use `reset_unifications` instead of creating new unification table

This eliminates a hot spot in NLL performance.

Emit noalias on &mut parameters by default

This used to be disabled due to LLVM bugs in the handling of
noalias information in conjunction with unwinding. However,
according to #31681 all known LLVM bugs have been fixed by
LLVM 6.0, so it's probably time to reenable this optimization.

-Z no-mutable-noalias is left as an escape-hatch to debug problems
suspected to stem from this change.

Revert #50105 until regression is fixed

Discovered at https://github.com/rust-lang/rust/pull/50105#issuecomment-388630750 it looks like this caused a regression with i686 musl, so let's revert in the meantime while a fix is worked out

 rustc_mir: allow promotion of promotable temps indexed at runtime.

Fixes #49955.

r? @nikomatsakis

Implement [T]::align_to

Note that this PR deviates from what is accepted by RFC slightly by making `align_offset` to return an offset in elements, rather than bytes. This is necessary to sanely support `[T]::align_to` and also simply makes more sense™. The caveat is that trying to align a pointer of ZST is now an equivalent to `is_aligned` check, rather than anything else (as no number of ZST elements will align a misaligned ZST pointer).

It also implements the `align_to` slightly differently than proposed in the RFC to properly handle cases where size of T and U aren’t co-prime.

Furthermore, a promise is made that the slice containing `U`s will be as large as possible (contrary to the RFC) – otherwise the function is quite useless.

The implementation uses quite a few underhanded tricks and takes advantage of the fact that alignment is a power-of-two quite heavily to optimise the machine code down to something that results in as few known-expensive instructions as possible. Currently calling `ptr.align_offset` with an unknown-at-compile-time `align` results in code that has just a single "expensive" modulo operation; the rest is "cheap" arithmetic and bitwise ops.

cc https://github.com/rust-lang/rust/issues/44488 @oli-obk

As mentioned in the commit message for align_offset, many thanks go to Chris McDonald.

Use EverInit instead of MaybeInit to determine initialization

Fixes #50461.
Fixes #50463.

Make the `const_err` lint `deny`-by-default

At best these things are runtime panics (debug mode) or overflows (release mode). More likely they are public constants that are unused in the crate declaring them.

This is not a breaking change, as dependencies won't break and root crates can `#![warn(const_err)]`, though I don't know why anyone would do that.

add auto-impl for primitive type

Part of #50431.

I have no clue how to test this though with the rustdoc test suite...

r? @QuietMisdreavus

Implement edition hygiene for keywords

Determine "keywordness" of an identifier in its hygienic context.
cc https://github.com/rust-lang/rust/pull/49611

I've resurrected `proc` as an Edition-2015-only keyword for testing purposes, but it should probably be buried again. EDIT: `proc` is removed again.

Stabilise inclusive_range_methods

r? @SimonSapin

Closes #49022.

Update RLS and Rustfmt

Fixes RLS build (The Rustfmt update is insignificant)

r? @alexcrichton

Rollup of 10 pull requests

Successful merges:

 - #50387 (Remove leftover tab in libtest outputs)
 - #50553 (Add Option::xor method)
 - #50610 (Improve format string errors)
 - #50649 (Tweak `nearest_common_ancestor()`.)
 - #50790 (Fix grammar documentation wrt Unicode identifiers)
 - #50791 (Fix null exclusions in grammar docs)
 - #50806 (Add `bless` x.py subcommand for easy ui test replacement)
 - #50818 (Speed up `opt_normalize_projection_type`)
 - #50837 (Revert #49767)
 - #50839 (Make sure people know the book is free oline)

Failed merges:

Streamline `StringReader::bump`

These patches make `bump` smaller and nicer. They speed up most runs for coercions and tuple-stress by 1--3%.

stop considering location when computing outlives relationships

This doesn't (yet?) use SEME regions, but it does ignore the location for outlives constraints. This makes (I believe) NLL significantly faster -- but we should do some benchmarks. It regresses the "get-default" family of use cases for NLL, which is a shame, but keeps the other benefits, and thus represents a decent step forward.

r? @pnkfelix

This used to be disabled due to LLVM bugs in the handling of
noalias information in conjunction with unwinding. However,
according to #31681 all known LLVM bugs have been fixed by
LLVM 6.0, so it's probably time to reenable this optimization.

Noalias annotations will not be emitted by default if either
-C panic=abort (as previously) or LLVM >= 6.0 (new).

-Z mutable-noalias=no is left as an escape-hatch to allow
debugging problems suspected to stem from this change.

Keep only the language item. This removes some indirection and makes
codegen worse for debug builds, but simplifies code significantly, which
is a good tradeoff to make, in my opinion.

Besides, the codegen can be improved even further with some constant
evaluation improvements that we expect to happen in the future.

Make sure people know the book is free oline

I've used the tutorial a number of times to relearn rust basics.  When i saw this for a moment I was sad thinking it had been taken offline.

Revert #49767

There was [some confusion](https://github.com/rust-lang/rust/pull/49767#issuecomment-389250815) and I accidentally merged a PR that wasn't ready.

Speed up `opt_normalize_projection_type`

`opt_normalize_projection_type` is hot in the serde and futures benchmarks in rustc-perf. These two patches speed up the execution of most runs for them by 2--4%.

Add `bless` x.py subcommand for easy ui test replacement

fixes #49815

r? @nikomatsakis

Fix null exclusions in grammar docs

The grammar documentation incorrectly says that comments, character literals,
and string literals may not include null.

Fix grammar documentation wrt Unicode identifiers

The grammar defines identifiers in terms of XID_start and XID_continue,
but this is referring to the unstable non_ascii_idents feature.
The documentation implies that non_ascii_idents is forthcoming, but this
is left over from pre-1.0 documentation; in reality, non_ascii_idents
has been without even an RFC for several years now, and will not be
stabilized anytime soon. Furthermore, according to the tracking issue at
https://github.com/rust-lang/rust/issues/28979 , it's highly
questionable whether or not this feature will use XID_start or
XID_continue even when or if non_ascii_idents is stabilized.
This commit fixes this by respecifying identifiers as the usual
[a-zA-Z_][a-zA-Z0-9_]*