Delay of iteration side effects is confusing #14666
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Seems like a good idea. I wonder if |
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(I'm working on this.) |
huonw
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Jul 9, 2014
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.
bors
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Jul 10, 2014
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.
|
This just saved my bacon, and is SUPER useful. ❤️ |
flip1995
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May 1, 2025
…14666) here is my small fix changelog: fix suggestion span to avoid showing macro name in `.div_ceil()` suggestion i changed this line `let divisor_snippet = snippet_with_applicability(cx, rhs.spansource_callsite(), "..", applicability);` to this line `let divisor_snippet = snippet_with_applicability(cx, rhs.span, "..", applicability);` to fix problem where this warning in macro expands like this ```rust 4 | let _ = (x + 7) / 8; | ^^^^^^^^^^^ help: consider using `.div_ceil()`: `x.div_ceil(y!())` ``` [play it yourself](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=397aa8cd2ffffb24a286fbddbc75446c) as you can see here it looks like `x.div_ceil(y!())` and contains macro signature so i fixed this problem, i will look closely if there any more problems like this and fix them in order **Related issue** fixes [rust-lang/rust-clippy#14665](rust-lang/rust-clippy#14665)
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This code produces no output:
It's because the iterator returned by
mapis never used. Switching to.map(f).last()produces the expected output.Perhaps iterator structs should be
#[must_use]?The text was updated successfully, but these errors were encountered: