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Delay of iteration side effects is confusing #14666

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kmcallister opened this issue Jun 5, 2014 · 3 comments · Fixed by #15561
Closed

Delay of iteration side effects is confusing #14666

kmcallister opened this issue Jun 5, 2014 · 3 comments · Fixed by #15561

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@kmcallister
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This code produces no output:

fn main() {
    let xs = &[1u,2u,3u];
    let f = |x: &uint| {
        println!("{:u}", *x);
    };
    xs.iter().map(f);
}

It's because the iterator returned by map is never used. Switching to .map(f).last() produces the expected output.

Perhaps iterator structs should be #[must_use]?
@huonw huonw added the A-libs label Jun 5, 2014
@huonw
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huonw commented Jun 15, 2014

Perhaps iterator structs should be #[must_use]?

Seems like a good idea. I wonder if #[must_use] could allow specifying a explanation that is printed with the other warnings, so they could have something like #[must_use="iterator adaptors execute lazily"].

@schmee schmee mentioned this issue Jun 15, 2014
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@huonw
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huonw commented Jul 9, 2014

(I'm working on this.)

huonw added a commit to huonw/rust that referenced this issue Jul 9, 2014
@huonw
core: add #[must_use] attributes to iterator adaptor structs.
27d18fb 
It can be a little unintuitive that something like `v.iter().map(|x|
println!("{}", x));` does nothing: the majority of the iterator adaptors
are lazy and do not execute anything until something calls `next`, e.g.
a `for` loop, `collect`, `fold`, etc.

The majority of such errors can be seen by someone writing something
like the above, i.e. just calling an iterator adaptor and doing nothing
with it (and doing this is certainly useless), so we can co-opt the
`must_use` lint, using the message functionality to give a hint to the
reason why.

Fixes rust-lang#14666.
@huonw huonw mentioned this issue Jul 9, 2014
Merged
bors added a commit that referenced this issue Jul 10, 2014
@bors
auto merge of #15561 : huonw/rust/must-use-iterators, r=alexcrichton
6372915 
Similar to the stability attributes, a type annotated with `#[must_use =
"informative snippet"]` will print the normal warning message along with
"informative snippet". This allows the type author to provide some
guidance about why the type should be used.

---

It can be a little unintuitive that something like `v.iter().map(|x|
println!("{}", x));` does nothing: the majority of the iterator adaptors
are lazy and do not execute anything until something calls `next`, e.g.
a `for` loop, `collect`, `fold`, etc.

The majority of such errors can be seen by someone writing something
like the above, i.e. just calling an iterator adaptor and doing nothing
with it (and doing this is certainly useless), so we can co-opt the
`must_use` lint, using the message functionality to give a hint to the
reason why.

Fixes #14666.
@emberian
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This just saved my bacon, and is SUPER useful. ❤️

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Allow a message on #[must_use] & mark iterator adaptor structs with it huonw/rust
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@kmcallister @emberian @huonw