`-Znext-solver` with TinyVec and Rand

[wyatt-herkamp]

I was trying the new trait resolver system and I am getting compiler issues with two libraries

https://github.com/rust-random/rand
https://github.com/Lokathor/tinyvec

Tinyvec

error[E0311]: the associated type `<arrayvec::ArrayVec<A> as core::ops::Index<I>>::Output` may not live long enough
    |
help: consider adding an explicit lifetime bound
   --> src\arrayvec.rs:175:11
    |
175 |   fn index<'a>(&'a self, index: I) -> &'a Self::Output where <arrayvec::ArrayVec<A> as core::ops::Index<I>>::Output: 'a {
    |           ++++  ++                     ++              ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

error[E0311]: the associated type `<arrayvec::ArrayVec<A> as core::ops::Index<I>>::Output` may not live long enough
    |
help: consider adding an explicit lifetime bound
   --> src\arrayvec.rs:183:15
    |
183 |   fn index_mut<'a>(&'a mut self, index: I) -> &'a mut Self::Output where <arrayvec::ArrayVec<A> as core::ops::Index<I>>::Output: 'a {
    |               ++++  ++                         ++                  ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

error[E0311]: the associated type `<SliceVec<'s, T> as core::ops::Index<I>>::Output` may not live long enough
   |
help: consider adding an explicit lifetime bound
  --> src\slicevec.rs:53:11
   |
53 |   fn index<'a>(&'a self, index: I) -> &'a Self::Output where <SliceVec<'s, T> as core::ops::Index<I>>::Output: 'a {
   |           ++++  ++                     ++              ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

error[E0311]: the associated type `<SliceVec<'s, T> as core::ops::Index<I>>::Output` may not live long enough
   |
help: consider adding an explicit lifetime bound
  --> src\slicevec.rs:64:15
   |
64 |   fn index_mut<'a>(&'a mut self, index: I) -> &'a mut Self::Output where <SliceVec<'s, T> as core::ops::Index<I>>::Output: 'a {
   |               ++++  ++                         ++                  ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Rand

error[E0311]: the associated type `<ReseedingCore<R, Rsdr> as BlockRngCore>::Results` may not live long enough
    |
help: consider adding an explicit lifetime bound
   --> src\rngs\adapter\reseeding.rs:172:16
    |
172 |     fn generate<'a>(&mut self, results: &'a mut Self::Results) where <ReseedingCore<R, Rsdr> as BlockRngCore>::Results: 'a {
    |                ++++                      ++                    +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

error[E0311]: the associated type `<ReseedingCore<R, Rsdr> as BlockRngCore>::Results` may not live long enough
    |
help: consider adding an explicit lifetime bound
   --> src\rngs\adapter\reseeding.rs:240:27
    |
240 ~     fn reseed_and_generate<'a>(
241 ~         &mut self, results: &'a mut <Self as BlockRngCore>::Results, global_fork_counter: usize,
242 ~     ) where <ReseedingCore<R, Rsdr> as BlockRngCore>::Results: 'a {
    |

Instead, this happened: explanation

Meta

rustc --version --verbose:

rustc 1.77.0-nightly (d6d7a9386 2023-12-22)
binary: rustc
commit-hash: d6d7a93866f2ffcfb51828b8859bdad760b54ce0
commit-date: 2023-12-22
host: x86_64-pc-windows-msvc
release: 1.77.0-nightly
LLVM version: 17.0.6

The two libraries are depends of my project. I tried compiling them by themselves. Still getting the error.

I am not sure if this is the correct way to report this bug.

#107374

[SNCPlay42]

Reduced from tinyvec (note this requires -Znext-solver=globally, not just -Znext-solver=coherence):

use core::ops::Index;

pub struct SliceVec<T> {
    data: T,
}

impl<T, I> Index<I> for SliceVec<T> {
    type Output = ();
    fn index(&self, index: I) -> &Self::Output {
        unimplemented!()
    }
}

The rand version looks basically the same - the associated type mentioned is an argument, not the return type.

[compiler-errors]

This is fixed by #119101

[compiler-errors]

Correction: This will be fixed by #120513, since we need to fix it for NLL too.

[wyatt-herkamp]

The code compiles now with the -Znext-solver flag!
Thank y'all.