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 :)

This makes sure that all bits in each IdxSet between the universe length
and the end of the word are all zero instead of being in an indeterminate state.

This fixes a crash with RUST_LOG=rustc_mir, and is probably a good idea
anyway.

vec![0; n], via implementations of SpecFromElem, has an optimization
that uses with_capacity_zeroed instead of with_capacity, which will use
calloc instead of malloc, and avoid an extra memset.

This adds the same optimization for vec![ptr::null(); n] and
vec![ptr::null_mut(); n], assuming their bit value is 0 (which is true
on all currently supported platforms).

This does so by adding an intermediate trait IsZero, which looks very
much like nonzero::Zeroable, but that one is on the way out, and doesn't
apply to pointers anyways.

Adding such a trait allows to avoid repeating the logic using
with_capacity_zeroed or with_capacity, or making the macro more complex
to support generics.

Similarly to vec![ptr::null{,_mut}(); n] in previous change, this adds
the optimization for vec!['\0'; n].

So we can cut some params by using stuff from the environment.

It was stabilized right before the beta branch was cut for 1.26.0.

See rust-lang#49523 (comment)

…gs, r=nikomatsakis

Small typeck refactorings

Some code improvements I noticed while reading the code.

Add more vec![... ; n] optimizations

vec![0; n], via implementations of SpecFromElem, has an optimization that uses with_capacity_zeroed instead of with_capacity, which will use calloc instead of malloc, and avoid an extra memset.

This PR adds the same optimization for ptr::null, ptr::null_mut, and None, when their in-memory representation is zeroes.

…r=QuietMisdreavus

Add support for variant and types fields for intra links

Part of rust-lang#43466.

r? @QuietMisdreavus

…ing-anchor, r=QuietMisdreavus

Add missing anchor for union type fields

r? @QuietMisdreavus

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

…sakis

avoid IdxSets containing garbage above the universe length

This makes sure that all bits in each IdxSet between the universe length
and the end of the word are all zero instead of being in an indeterminate state.

This fixes a crash with RUST_LOG=rustc_mir, and is probably a good idea
anyway.

r? @nikomatsakis - I think you are responsible for this code area now?

…sxcv

Fix typo

In `libstd/io/buffered.rs` one example was marked as `no_ru` instead of `no_run`. I assume this is a typo.

…etrochenkov

run-pass/attr-stmt-expr: expand test cases

Follow-up to rust-lang#49124 (comment)

r? @petrochenkov

Fix "since" version for getpid feature.

It was stabilized right before the beta branch was cut for 1.26.0.

See rust-lang#49523 (comment)

This will need to be backported to beta (1.26.0)