`while_let_loop` suggests wrong name to use in pattern

[domenicquirl]

When running clippy on this code

fn foo() -> Option<usize> {
    todo!()
}

fn main() {
    loop {
        let x = match foo() {
            Some(n) => n,
            None => break,
        };
        println!("{}", x);
    }
}

it (rightly) suggests that while let could be used instead. However, the suggested fix is try: 'while let Some(n) = foo() { .. }' (playground link), taking the n from the pattern inside the match instead of the x from let x. Carrying out this replacement introduces compilation errors, as x is now undefined.

Expected behaviour:

Suggest trying while let Some(x), taking the variable name that binds the result of the match expression.

Meta

• cargo clippy -V: clippy 0.1.52 (9bc8c42b 2021-05-09)
• rustc -Vv:

  rustc 1.52.1 (9bc8c42bb 2021-05-09)
  binary: rustc
  commit-hash: 9bc8c42bb2f19e745a63f3445f1ac248fb015e53
  commit-date: 2021-05-09
  host: x86_64-pc-windows-msvc
  release: 1.52.1
  LLVM version: 12.0.0
  

[kraktus]

This is basically the same issue as #675, and that is why ellipsis are used in the first place.

For reference, the fix of #675:

LL | /     loop {
LL | |         let (e, l) = match "".split_whitespace().next() {
LL | |             Some(word) => (word.is_empty(), word.len()),
LL | |             None => break,
...  |
LL | |         let _ = (e, l);
LL | |     }
   | |_____^ help: try: `while let Some(word) = "".split_whitespace().next() { .. }`

[ada4a]

After #721, which fixed the aforementioned #675, Clippy now suggests rebinding n to x, which at least makes the code compile again:

warning: this loop could be written as a `while let` loop
  --> src/main.rs:9:5
   |
 9 | /     loop {
10 | |         let x = match foo() {
11 | |             Some(n) => n,
12 | |             None => break,
13 | |         };
14 | |         println!("{}", x);
15 | |     }
   | |_____^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#while_let_loop
note: the lint level is defined here
  --> src/main.rs:2:9
   |
 2 | #![warn(clippy::while_let_loop)]
   |         ^^^^^^^^^^^^^^^^^^^^^^
help: try
   |
 9 ~     while let Some(n) = foo() {
10 +         let x = n;
11 +         ..
12 +     }
   |

Should this be closed, similarly to #675?

[domenicquirl]

I don't think #721 was what fixed the lint to produce that output, since that predates the issue by several years and only changed the output to omit the loop body / replace it with {..}. However, #14756 recently addressed this together with #362, which was another case of the lint's suggestion failing to compile due to a missing binding. I still don't think the suggestion is optimal, but at least you can now apply the suggestion and the resulting code will compile 👍🏻