Fix a whitespace typo
serialize: fix a warning
syntax: add some more extension helper methods
syntax: allow quasiquoter to inline `Vec<Stmt>`s
syntax: promote a comment on PatEnum into a docstring
Fix a bug pretty printing `match { 5i } { _ => { } }`

This also always puts a trailing comma on the last non-block expr.
syntax: add support for quoting arms
remove seek from std::io::MemWriter, add SeekableMemWriter to librustc

Not all users of MemWriter need to seek, but having MemWriter
seekable adds between 3-29% in overhead in certain circumstances.
This fixes that performance gap by making a non-seekable MemWriter,
and creating a new SeekableMemWriter for those circumstances when
that functionality is actually needed.

```
test io::mem::test::bench_buf_reader                        ... bench:       682 ns/iter (+/- 85)
test io::mem::test::bench_buf_writer                        ... bench:       580 ns/iter (+/- 57)
test io::mem::test::bench_mem_reader                        ... bench:       793 ns/iter (+/- 99)
test io::mem::test::bench_mem_writer_001_0000               ... bench:        48 ns/iter (+/- 27)
test io::mem::test::bench_mem_writer_001_0010               ... bench:        65 ns/iter (+/- 27) = 153 MB/s
test io::mem::test::bench_mem_writer_001_0100               ... bench:       132 ns/iter (+/- 12) = 757 MB/s
test io::mem::test::bench_mem_writer_001_1000               ... bench:       802 ns/iter (+/- 151) = 1246 MB/s
test io::mem::test::bench_mem_writer_100_0000               ... bench:       481 ns/iter (+/- 28)
test io::mem::test::bench_mem_writer_100_0010               ... bench:      1957 ns/iter (+/- 126) = 510 MB/s
test io::mem::test::bench_mem_writer_100_0100               ... bench:      8222 ns/iter (+/- 434) = 1216 MB/s
test io::mem::test::bench_mem_writer_100_1000               ... bench:     82496 ns/iter (+/- 11191) = 1212 MB/s
test io::mem::test::bench_seekable_mem_writer_001_0000      ... bench:        48 ns/iter (+/- 2)
test io::mem::test::bench_seekable_mem_writer_001_0010      ... bench:        64 ns/iter (+/- 2) = 156 MB/s
test io::mem::test::bench_seekable_mem_writer_001_0100      ... bench:       129 ns/iter (+/- 7) = 775 MB/s
test io::mem::test::bench_seekable_mem_writer_001_1000      ... bench:       801 ns/iter (+/- 159) = 1248 MB/s
test io::mem::test::bench_seekable_mem_writer_100_0000      ... bench:       711 ns/iter (+/- 51)
test io::mem::test::bench_seekable_mem_writer_100_0010      ... bench:      2532 ns/iter (+/- 227) = 394 MB/s
test io::mem::test::bench_seekable_mem_writer_100_0100      ... bench:      8962 ns/iter (+/- 947) = 1115 MB/s
test io::mem::test::bench_seekable_mem_writer_100_1000      ... bench:     85086 ns/iter (+/- 11555) = 1175 MB/s
```

[breaking-change]
std: Make MemWriter clonable
auto merge of #16037 : erickt/rust/quote_arm, r=acrichto

This adds support for `quote_arm!(cx, $pat => $expr)`, and `macro_rules!(($a:arm) => (...))`. It also fixes a bug in pretty printing, where this would generate invalid code:

```
match { 5i } {
    1 => 2,
    _ => 3,
}
```

It would generate this code:

```
match { 5i } {
    1 => 2
    _ => 3
}
```

Finally, it adds a couple helper methods to `ExtCtxt`.
rustrt: Allow dropping a brand-new Task

When a new task fails to spawn, it triggers a task failure of the spawning task.
This ends up causing runtime aborts today because of the destructor bomb in the
Task structure. The bomb doesn't actually need to go off until *after* the task
has run at least once.

This now prevents a runtime abort when a native thread fails to spawn.
native: Don't deadlock the runtime on spawn failure

Previously, the call to bookkeeping::increment() was never paired with a
decrement when the spawn failed (due to unwinding). This fixes the problem by
returning a "bomb" from increment() which will decrement on drop, and then
moving the bomb into the child task's procedure which will be dropped naturally.
auto merge of #15915 : erickt/rust/master, r=alexcrichton

std: rename MemWriter to SeekableMemWriter, add seekless MemWriter

Not all users of MemWriter need to seek, but having MemWriter seekable adds between 3-29% in overhead in certain circumstances. This fixes that performance gap by making a non-seekable MemWriter, and creating a new SeekableMemWriter for those circumstances when that functionality is actually needed.

```
test io::mem::test::bench_buf_reader                        ... bench:       682 ns/iter (+/- 85)
test io::mem::test::bench_buf_writer                        ... bench:       580 ns/iter (+/- 57)
test io::mem::test::bench_mem_reader                        ... bench:       793 ns/iter (+/- 99)
test io::mem::test::bench_mem_writer_001_0000               ... bench:        48 ns/iter (+/- 27)
test io::mem::test::bench_mem_writer_001_0010               ... bench:        65 ns/iter (+/- 27) = 153 MB/s
test io::mem::test::bench_mem_writer_001_0100               ... bench:       132 ns/iter (+/- 12) = 757 MB/s
test io::mem::test::bench_mem_writer_001_1000               ... bench:       802 ns/iter (+/- 151) = 1246 MB/s
test io::mem::test::bench_mem_writer_100_0000               ... bench:       481 ns/iter (+/- 28)
test io::mem::test::bench_mem_writer_100_0010               ... bench:      1957 ns/iter (+/- 126) = 510 MB/s
test io::mem::test::bench_mem_writer_100_0100               ... bench:      8222 ns/iter (+/- 434) = 1216 MB/s
test io::mem::test::bench_mem_writer_100_1000               ... bench:     82496 ns/iter (+/- 11191) = 1212 MB/s
test io::mem::test::bench_seekable_mem_writer_001_0000      ... bench:        48 ns/iter (+/- 2)
test io::mem::test::bench_seekable_mem_writer_001_0010      ... bench:        64 ns/iter (+/- 2) = 156 MB/s
test io::mem::test::bench_seekable_mem_writer_001_0100      ... bench:       129 ns/iter (+/- 7) = 775 MB/s
test io::mem::test::bench_seekable_mem_writer_001_1000      ... bench:       801 ns/iter (+/- 159) = 1248 MB/s
test io::mem::test::bench_seekable_mem_writer_100_0000      ... bench:       711 ns/iter (+/- 51)
test io::mem::test::bench_seekable_mem_writer_100_0010      ... bench:      2532 ns/iter (+/- 227) = 394 MB/s
test io::mem::test::bench_seekable_mem_writer_100_0100      ... bench:      8962 ns/iter (+/- 947) = 1115 MB/s
test io::mem::test::bench_seekable_mem_writer_100_1000      ... bench:     85086 ns/iter (+/- 11555) = 1175 MB/s
```
green: Prevent runtime corruption on spawn failure

Like with libnative, when a green task failed to spawn it would leave the world
in a corrupt state where the local scheduler had been dropped as well as the
local task. Also like libnative, this patch sets up a "bomb" which when it goes
off will restore the state of the world.
auto merge of #15944 : alexcrichton/rust/task-dont-die, r=brson

Previously both spawning mechanisms were not resilient to task failures which were initiated from the task spawning infrastructure.

Closes #15895