Add #[must_use] to a few standard library methods

[scottmcm]

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 :)

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 #48926

[scottmcm]

Looks like highfive was asleep for this one, so picking, from the must_use tracking issue,

r? @nikomatsakis

[nikomatsakis]

@bors r+

[bors]

📌 Commit fb7deda has been approved by nikomatsakis

[cramertj]

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.

[ExpHP]

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?