Allow `!` as the return type of proc/closure literals

Fixes #13490.
Change rustc to say `array` instead of `vector`
core: Make TwoWaySearcher reset its prefix memory when shifting by byteset

Closes #16878.
Add many comments to TwoWaySearcher.
Add some tests for closures that return `!`
Change method lookup to require invariance for mutable references

Fixes #17033.
Fixes #15965.
Add unnecessary_import_braces lint.

The lint checks any unnecessary braces around one imported item like
`use std::num::{abs};`.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Disable ASLR on Windows, for now.
Escape backslashes in debugger scripts.
Use DWARF4 on Windows.
Fix #17156
mk: Fix defs for i686-w64-mingw32
Make debug message about resolving `extern crate` statements more helpful
Fix check for existing crate when using --extern

When checking for an existing crate, compare against the
`crate_metadata::name` field, which is the crate name which
was requested during resolution, rather than the result of the
`crate_metadata::name()` method, which is the crate name within
the crate metadata, as these may not match when using the --extern
option to `rustc`.

This fixes spurious "multiple crate version" warnings under the
following scenario:

- The crate `foo`, is referenced multiple times
- `--extern foo=./path/to/libbar.rlib` is specified to rustc
- The internal crate name of `libbar.rlib` is not `foo`

The behavior surrounding `Context::should_match_name` and the
comments in `loader.rs` both lead me to believe that this scenario
is intended to work.

Fixes #17186
remove dead and broken tvec ~[T] code path

`Box<[T]>` is created by allocating `Box<[T, ..n]>` and coercing it so
this code path is never used. It's also broken because it clamps the
capacity of the memory allocations to 4 elements and that's incompatible
with sized deallocation. This dates back to when `~[T]` was a growable
vector type implemented as:

*{ { tydesc, ref_count, prev, next }, { length, capacity, data[] } }

Since even empty vectors had to allocate, it started off the capacity of
all vectors at 4 as a heuristic. It's not possible to grow `Box<[T]>`
and there is no need for a memory allocation when it's empty, so it
would be a terrible heuristic today even if it worked.
Document why `core::str::Searcher::new` doesn't overflow
Updated jemalloc
Remove build system support for i686-pc-mingw32 triple in favor if i686-w64-mingw32
Fix bad error message copypasta
Add a Rust string ostream for LLVM