Group "missing variable bind" spans in `or` matches and clarify wording
for the two possible cases: when a variable from the first pattern is
not in any of the subsequent patterns, and when a variable in any of the
other patterns is not in the first one.

Before:

```
error[E0408]: variable `a` from pattern #1 is not bound in pattern rust-lang#2
  --> file.rs:10:23
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                       ^^^^^^^^^^^ pattern doesn't bind `a`

error[E0408]: variable `b` from pattern rust-lang#2 is not bound in pattern #1
  --> file.rs:10:32
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                                ^ pattern doesn't bind `b`

error[E0408]: variable `a` from pattern #1 is not bound in pattern rust-lang#3
  --> file.rs:10:37
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                                     ^^^^^^^^ pattern doesn't bind `a`

error[E0408]: variable `d` from pattern #1 is not bound in pattern rust-lang#3
  --> file.rs:10:37
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                                     ^^^^^^^^ pattern doesn't bind `d`

error[E0408]: variable `c` from pattern rust-lang#3 is not bound in pattern #1
  --> file.rs:10:43
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                                           ^ pattern doesn't bind `c`

error[E0408]: variable `d` from pattern #1 is not bound in pattern rust-lang#4
  --> file.rs:10:48
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                                                ^^^^^^^^ pattern doesn't bind `d`

error: aborting due to 6 previous errors
```

After:

```
error[E0408]: variable `a` is not bound in all patterns
  --> file.rs:20:37
   |
20 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => {
intln!("{:?}", a); }
   |               -       ^^^^^^^^^^^   ^^^^^^^^         - variable
t in all patterns
   |               |       |             |
   |               |       |             pattern doesn't bind `a`
   |               |       pattern doesn't bind `a`
   |               variable not in all patterns

error[E0408]: variable `d` is not bound in all patterns
  --> file.rs:20:37
   |
20 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => {
intln!("{:?}", a); }
   |                  -          -       ^^^^^^^^   ^^^^^^^^ pattern
esn't bind `d`
   |                  |          |       |
   |                  |          |       pattern doesn't bind `d`
   |                  |          variable not in all patterns
   |                  variable not in all patterns

error[E0408]: variable `b` is not bound in all patterns
  --> file.rs:20:37
   |
20 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => {
intln!("{:?}", a); }
   |         ^^^^^^^^^^^            -    ^^^^^^^^   ^^^^^^^^ pattern
esn't bind `b`
   |         |                      |    |
   |         |                      |    pattern doesn't bind `b`
   |         |                      variable not in all patterns
   |         pattern doesn't bind `b`

error[E0408]: variable `c` is not bound in all patterns
  --> file.rs:20:48
   |
20 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => {
intln!("{:?}", a); }
   |         ^^^^^^^^^^^   ^^^^^^^^^^^         -    ^^^^^^^^ pattern
esn't bind `c`
   |         |             |                   |
   |         |             |                   variable not in all
tterns
   |         |             pattern doesn't bind `c`
   |         pattern doesn't bind `c`

error: aborting due to 4 previous errors
```

* Have only one presentation for binding consistency errors
* Point to same binding in multiple patterns when possible
* Check inconsistent bindings in all arms
* Simplify wording of diagnostic message
* Sort emition and spans of binding errors for deterministic output

When declaring nested unsafe blocks (`unsafe {unsafe {}}`) that trigger
the "unnecessary `unsafe` block" error, point out the enclosing `unsafe
block` or `unsafe fn` that makes it unnecessary.

Fix rust-lang#40221 and add unittest.

… r? @steveklabnik

…turner

Point to enclosing block/fn on nested unsafe

When declaring nested unsafe blocks (`unsafe {unsafe {}}`) that trigger
the "unnecessary `unsafe` block" error, point out the enclosing `unsafe
block` or `unsafe fn` that makes it unnecessary.

<img width="621" alt="" src="https://cloud.githubusercontent.com/assets/1606434/22139922/26ad468a-de9e-11e6-8884-2945be882ea8.png">

Fixes rust-lang#39144.

…rner

Clean up "pattern doesn't bind x" messages

Group "missing variable bind" spans in `or` matches and clarify wording
for the two possible cases: when a variable from the first pattern is
not in any of the subsequent patterns, and when a variable in any of the
other patterns is not in the first one.

Before:

```rust
error[E0408]: variable `a` from pattern #1 is not bound in pattern rust-lang#2
  --> file.rs:10:23
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                       ^^^^^^^^^^^ pattern doesn't bind `a`

error[E0408]: variable `b` from pattern rust-lang#2 is not bound in pattern #1
  --> file.rs:10:32
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                                ^ pattern doesn't bind `b`

error[E0408]: variable `a` from pattern #1 is not bound in pattern rust-lang#3
  --> file.rs:10:37
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                                     ^^^^^^^^ pattern doesn't bind `a`

error[E0408]: variable `d` from pattern #1 is not bound in pattern rust-lang#3
  --> file.rs:10:37
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                                     ^^^^^^^^ pattern doesn't bind `d`

error[E0408]: variable `c` from pattern rust-lang#3 is not bound in pattern #1
  --> file.rs:10:43
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                                           ^ pattern doesn't bind `c`

error[E0408]: variable `d` from pattern #1 is not bound in pattern rust-lang#4
  --> file.rs:10:48
   |
10 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                                                ^^^^^^^^ pattern doesn't bind `d`

error: aborting due to 6 previous errors
```

After:

```rust
error[E0408]: variable `d` is not bound in all patterns
  --> $DIR/issue-39698.rs:20:37
   |
20 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |                  -          -       ^^^^^^^^   ^^^^^^^^ pattern doesn't bind `d`
   |                  |          |       |
   |                  |          |       pattern doesn't bind `d`
   |                  |          variable not in all patterns
   |                  variable not in all patterns

error[E0408]: variable `c` is not bound in all patterns
  --> $DIR/issue-39698.rs:20:48
   |
20 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |         ^^^^^^^^^^^   ^^^^^^^^^^^         -    ^^^^^^^^ pattern doesn't bind `c`
   |         |             |                   |
   |         |             |                   variable not in all patterns
   |         |             pattern doesn't bind `c`
   |         pattern doesn't bind `c`

error[E0408]: variable `a` is not bound in all patterns
  --> $DIR/issue-39698.rs:20:37
   |
20 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |               -       ^^^^^^^^^^^   ^^^^^^^^         - variable not in all patterns
   |               |       |             |
   |               |       |             pattern doesn't bind `a`
   |               |       pattern doesn't bind `a`
   |               variable not in all patterns

error[E0408]: variable `b` is not bound in all patterns
  --> $DIR/issue-39698.rs:20:37
   |
20 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |         ^^^^^^^^^^^            -    ^^^^^^^^   ^^^^^^^^ pattern doesn't bind `b`
   |         |                      |    |
   |         |                      |    pattern doesn't bind `b`
   |         |                      variable not in all patterns
   |         pattern doesn't bind `b`

error: aborting due to 4 previous errors
```

Fixes rust-lang#39698.

…frewsxcv

add unstable book to the bookshelf

r? @frewsxcv @GuillaumeGomez

Issue rust-lang#39688 - Help people find String::as_bytes() for UTF-8

Added in links for the inverse functions so people will know that as_bytes() is the inverse of from_utf8() and vice versa.
?r @steveklabnik

Fix ICE: don't use `struct_variant` on enums

Fix rust-lang#40221 and add unittest.