use<> bounds ("precise capture") require all type parameters to be listed

[nikomatsakis]

The current implementation of precise capture requires that use<> bounds include all type parameters in scope. This is a temporary limitation that should be lifted. Example:

#![allow(warnings)]

fn main() {
    let mut data = vec![1, 2, 3];
    let mut i = indices(&data);
    data.push(4);
    i.next();
}

fn indices<T>(
    slice: &[T],
) -> impl Iterator<Item = usize> + use<> {
    0 .. slice.len()
}

This code should compile, but it current gives an error (playground):

error: `impl Trait` must mention all type parameters in scope in `use<...>`
  --> src/main.rs:12:6
   |
10 | fn indices<T>(
   |            - type parameter is implicitly captured by this `impl Trait`
11 |     slice: &[T],
12 | ) -> impl Iterator<Item = usize> + use<> {
   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: currently, all type parameters are required to be mentioned in the precise captures list

[compiler-errors]

There are certainly some cases where it will be impossible to "fix" this, though. I expect that all non-method type and const parameters will always need to be captured for RPITITs to be sound. We should probably explore which parameters are obligatory to capture in what scenarios.

[nikomatsakis]

Closing in favor of #130043

[theemathas]

I think that this example code is not very good, since changing the use<> to use<T> causes the code to compile.

[compiler-errors]

@theemathas: The whole point of the example is to return an opaque that does not capture T. This has observable side-effects.