collapsible_match suggests an incorrect behavior change

[dtolnay]

fn next() -> Option<Enum> {
    None
}

enum Enum {
    Yes,
    No,
}

fn main() {
    while let Some(thing) = next() {
        if let Enum::Yes = thing {
            println!("yes");
        }
    }
}

$ cargo clippy

warning: unnecessary nested `if let` or `match`
  --> src/main.rs:12:9
   |
12 | /         if let Enum::Yes = thing {
13 | |             println!("yes");
14 | |         }
   | |_________^
   |
   = note: `#[warn(clippy::collapsible_match)]` on by default
help: the outer pattern can be modified to include the inner pattern
  --> src/main.rs:11:20
   |
11 |     while let Some(thing) = next() {
   |                    ^^^^^ replace this binding
12 |         if let Enum::Yes = thing {
   |                ^^^^^^^^^ with this pattern
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match

Clippy apparently wants this loop written as:

    while let Some(Enum::Yes) = next() {
        println!("yes");
    }

which is just not the same meaning.

Meta

• cargo clippy -V: e.g. clippy 0.1.56 (0035d9d 2021-08-16)
• rustc -Vv:

  rustc 1.56.0-nightly (0035d9dce 2021-08-16)
  binary: rustc
  commit-hash: 0035d9dcecee49d1f7349932bfa52c05a6f83641
  commit-date: 2021-08-16
  host: x86_64-unknown-linux-gnu
  release: 1.56.0-nightly
  LLVM version: 12.0.1
  

[camsteffen]

@rustbot claim

[teor2345]

We had a similar issue with an outer match:

fn next() -> Option<Enum> {
    None
}

enum Enum {
    Yes,
    No,
}

fn main() {
    match next() {
        Some(thing) => {
            if let Enum::Yes = thing {
                println!("yes");
            }
        }
        _ => panic!("unexpected None"),
    }
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=c09bd33fcac6b1c5f491e995f74503ce

Specifically, the original code does nothing on Enum::No, but the suggested code panics.