Inconsistent lifetime inference when using impl Trait reference in function return position

[jflatow]

Given that this is using an unstable feature, I'm not sure if a bug report is appropriate, but I thought it might be useful information for those working on #99697.

I tried this code:

#![feature(return_position_impl_trait_in_trait)]

pub trait A<'a, Q: P<'a>>: Sized {
    fn m(&self) -> Option<&'a impl M<'a, Q, Self>>; // impl ref fails, generic succeeds
}

pub enum E<'a, Q: P<'a>, N: M<'a, Q, Self>> {
    O(&'a N, Q),
}

impl<'a, Q: P<'a>, N: M<'a, Q, Self>> A<'a, Q> for E<'a, Q, N> {
    fn m(&self) -> Option<&'a impl M<'a, Q, Self>> {
        match self {
            Self::O(n, _) => Some(*n),
        }
    }
}

pub trait M<'a, Q: P<'a>, B: A<'a, Q>>: Sized {}

pub trait S<'a>: Sized {
    type A;

    fn s<'b>(&'a self, u: &'b Self::A) where 'a: 'b;
}

pub trait P<'a>: Sized + 'a {
    type M: M<'a, Self, <Self as S<'a>>::A>;
}

impl<'a, T: P<'a>> S<'a> for T {
    type A = E<'a, Self, T::M>;

    fn s<'b>(&'a self, u: &'b Self::A) where 'a: 'b {
        u.m();
    }
}

I expected this to compile. If I add a generic N: M<...> above and return a reference to it instead in the trait, it does work.

Instead when using the &'a impl M<...>, the compiler insists that 'b needs to live longer than 'a, which I think is an overly restrictive inference that prevents efficient memory management as it cannot be bypassed.

Meta

rustc +nightly --version --verbose:

rustc 1.75.0-nightly (187b8131d 2023-10-03)
binary: rustc
commit-hash: 187b8131d4f760f856b214fce34534903276f2ef
commit-date: 2023-10-03
host: x86_64-apple-darwin
release: 1.75.0-nightly
LLVM version: 17.0.2

Backtrace

error: lifetime may not live long enough
  --> lifetime.rs:35:9
   |
31 | impl<'a, T: P<'a>> S<'a> for T {
   |      -- lifetime `'a` defined here
...
34 |     fn s<'b>(&'a self, u: &'b Self::A) where 'a: 'b {
   |          -- lifetime `'b` defined here
35 |         u.m();
   |         ^^^^^ argument requires that `'b` must outlive `'a`
   |
   = help: consider adding the following bound: `'b: 'a`

error: aborting due to previous error

[compiler-errors]

This is consistent with the capture rules for RPITITs. The problem here is that the signature for:

fn m(&self) -> Option<&'a impl M<'a, Q, Self>> {

requires that 'a outlives all of the lifetimes captured by the impl M<...>, including the elided lifetime for &self.

This can be fixed by adding an explicit outlives bound on the impl M<...>, like impl M<...> + 'a:

fn m(&self) -> Option<&'a (impl M<'a, Q, Self> + 'a)> {

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e1fceffe62d5a0fb4666389a84edda94

[jflatow]

Thanks that is tricky and great to know!