Add #[must_use] to a few standard library methods #49533
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Chosen to start a precedent of using it on ones that are potentially-expensive and where using it for side effects is particularly discouraged. Discuss :)
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Looks like highfive was asleep for this one, so picking, from the |
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@bors r+ |
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📌 Commit fb7deda has been approved by |
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Apr 2, 2018
| @@ -1368,6 +1368,7 @@ pub trait Iterator { | |||
| /// [`Result`]: ../../std/result/enum.Result.html | |||
| #[inline] | |||
| #[stable(feature = "rust1", since = "1.0.0")] | |||
| #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"] | |||
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I think this is going to be annoying for the "collect to unit" use-case. I don't know how much we want to support that, but I thought it worth pointing out.
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AFAICT Unit-returning functions never trigger the warning, even if they are the result of monomorphising a function generic in its output type. Also, there's a use case for that FromIterator implementation besides just making collect::<Result<()>> work?
kennytm
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Apr 3, 2018
Add #[must_use] to a few standard library methods
Chosen to start a precedent of using it on ones that are potentially-expensive and where using it for side effects is particularly discouraged.
Discuss :)
```rust
warning: unused return value of `std::iter::Iterator::collect` which must be used: if you really need to exhaust the iterator, consider `.for_each(drop)` instead
--> $DIR/fn_must_use_stdlib.rs:19:5
|
LL | "1 2 3".split_whitespace().collect::<Vec<_>>();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: unused return value of `std::borrow::ToOwned::to_owned` which must be used: cloning is often expensive and is not expected to have side effects
--> $DIR/fn_must_use_stdlib.rs:21:5
|
LL | "hello".to_owned();
| ^^^^^^^^^^^^^^^^^^^
warning: unused return value of `std::clone::Clone::clone` which must be used: cloning is often expensive and is not expected to have side effects
--> $DIR/fn_must_use_stdlib.rs:23:5
|
LL | String::from("world").clone();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
cc rust-lang#48926
kennytm
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Apr 3, 2018
Add #[must_use] to a few standard library methods
Chosen to start a precedent of using it on ones that are potentially-expensive and where using it for side effects is particularly discouraged.
Discuss :)
```rust
warning: unused return value of `std::iter::Iterator::collect` which must be used: if you really need to exhaust the iterator, consider `.for_each(drop)` instead
--> $DIR/fn_must_use_stdlib.rs:19:5
|
LL | "1 2 3".split_whitespace().collect::<Vec<_>>();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: unused return value of `std::borrow::ToOwned::to_owned` which must be used: cloning is often expensive and is not expected to have side effects
--> $DIR/fn_must_use_stdlib.rs:21:5
|
LL | "hello".to_owned();
| ^^^^^^^^^^^^^^^^^^^
warning: unused return value of `std::clone::Clone::clone` which must be used: cloning is often expensive and is not expected to have side effects
--> $DIR/fn_must_use_stdlib.rs:23:5
|
LL | String::from("world").clone();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
cc rust-lang#48926
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Apr 3, 2018
Rollup of 14 pull requests Successful merges: - #49179 (Handle future deprecation annotations ) - #49512 (Add support for variant and types fields for intra links) - #49516 (Add missing anchor for union type fields) - #49532 (Add test for rustdoc ignore test) - #49533 (Add #[must_use] to a few standard library methods) - #49570 (avoid IdxSets containing garbage above the universe length) - #49594 (Add some performance guidance to std::fs and std::io docs) - #49599 (Fix typo) - #49603 (Fix url for intra link provided method) - #49609 (run-pass/attr-stmt-expr: expand test cases) - #49612 (Fix "since" version for getpid feature.) - #49618 (Fix build error when compiling libcore for 16bit targets) - #49619 (tweak core::fmt docs) - #49623 (update mdbook) Failed merges:
kennytm
added a commit
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Apr 4, 2018
Add #[must_use] to a few standard library methods
Chosen to start a precedent of using it on ones that are potentially-expensive and where using it for side effects is particularly discouraged.
Discuss :)
```rust
warning: unused return value of `std::iter::Iterator::collect` which must be used: if you really need to exhaust the iterator, consider `.for_each(drop)` instead
--> $DIR/fn_must_use_stdlib.rs:19:5
|
LL | "1 2 3".split_whitespace().collect::<Vec<_>>();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: unused return value of `std::borrow::ToOwned::to_owned` which must be used: cloning is often expensive and is not expected to have side effects
--> $DIR/fn_must_use_stdlib.rs:21:5
|
LL | "hello".to_owned();
| ^^^^^^^^^^^^^^^^^^^
warning: unused return value of `std::clone::Clone::clone` which must be used: cloning is often expensive and is not expected to have side effects
--> $DIR/fn_must_use_stdlib.rs:23:5
|
LL | String::from("world").clone();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
cc rust-lang#48926
bors
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that referenced
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Apr 4, 2018
Rollup of 25 pull requests Successful merges: - #49179 (Handle future deprecation annotations ) - #49512 (Add support for variant and types fields for intra links) - #49515 (fix targetted value background) - #49516 (Add missing anchor for union type fields) - #49532 (Add test for rustdoc ignore test) - #49533 (Add #[must_use] to a few standard library methods) - #49540 (Fix miri Discriminant() for non-ADT) - #49559 (Introduce Vec::resize_with method (see #41758)) - #49570 (avoid IdxSets containing garbage above the universe length) - #49577 (Stabilize String::replace_range) - #49599 (Fix typo) - #49603 (Fix url for intra link provided method) - #49607 (Stabilize iterator methods in 1.27) - #49609 (run-pass/attr-stmt-expr: expand test cases) - #49612 (Fix "since" version for getpid feature.) - #49618 (Fix build error when compiling libcore for 16bit targets) - #49619 (tweak core::fmt docs) - #49637 (Stabilize parent_id()) - #49639 (Update Cargo) - #49628 (Re-write the documentation index) - #49594 (Add some performance guidance to std::fs and std::io docs) - #49625 (miri: add public alloc_kind accessor) - #49634 (Add a test for the fix to issue #43058) - #49641 (Regression test for #46314) - #49547 (Unignore borrowck test) Failed merges:
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Chosen to start a precedent of using it on ones that are potentially-expensive and where using it for side effects is particularly discouraged.
Discuss :)
cc #48926