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