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Rollup merge of rust-lang#49533 - scottmcm:more-must-use, r=nikomatsakis
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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