Deemphasize the secondary messages so that all other highlights stand
out more.

LLVM isn't able to remove the alloca for the unaligned block in the SIMD tail in some cases, so doing this helps SRoA work in cases where it currently doesn't.  Found in the `replace_with` RFC discussion.

… values

The documentation of Unique::empty() and NonNull::dangling() could
potentially suggest that they work as sentinel values indicating a
not-yet-initialized pointer. However, they both declare a non-null
pointer equal to the alignment of the type, which could potentially
reference a valid value of that type (specifically, the first such valid
value in memory). Explicitly document that the return value of these
functions does not work as a sentinel value.

Due to a change in how mdbook generates section anchors, headers
with non-alphabetic characters now start with "a".

Closes rust-lang#34784

Closes rust-lang#33264

Closes rust-lang#44005

Closes rust-lang#42060

Closes rust-lang#43196

…excrichton

Deprecation of str::slice_unchecked(_mut)

Closes rust-lang#51715

I am not sure if 1.28.0 or 1.29.0 should be used for deprecation version, for now it's 1.28.0.

Additionally I've replaced `slice_unchecked` uses with `get_unchecked`. The only places where this method is still used are `src/liballoc/tests/str.rs` and `src/liballoc/tests/str.rs`.

mem::swap the obvious way for types smaller than the SIMD optimization's block size

LLVM isn't able to remove the alloca for the unaligned block in the post-SIMD tail in some cases, so doing this helps SRoA work in cases where it currently doesn't.  Found in the `replace_with` RFC discussion.

Examples of the improvements:
<details>
 <summary>swapping `[u16; 3]` takes 1/3 fewer instructions and no stackalloc</summary>

```rust
type Demo = [u16; 3];
pub fn swap_demo(x: &mut Demo, y: &mut Demo) {
    std::mem::swap(x, y);
}
```

nightly:
```asm
_ZN4blah9swap_demo17ha1732a9b71393a7eE:
.seh_proc _ZN4blah9swap_demo17ha1732a9b71393a7eE
	sub	rsp, 32
	.seh_stackalloc 32
	.seh_endprologue
	movzx	eax, word ptr [rcx + 4]
	mov	word ptr [rsp + 4], ax
	mov	eax, dword ptr [rcx]
	mov	dword ptr [rsp], eax
	movzx	eax, word ptr [rdx + 4]
	mov	word ptr [rcx + 4], ax
	mov	eax, dword ptr [rdx]
	mov	dword ptr [rcx], eax
	movzx	eax, word ptr [rsp + 4]
	mov	word ptr [rdx + 4], ax
	mov	eax, dword ptr [rsp]
	mov	dword ptr [rdx], eax
	add	rsp, 32
	ret
	.seh_handlerdata
	.section	.text,"xr",one_only,_ZN4blah9swap_demo17ha1732a9b71393a7eE
	.seh_endproc
```

this PR:
```asm
_ZN4blah9swap_demo17ha1732a9b71393a7eE:
	mov	r8d, dword ptr [rcx]
	movzx	r9d, word ptr [rcx + 4]
	movzx	eax, word ptr [rdx + 4]
	mov	word ptr [rcx + 4], ax
	mov	eax, dword ptr [rdx]
	mov	dword ptr [rcx], eax
	mov	word ptr [rdx + 4], r9w
	mov	dword ptr [rdx], r8d
	ret
```
</details>

<details>
 <summary>`replace_with` optimizes down much better</summary>

Inspired by rust-lang/rfcs#2490,

```rust
fn replace_with<T, F>(x: &mut Option<T>, f: F)
    where F: FnOnce(Option<T>) -> Option<T>
{
    *x = f(x.take());
}

pub fn inc_opt(mut x: &mut Option<i32>) {
    replace_with(&mut x, |i| i.map(|j| j + 1));
}
```

Rust 1.26.0:
```asm
_ZN4blah7inc_opt17heb0acb64c51777cfE:
	mov	rax, qword ptr [rcx]
	movabs	r8, 4294967296
	add	r8, rax
	shl	rax, 32
	movabs	rdx, -4294967296
	and	rdx, r8
	xor	r8d, r8d
	test	rax, rax
	cmove	rdx, rax
	setne	r8b
	or	rdx, r8
	mov	qword ptr [rcx], rdx
	ret
```

Nightly (better thanks to ScalarPair, maybe?):
```asm
_ZN4blah7inc_opt17h66df690be0b5899dE:
	mov	r8, qword ptr [rcx]
	mov	rdx, r8
	shr	rdx, 32
	xor	eax, eax
	test	r8d, r8d
	setne	al
	add	edx, 1
	mov	dword ptr [rcx], eax
	mov	dword ptr [rcx + 4], edx
	ret
```

This PR:
```asm
_ZN4blah7inc_opt17h1426dc215ecbdb19E:
	xor	eax, eax
	cmp	dword ptr [rcx], 0
	setne	al
	mov	dword ptr [rcx], eax
	add	dword ptr [rcx + 4], 1
	ret
```

Where that add is beautiful -- using an addressing mode to not even need to explicitly go through a register -- and the remaining imperfection is well-known (rust-lang#49420 (comment)).
</details>

Reword when `_` couldn't be inferred

r? @nikomatsakis

Add tests for ICEs which no longer repro

Adds tests for some ICEs which no longer repro and closes the associated issues.

Avoid using `#[macro_export]` for documenting builtin macros

Use a special `rustc_*` attribute instead.

cc rust-lang#52234

Typo

Fix links in rustdoc book.

Due to a change in how mdbook generates section anchors, headers
with non-alphabetic characters now start with "a".

Add missing backtick in UniversalRegions doc comment

Add CI test harness for `thumb*` targets. [IRR-2018-embedded]

This pull request will do the following (rather trivial) changes:
- Fix rust-lang#52163. In other words, we enabled `./x.py test src/test/run-make` for `no_std` targets.
- Modify `dist-various-1` Dockerfile.
  - CI now performs `run-make` test run on the targets below:
    - `thumbv6m-none-eabi`
    - `thumbv7m-none-eabi`
    - `thumbv7em-none-eabi`
    - `thumbv7em-none-eabihf`.
- ~~Add `thumb-none` Dockerfile.~~
  - ~~Initially, `thumbv7m-none-eabi`, `thumbv7em-none-eabi` and `thumbv7em-none-eabihf` are included as the tested target. `thumbv6m-none-eabi` is disabled for now because LLVM support is not certain.~~
- ~~Add `thumb-none` to .travis.yml~~

Note:
- `run-make` tests are not implemented yet. This PR is test harness only.

The amount of change is very small, but I'd like to open the pull request while the change is trivial.
Because I'm not very used to pull request process, I want to make a small progress first.  This PR will be a foundation for later additions.

…, r=GuillaumeGomez

Remove highlighting from secondary messages

Deemphasize the secondary messages so that all other highlights stand
out more.

<img width="684" alt="" src="https://user-images.githubusercontent.com/1606434/41261199-7b4fe96e-6d8f-11e8-8619-04d170617df2.png">

… r=Mark-Simulacrum

Document that Unique::empty() and NonNull::dangling() aren't sentinel values

The documentation of Unique::empty() and NonNull::dangling() could
potentially suggest that they work as sentinel values indicating a
not-yet-initialized pointer. However, they both declare a non-null
pointer equal to the alignment of the type, which could potentially
reference a valid value of that type (specifically, the first such valid
value in memory). Explicitly document that the return value of these
functions does not work as a sentinel value.