This will check for regression on issue rust-lang#32556

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.

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

See rust-lang#49523 (comment)

Fixes rust-lang#49617

Remove an outdated claim about passing something or other to a function. Also swap the variable names in the example.

Swap the variable names in the example.

…isdreavus,GuillaumeGomez

Handle future deprecation annotations

This adds special handling to the `since` parameter of the `deprecated` attribute: in particular, if the `since` version exceeds the version of the compiler, the deprecation notice will not be printed; but a note is added to the documentation stating that the item will be deprecated in a later version.

(I've used `since` for this, rather than adding a new attribute, because it's more seamless and, I feel, intuitive. Plus it involves less code churn.)

![image](https://user-images.githubusercontent.com/3943692/37611317-ef5cdf16-2b99-11e8-8251-e35e8f7b0137.png)
![image](https://user-images.githubusercontent.com/3943692/37611323-f748c2d0-2b99-11e8-966b-11408c73d416.png)

This is a prerequisite for doing things renaming methods in the standard library (e.g. rust-lang#30459). Resolves rust-lang#30785.

…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

…n-test, r=GuillaumeGomez

Add test for rustdoc ignore test

This will check for regression on issue rust-lang#32556

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?

Add some performance guidance to std::fs and std::io docs

Adds more documentation about performance to various "read" functions in `fs` and `io`, and to `BufReader`/`BufWriter`, with the goal of helping developers choose the best option for a given task.

r? @steveklabnik

…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.

…-provided-method, r=QuietMisdreavus

Fix url for intra link provided method

Fixes rust-lang#49582.

r? @QuietMisdreavus

…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)

Fix build error when compiling libcore for 16bit targets

Fixes rust-lang#49617

cc @SimonSapin

This includes search for all books, a long-requested feature!

tweak core::fmt docs

Remove an outdated (pre-1.0!) claim about passing something or other to a function. Also swap the variable names in the example.

…aumeGomez

update mdbook

This includes search for all books, a long-requested feature!