while_let_on_iterator suggestion doesn't compile

[jyn514]

I tried this code:

impl<T: Stackable> Drop for IntoIter<T> {
    fn drop(&mut self) {
        unsafe {
            while let Some(_) = self.next() {}
            OPENSSL_sk_free(self.stack as *mut _);
        }
    }
}

I expected to see this happen: Clippy suggests code that compiles:

diff --git a/boring/src/stack.rs b/boring/src/stack.rs
index 043ee178..70f366f6 100644
--- a/boring/src/stack.rs
+++ b/boring/src/stack.rs
@@ -131,7 +131,7 @@ pub struct IntoIter<T: Stackable> {
 impl<T: Stackable> Drop for IntoIter<T> {
     fn drop(&mut self) {
         unsafe {
-            while let Some(_) = self.next() {}
+            for _ in &mut *self {}
             OPENSSL_sk_free(self.stack as *mut _);
         }
     }

Instead, this happened: Clippy's suggestion doesn't compile.

warning: this loop could be written as a `for` loop
   --> boring/src/stack.rs:134:13
    |
134 |             while let Some(_) = self.next() {}
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for _ in &mut self`
    |
    = note: `#[warn(clippy::while_let_on_iterator)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#while_let_on_iterator

error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
   --> boring/src/stack.rs:134:22
    |
134 |             for _ in &mut self {}
    |                      ^^^^^^^^^
    |                      |
    |                      cannot borrow as mutable
    |                      try removing `&mut` here

(for _ in self {} doesn't compile either because it moves self, and it's used again afterwards.)

Meta

• cargo clippy -V: clippy 0.1.54 (a178d03 2021-07-26)
• rustc -Vv:

rustc 1.54.0 (a178d0322 2021-07-26)
binary: rustc
commit-hash: a178d0322ce20e33eac124758e837cbd80a6f633
commit-date: 2021-07-26
host: x86_64-unknown-linux-gnu
release: 1.54.0
LLVM version: 12.0.1

[SamRodri]

I got the same false positive in a tree iterator code, reduced:

pub struct Foo<I> {
    iter: I
}
impl<I: Iterator<Item=u8>> Foo<I> {
    pub fn test(&mut self) {
        while let Some(_) = self.iter.next() {
            for _ in &mut self.iter {
            }
        }
    }

    // pub fn incorrect_suggestion(&mut self) {
    //     for _ in &mut self.iter {
    //         for _ in &mut self.iter {
    //         }
    //     }
    // }
}

I don’t think there is a correct way to use a for loop in this case.

[pickfire]

Same issue here https://github.com/helix-editor/helix/pull/525/files#diff-0dd7da00658aae6b00b285feac8d5a8a9f41717a1000be65fac7fa2874d322f7R248, we just had #[allow(clippy::while_let_on_iterator)].