rust-lang / rust-lang/rust

Random access impl of `Take::for_each`/`Take::fold` suppresses side effects of `next` call on empty iterator

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#161,350 5 comments 0 reactions 1 assignee View on GitHub

@Indrapal-70 is already working on this.

Since Aug 20, 2026.

A-docs A-iterators C-bug T-libs
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Rust
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Description

I tried this code:

use std::fmt::Debug;

fn main() {
    eprintln!("does_print:");
    consume(does_print());
    eprintln!("does_not_print:");
    consume(does_not_print());
}

fn does_not_print() -> impl Iterator<Item: Debug> {
    [1, 2, 3].iter()
}
fn does_print() -> impl Iterator<Item: Debug> {
    [1, 2, 3].iter().filter(|_| true) // use any noop adapter without `TrustedRandomAccessNoCoerce` here
}

fn consume(iter: impl Iterator<Item: Debug>) {
    // the skip covers all the elements, meaning `take` is called on an empty iterator
    iter.map(|it| dbg!(it)).skip(3).take(100).for_each(|_| {});
}

I expected to see this happen:

Either both iterations print, or neither does, since the only difference is that one iterator is being adapted with filter(true), which is a noop.

Instead, this happened:

does_print:
[src/main.rs:11:19] it = 1
[src/main.rs:11:19] it = 2
[src/main.rs:11:19] it = 3
does_not_print:

@rustbot label A-iterators T-libs

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