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Allow a message on #[must_use] & mark iterator adaptor structs with it #15561

Merged
merged 2 commits into from
Jul 10, 2014
Merged

Allow a message on #[must_use] & mark iterator adaptor structs with it #15561

merged 2 commits into from
Jul 10, 2014

Commits on Jul 9, 2014

  1. lint: extend #[must_use] to handle a message. 

    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.
    @huonw
    huonw committed Jul 9, 2014
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  2. core: add #[must_use] attributes to iterator adaptor structs. 

    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 committed Jul 9, 2014
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