### Author:  [@BatmanAoD](https://internals.rust-lang.org/u/BatmanAoD)
#### Post date: [February 13, 2019, 8:01pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/3 "2019-02-13T20:01:44Z")
Looks like work was abandoned because there was no one to spearhead it, and the proposal ran into some issues by assuming that `const fn` is necessarily “pure”. My proposal overlaps with that one, but isn’t identical (and explicitly wouldn’t run into the same issue with `Drop`, since I specified that it would _only_ apply to functions where the arguments are either `&` or `Copy`).
Should I just open a new RFC?
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### Author:  [@CAD97](https://internals.rust-lang.org/u/CAD97)
#### Post date: [February 13, 2019, 8:42pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/4 "2019-02-13T20:42:39Z")
Note that [`&_` is `Copy`](https://doc.rust-lang.org/std/marker/trait.Copy.html#impl-Copy-63) as well.
I’m pretty sure that `clippy` would accept a `pedantic` lint for “immediately discarded return value from function taking only `Copy` arguments”.
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### Author:  [@mcy](https://internals.rust-lang.org/u/mcy)
#### Post date: [February 13, 2019, 9:01pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/5 "2019-02-13T21:01:00Z")
This is not strong enough to say “this is a pure function”; consider a function that takes no arguments and locks a global, not-RAII mutex in a system library.
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### Author:  [@dhm](https://internals.rust-lang.org/u/dhm)
#### Post date: [February 13, 2019, 9:09pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/6 "2019-02-13T21:09:07Z")
Excluding the `()`, this sounds like a very decent lint, but some “pure” false positives remain:
- aliasing does not always imply immutability. Take, for instance, [`::std::sync::atomic::AtomicUsize::fetch_add`](https://doc.rust-lang.org/std/sync/atomic/struct.AtomicUsize.html#method.fetch_add) ;
- some form of unfallible global state, like a global mutable var (using `unsafe` or inner mutability), or printing to `stdout` through its panicking version (e.g. `print[ln]!`)
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### Author:  [@scottmcm](https://internals.rust-lang.org/u/scottmcm)
#### Post date: [February 13, 2019, 9:09pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/7 "2019-02-13T21:09:12Z")
You might be interested in the discussion on this issue:
>
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### Author:  [@BatmanAoD](https://internals.rust-lang.org/u/BatmanAoD)
#### Post date: [February 13, 2019, 9:59pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/8 "2019-02-13T21:59:57Z")
> [@mcy](#):
>
> This is not strong enough to say “this is a pure function”...
That's why I included the caveat about the term "pure".
> [@dhm](#):
>
> ...some “pure” false positives remain
I understand; but return values are easy enough to _explicitly_ ignore (`let _ = `) that I personally don't really think false positives are a problem. Of course, that's purely a matter of opinion.
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### Author:  [@dhm](https://internals.rust-lang.org/u/dhm)
#### Post date: [February 13, 2019, 10:33pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/9 "2019-02-13T22:33:01Z")
> [@BatmanAoD](#):
>
> return values are easy enough to _explicitly_ ignore ( `let _ =` )
Good point
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### Author:  [@withoutboats](https://internals.rust-lang.org/u/withoutboats)
#### Post date: [February 14, 2019, 4:04pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/10 "2019-02-14T16:04:41Z")
It seems much more achievable to tag some of these specific functions in std that are liable to be mis-used as methods as `#[must_use]` now that we have that feature on functions.
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### Author:  [@BatmanAoD](https://internals.rust-lang.org/u/BatmanAoD)
#### Post date: [February 14, 2019, 8:28pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/11 "2019-02-14T20:28:31Z")
You’re probably already aware, but that’s the main topic in the GitHub issue thread above. One complaint about that is that the annotations are noisy, which is true.
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### Author:  [@withoutboats](https://internals.rust-lang.org/u/withoutboats)
#### Post date: [February 14, 2019, 9:03pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/12 "2019-02-14T21:03:50Z")
I think the standard library is doomed to noisy attributes by its place in the ecosystem (we’ve already committed to a stable/unstable attribute on every single item). I don’t think we should put a must\_use on every “pure” function; I think we should put them on ones that seem plausible to mistake as mutating methods (like round). And that’s something we can do _now_, whereas these kinds of abstractions run into problems around fuzziness as we’ve seen before we even get into bothering to implement them.
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### Author:  [@BatmanAoD](https://internals.rust-lang.org/u/BatmanAoD)
#### Post date: [February 14, 2019, 10:02pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/13 "2019-02-14T22:02:52Z")
Fair enough. So would the appropriate action be to just start opening PRs that add such annotations?
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### Author:  [@josh](https://internals.rust-lang.org/u/josh)
#### Post date: [February 14, 2019, 11:47pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/14 "2019-02-14T23:47:01Z")
I’d certainly love to see such PRs.
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### Author:  [@scottmcm](https://internals.rust-lang.org/u/scottmcm)
#### Post date: [February 15, 2019, 12:01am UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/15 "2019-02-15T00:01:23Z")
That’s how I got some added 🙂 I’d suggest picking a theme to justify why those particular ones.
>
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### Author:  [@Finn](https://internals.rust-lang.org/u/Finn)
#### Post date: [February 15, 2019, 12:22pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/16 "2019-02-15T12:22:39Z")
It might have been better if `#[must_use]` would be the default for other types than `()`. Now I wonder how often people intentionally allow return values to be implicitly discarded.
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### Author:  [@Nemo157](https://internals.rust-lang.org/u/Nemo157)
#### Post date: [February 15, 2019, 12:27pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/17 "2019-02-15T12:27:02Z")
If you want to see how annoying this is go to a crate and run `RUSTFLAGS='-W unused-results' cargo build`, I have tried having this enabled on projects in the past and found it to be too noisy to be worth doing.
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### Author:  [@system](https://internals.rust-lang.org/u/system)
#### Post date: [May 16, 2019, 12:27pm UTC](https://internals.rust-lang.org/t/lint-for-unused-return-values-of-pure-functions/9435/18 "2019-05-16T12:27:03Z")
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