`manual_flatten` doesn't follow its suggestion to remove `if let` statement leading to broken code

[ulrichstark]

Summary

Clippy lint manual_flatten doesn't follow its own suggestion to remove if let statement leading to broken code.
Similar to #7514, but without references or slices.

Reproducer

I tried this code:

fn main() {
    let results: Vec<Result<String, ()>> = vec![];

    for result in results {
        if let Ok(value) = result {
            println!("{value}");
        }
    }
}

I expected to see this happen:

for result in results.into_iter().flatten() {
    println!("{result}");
}

Instead, this happened:

for result in results.into_iter().flatten() {
    if let Ok(value) = result { // broken because result is now of type String
        println!("{value}");
    }
}

Version

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-pc-windows-msvc
release: 1.86.0
LLVM version: 19.1.7

Additional Labels

@rustbot label +I-suggestion-causes-error

[donkomura]

@rustbot claim

[donkomura]

Hi, @ulrichstark .
Could you tell me the clippy suggestions?
I tried running Clippy, and the suggestion can be compiled.

Rust Playground

[ulrichstark]

I'm using rust-analyzer's quick fix "try: results.into_iter().flatten()" in VS Code.
It's only replacing results with results.into_iter().flatten(), but not removing the if.
Is this instead just an issue with rust-analyzer only applying the first suggestion of clippy?

Clippy suggestion:

warning: unnecessary `if let` since only the `Ok` variant of the iterator element is used
 --> src\main.rs:4:5
  |
4 |       for result in results {
  |       ^             ------- help: try: `results.into_iter().flatten()`
  |  _____|
  | |
5 | |         if let Ok(value) = result {
6 | |             println!("{value}");
7 | |         }
8 | |     }
  | |_____^
  |
help: ...and remove the `if let` statement in the for loop
 --> src\main.rs:5:9
  |
5 | /         if let Ok(value) = result {
6 | |             println!("{value}");
7 | |         }
  | |_________^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_flatten
  = note: `#[warn(clippy::manual_flatten)]` on by default