Switch `#[derive(PartialEq)]` to have separate lhs and rhs lifetimes

[programmerjake]

As far as I can tell the following code shouldn't cause a lifetime error but it does. If Iter::Item is changed to &'a () instead of PhantomData<&'a ()> the error goes away.

use core::marker::PhantomData;

fn g<A: Iterator, B: Iterator>(a: A, b: B) -> A
where
    A::Item: PartialEq<B::Item>,
{
    todo!()
}

pub struct Iter<'a>(&'a ());

impl<'a> Iterator for Iter<'a> {
    type Item = PhantomData<&'a ()>;
    fn next(&mut self) -> Option<Self::Item> {
        todo!()
    }
}

pub fn f<'a, B: AsRef<()>>(a: &'a (), b: B) -> &'a () {
    g(Iter(a), Iter(b.as_ref())).0
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0309]: the parameter type `B` may not live long enough
  --> src/lib.rs:20:21
   |
19 | pub fn f<'a, B: AsRef<()>>(a: &'a (), b: B) -> &'a () {
   |              -- help: consider adding an explicit lifetime bound...: `B: 'a +`
20 |     g(Iter(a), Iter(b.as_ref())).0
   |                     ^ ...so that the type `B` is not borrowed for too long

error[E0309]: the parameter type `B` may not live long enough
  --> src/lib.rs:20:23
   |
19 | pub fn f<'a, B: AsRef<()>>(a: &'a (), b: B) -> &'a () {
   |              -- help: consider adding an explicit lifetime bound...: `B: 'a +`
20 |     g(Iter(a), Iter(b.as_ref())).0
   |                       ^^^^^^ ...so that the reference type `&B` does not outlive the data it points at

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0309`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

[programmerjake]

@rustbot label: +C-bug

[eggyal]

The sole implementation of PartialEq for PhantomData<T> is where Rhs is also PhantomData<T>, so the call to g requires the lifetimes to match.

[programmerjake]

hmm, I had changed it to PhantomData in an attempt to reduce the problem, it had before been an enum like so:

#[derive(PartialEq)]
pub enum MyItem<'a> {
    Empty,
    Thing(&'a [u8]),
}

I'd expect to be able to compare two different instances of MyItem with differing lifetimes...

[programmerjake]

That produces

#[automatically_derived]
#[allow(unused_qualifications)]
impl <'a> ::core::cmp::PartialEq for MyItem<'a> {
    // ...
}

So, I think #[derive(PartialEq)] should instead let the lifetimes be unequal when comparing, producing:

#[automatically_derived]
#[allow(unused_qualifications)]
impl<'a, 'b> ::core::cmp::PartialEq<MyItem<'b>> for MyItem<'a> {
    // ...
}

[eggyal]

I think whether two values that have different lifetimes can be considered equal or not is application-dependent. If, in your case, they are equal then you merely need implement PartialEq yourself rather than use the default derive implementation.

[programmerjake]

Hmm, I'd argue that you'd have some field's PartialEq that requires matching lifetimes if you wanted to have derive(PartialEq) require matching lifetimes...I think the default should be to just match the PartialEq implementations of all fields:

impl<'lhs1, 'rhs1> PartialEq<Type<'rhs1>> for Type<'lhs1>
where
    Field1<'lhs1>: PartialEq<Field1<'rhs1>>,
    Field2<'lhs1>: PartialEq<Field2<'rhs1>>,
    Field3<'lhs1>: PartialEq<Field3<'rhs1>>,
{
    // ...
}

[eggyal]

@rustbot label: +A-lifetimes +A-macros -C-bug +C-enhancement +T-libs

[eggyal]

See also #27950 and, more generally, #26925.