Rollup merge of #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 https://github.com/rust-lang/rust/issues/48926

1 files changed, 1 insertions, 0 deletions

diff --git a/src/libcore/iter/iterator.rs b/src/libcore/iter/iterator.rs
index f039d1730eb..75138f31a9d 100644
--- a/src/libcore/iter/iterator.rs
+++ b/src/libcore/iter/iterator.rs
@@ -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"]
     fn collect<B: FromIterator<Self::Item>>(self) -> B where Self: Sized {
         FromIterator::from_iter(self)
     }