This matches std::collections

Add a fallback for stacktrace printing for older Windows versions.

Some time ago we switched stack inspection functions of dbghelp.dll to their newer alternatives that also capture inlined context.
Unfortunately, said new alternatives are not present in older dbghelp.dll versions.
In particular Windows 7 at the time of writing has dbghelp.dll version 6.1.7601 from 2010, that lacks StackWalkEx and friends.

Tested on my Windows 7 - both msvc and gnu versions produce a readable stacktrace.

Fixes #50138

park/park_timeout: prohibit spurious wakeups in next park

<pre><code>
// The implementation currently uses the trivial strategy of a Mutex+Condvar
// with wakeup flag, which does not actually allow spurious wakeups.
</pre></code>

Because does not actually allow spurious wakeups.
so we have let thread.inner.cvar.wait(m) in the loop to prohibit spurious wakeups.
but if notified after we locked, this notification doesn't be consumed, it return, the next park will consume this notification...this is also 'spurious wakeup' case, 'one unpark() wakeups two  park()'.

We should improve this situation：
`thread.inner.state.store(EMPTY, SeqCst);`

Arc: remove unused allocation from Weak::new()

It seems non-obvious that calling `Weak::new()` actually allocates space for the entire size of `T`, even though you can **never** access that data from such a constructed weak pointer. Besides that, if someone were to create many `Weak:new()`s, they could be unknowingly wasting a bunch of memory.

This change makes it so that `Weak::new()` allocates no memory at all. Instead, it is created with a null pointer. The only things done with a `Weak` are trying to upgrade, cloning, and dropping, meaning there are very few places that the code actually needs to check if the pointer is null.

Fix macro missing from doc search

Fixes #51095.

r? @QuietMisdreavus

Rollup of 6 pull requests

Successful merges:

 - #51636 (Refactor error reporting of constants)
 - #51765 (Use assert_eq! in copy_from_slice)
 - #51822 (Provide existing ref suggestions for more E0308 errors)
 - #51839 (Detect overflows of non u32 shifts)
 - #51868 (Remove process::id from 'Stabilized APIs' in 1.27.0 release notes)
 - #51875 (Explicitely disable WASM code generation for Emscripten)

Failed merges:

r? @ghost

Explicitely disable WASM code generation for Emscripten

Emscripten changed the default behavior recently:
https://github.com/kripken/emscripten/blob/bd050e64bb0d9952df1344b8ea9356252328ad83/ChangeLog.markdown#v1381-05172018

It now defaults to WebAssembly and requires an explicit flag to generate asm.js.
WASM=0 is also valid for older emcc and thus doesn't break it.

Remove process::id from 'Stabilized APIs' in 1.27.0 release notes

process::id was already stabilized in 1.26.0 and is marked as such in the docs.

Detect overflows of non u32 shifts

Provide existing ref suggestions for more E0308 errors

Use assert_eq! in copy_from_slice

This will print both lengths when the assertion fails instead of just saying that they're different.

Output of current stable and nightly (modulo the exact line number):
```
thread 'main' panicked at 'destination and source slices have different lengths', libcore/slice/mod.rs:1645:9
```

Output after this PR:
```
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `123`,
 right: `456`: destination and source slices have different lengths', libcore/slice/mod.rs:1645:9
```

Note that I have not run the tests locally.

Refactor error reporting of constants

cc @EddyB

This PR should not change any behaviour. It solely simplifies the internal handling of the errors

.. and pass them around in BacktraceContext.

.. rather than having them be one giant match statement.

PR #47252 switched stack inspection functions of dbghelp.dll
to their newer alternatives that also capture inlined context.
Unfortunately, said new alternatives are not present in older
dbghelp.dll versions.
In particular Windows 7 at the time of writing has dbghelp.dll
version 6.1.7601 from 2010, that lacks StackWalkEx and friends.

Fixes #50138

translate / export weak lang items

see #51671 for details

fixes #51671
fixes #51342

r? @michaelwoerister or @alexcrichton

Optimize RefCell refcount tracking

Address the performance concern raised in https://github.com/rust-lang/rust/pull/51466#issuecomment-398255276

cc @dtolnay  @nnethercote @rust-lang/wg-compiler-performance

cc @RalfJung @jhjourdan for soundness concerns

Can somebody kick off a perf run on this? I'm not sure how that's done, but I understand it has to be started manually.

The idea of this change is to switch to representing mutable refcount as values below 0 to eliminate some branching that was required with the old algorithm.

Emscripten changed the default behavior recently:
https://github.com/kripken/emscripten/blob/bd050e64bb0d9952df1344b8ea9356252328ad83/ChangeLog.markdown#v1381-05172018

It now defaults to WebAssembly and requires an explicit flag to generate asm.js.
WASM=0 is also valid for older emcc and thus doesn't break it.

Closes #51856

 Make FileMap::{lines, multibyte_chars, non_narrow_chars} non-mutable.

This PR removes most of the interior mutability from `FileMap`, which should be beneficial, especially in a multithreaded setting. This is achieved by initializing the state in question when the filemap is constructed instead of during lexing. Hopefully this doesn't degrade performance.

cc @wesleywiser

Use `Ident`s in HIR and remove emulation of hygiene with gensyms

continuation of https://github.com/rust-lang/rust/pull/51072, part of https://github.com/rust-lang/rust/issues/49300

Not all `Name`s in HIR are replaced with `Ident`s, only those needed for hygiene or already having attached spans.

The method relied on the FileMap still being under construction in
order for it to do what the name promises. It's now independent of
the current state.