Implicitly copyable iterators are mega confusing #18045
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In a garbage collected language, adaptors would always take the wrapped iterator by-reference. It makes more sense that way, but it doesn't work well in Rust. Rust's iterator adaptors need to be able to take the wrapped iterator by-value, and the natural way to deal with it is only having by-value adaptors and then taking advantage of references being values ( |
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I think it would be wrong to avoid |
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Sure, I agree with all that. I just think there's a usability / confusion issue here that we need to resolve. Opt-in |
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Hm, I hadn't thought about putting these iterators in |
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cc @aturon |
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@kmcallister: It would end up causing warnings for typical iterator chains. I never liked that I had to use by-value for the parameters but I don't really see an alternative. |
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@kmcallister Opt-in I think that PR probably makes everything in |
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@aturon: It doesn't address the problem. It would be incorrect to leave out of a |
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It's hardly an issue specific to iterator types anyway... |
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This code still acts the same today, even after opt-in |
As per [RFC rust-lang#235][rfc], you can now do: [rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0235-collections-conventions.md#intoiterator-and-iterable ``` rust let mut v = vec![1]; // iterate over immutable references for x in &v { assert_eq!(x, &1); } // iterate over mutable references for x in &mut v { assert_eq!(x, &mut 1); } // iterate over values, this consumes `v` for x in v { assert_eq!(x, 1); } ``` [breaking-change]s For loops now "consume" (move) the iterator, this breaks iterating over mutable references to iterators, and also breaks multiple iterations over the same iterator: ``` rust fn foo(mut it: &mut Iter) { // `Iter` implements `Iterator` for x in it { .. } //~ error: `&mut Iter` doesn't implement Iterator } fn bar() { for x in it { .. } //~ note: `it` moved here for x in it { .. } //~ error: `it` has been moved } ``` Both cases can be fixed using the `by_ref()` adapter to create an iterator from the mutable reference: ``` rust fn foo(mut it: &mut Iter) { for x in it.by_ref() { .. } } fn bar() { for x in it.by_ref() { .. } for x in it { .. } } ``` This PR also makes iterator non-implicitly copyable, as this was source of subtle bugs in the libraries. You can still use `clone()` to explictly copy the iterator. Finally, since the for loops are implemented in the frontend and use global paths to `IntoIterator`, `Iterator` and `Option` variants, users of the `core` crate will have to use add an `std` module to the root of their crate to be able to use for loops: ``` rust #![no_std] extern crate core; fn main() { for x in 0..10 {} } #[doc(hidden)] mod std { // these imports are needed to use for-loops pub use core::iter; pub use core::option; } ``` --- r? @nikomatsakis @aturon cc rust-lang#18424 closes rust-lang#18045
fix: Fix lowering of for loops dropping the loop block
For example, this code:
compiles and prints
Helten times, rather than the expectedHel,lo,,wo, etc.The fact that
streamisn'tmutis a clue that this code doesn't work as expected.I guess the solution is
stream.as_ref().take(3)but the "default" behavior here is really confusing.The text was updated successfully, but these errors were encountered: