::cmp::Ord::cmp(&self.0, &other.0) {
            ::core::cmp::Ordering::Equal =>
                ::core::cmp::Ord::cmp(&self.1, &other.1),
            cmp => cmp,
        }
    }
}
#[automatically_derived]
impl ::core::cmp::PartialOrd for Foo {
    #[inline]
    fn partial_cmp(&self, other: &Foo)
        -> ::core::option::Option<::core::cmp::Ordering> {
        let _: ::core::cmp::AssertParamIsPartialOrd<i32>;
        let _: ::core::cmp::AssertParamIsPartialOrd<u32>;
        Some(::core::cmp::Ord::cmp(self, other))
    }
}

Re-using the logic from Ord and making it easier for the optimizer to know that Foo::partial_cmp never actually returns None.

Like with Clone, this must only be done for fully concrete (non-generic) types, where the fact that Ord is also derived is enough to guarantee that we can call Ord from PartialOrd (as there will be a compiler error in the derived Ord if not).

[scottmcm]

Thinking further, I don't know if the AssertParamIsPartialOrd is needed -- the Ord derive will use Ord on all the fields, which will make sure they're PartialOrd, and that might be sufficient.