manual_flatten suggestin make code not compilable

[xNxExOx]

Lint name: manual_flatten

I tried this code:
Code is simplified so workers are never set, but to trigger the incorrect sugestion it is enough.
The full code starts same with all workers doing nothing, and later in the loop assign work to them, and here early in the loop let them finish the work if it is the right time.

fn main() {
    let mut time = 0;
    let mut workers : [Option<(u8, i32)>; 5] = [None, None, None, None, None];
    loop {
        for w in &mut workers {
            if let Some((_to, when)) = w {
                if time == *when {
                    // ...
                    *w = None;
                }
            }
        }
        // ...
        time += 1;
    }
}

I expected to see this happen: No suggestion probably.

Instead, this happened:

warning: unnecessary `if let` since only the `Some` variant of the iterator element is used
  --> src\main.rs:6:9
   |
6  |           for w in &mut workers {
   |           ^        ------------ help: try: `workers.iter_mut().flatten()`
   |  _________|
   | |
7  | |             if let Some((_to, when)) = w {
8  | |                 if time == *when {
9  | |                     *w = None;
10 | |                 }
11 | |             }
12 | |         }
   | |_________^
   |
   = note: `#[warn(clippy::manual_flatten)]` on by default
help: ...and remove the `if let` statement in the for loop
  --> src\main.rs:7:13
   |
7  | /             if let Some((_to, when)) = w {
8  | |                 if time == *when {
9  | |                     *w = None;
10 | |                 }
11 | |             }
   | |_____________^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_flatten

warning: 1 warning emitted

In this simplified example line 9 *w = None; is visible.
Am I missing a way how to handle this line? I asked on Rust community server and no one find a way how to make this work, so I think it must be false positive.

Meta

• cargo clippy -V: clippy 0.1.52 (fe1bf8e 2021-02-23)
• rustc -Vv:

rustc 1.52.0-nightly (fe1bf8e05 2021-02-23)
binary: rustc
commit-hash: fe1bf8e05c39bdcc73fc09e246b7209444e389bc
commit-date: 2021-02-23
host: x86_64-pc-windows-msvc
release: 1.52.0-nightly
LLVM version: 11.0.1

[xNxExOx]

manual_flatten interaction with nested matches could better #6776 is probably related, but that code will still compile, just the simplification of it is not as good as could be expected, by a bit misleading message. So I do not think they are same and should not be merged together. (just my opinion)

[camsteffen]

I agree. This is a separate issue since we should not lint if w is used in any way. This should be an easy fix with LocalUsedVisitor.

[chris-oo]

I have some code that I believe is hitting this exact issue. Here's another example that triggers another false positive with this same lint. It's stripped down, but essentially it seems like clippy is not detecting cases where the outer option variable is actually being used.

fn main() {
    let mut vec = vec![Some(10), None, Some(20)];
    
    for entry in vec.iter_mut() {
        if let Some(number) = entry {
            if *number < 15 {
                let number = entry.take().unwrap();
                print!("removed {}", number);
            }
        }
    }
}