Add #[context()] for the function which has the return type impl Iterator<Item = xxx>, get error[E0562]

[HuijingHei]

When using

Please indicate the precise nightly build you are using; try rustc --version

# rustc --version
rustc 1.74.0 (79e9716c9 2023-11-13) (Fedora 1.74.0-1.fc39)

What I tried to do

Please include sample code or links to your project!

If add #[context()] for the function which has the return type impl Iterator<Item = xxx>, get error[E0562]: impl Trait only allowed in function and inherent method return types, not in closure return types.

use fn_error_context::context;

#[context("Scan all tmpfiles conf and save entries")]
fn empty_iterators() -> impl Iterator<Item = String> {
    std::iter::empty::<_>()
}

fn main() {   
    let _ = empty_iterators();   
}

What happened

Describe what happened in as much detail as possible.
For compiler ICEs, a good idea would be to include the output with RUST_BACKTRACE=1.

When run above code, get error:

error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in closure return types
 --> src/main.rs:4:25
  |
4 | fn empty_iterators() -> impl Iterator<Item = String> {
  |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0562`.

What I expected

When remove #[context()] line, no error. But it should support.

[cgwalters]

I think the tricky thing here is that due to the use of the fn-error-context crate it's not immediately obvious (to someone starting work on a codebase that uses the crate without being familiar with its implementation) that the macro rewrote the function to use an inner closure.

Hmm, I wonder if it'd be possible to change that crate to use an inner fn instead of a closure...thinking about it briefly it seems likely.

I'm not a maintainer for this repository but I suspect there's not much immediately actionable for the Rust trait maintainers.

Ah, there's already an issue: andrewhickman/fn-error-context#7 so I think we should probably discuss there.

[HuijingHei]

Thanks @cgwalters for the clarification, then I will close this issue and track in andrewhickman/fn-error-context#7