The existing APIs for spawning processes took strings for the command
and arguments, but the underlying system may not impose utf8 encoding,
so this is overly limiting.

The assumption we actually want to make is just that the command and
arguments are viewable as [u8] slices with no interior NULLs, i.e., as
CStrings. The ToCStr trait is a handy bound for types that meet this
requirement (such as &str and Path).

However, since the commands and arguments are often a mixture of
strings and paths, it would be inconvenient to take a slice with a
single T: ToCStr bound. So this patch revamps the process creation API
to instead use a builder-style interface, called `Command`, allowing
arguments to be added one at a time with differing ToCStr
implementations for each.

The initial cut of the builder API has some drawbacks that can be
addressed once issue #13851 (libstd as a facade) is closed. These are
detailed as FIXMEs.

Closes #11650.

[breaking-change]

This is a stopgap until DST (#12938) lands.

Until DST lands, we cannot decompose &str into & and str, so we cannot
usefully take ToCStr arguments by reference (without forcing an
additional & around &str). So we are instead temporarily adding an
instance for &Path and StrBuf, so that we can take ToCStr as owned. When
DST lands, the &Path instance should be removed, the string instances
should be revisted, and arguments bound by ToCStr should be passed by
reference.

FIXMEs have been added accordingly.

Due to our excellent macro hygiene, this involves having a global path and a
hidden module in libcore itself.

Explain why it should not exist, and the plan of attack for removing it.

There's no need to include this specific flag just for android. We can
already deal with what it tries to solve by using -C linker=/path/to/cc
and -C ar=/path/to/ar. The Makefiles for rustc already set this up when
we're crosscompiling.

I did add the flag to compiletest though so it can find gdb. Though, I'm
pretty sure we don't run debuginfo tests on android anyways right now.

[breaking-change]

Reader.read_at_least() ensures that at least a given number of bytes
have been read. The most common use-case for this is ensuring at least 1
byte has been read. If the reader returns 0 enough times in a row, a new
error kind NoProgress will be returned instead of looping infinitely.

This change is necessary in order to properly support Readers that
repeatedly return 0, either because they're broken, or because they're
attempting to do a non-blocking read on some resource that never becomes
available.

Also add .push() and .push_at_least() methods. push() is like read() but
the results are appended to the passed Vec.

Remove Reader.fill() and Reader.push_exact() as they end up being thin
wrappers around read_at_least() and push_at_least().

[breaking-change]

This implements set_timeout() for std::io::Process which will affect wait()
operations on the process. This follows the same pattern as the rest of the
timeouts emerging in std::io::net.

The implementation was super easy for everything except libnative on unix
(backwards from usual!), which required a good bit of signal handling. There's a
doc comment explaining the strategy in libnative. Internally, this also required
refactoring the "helper thread" implementation used by libnative to allow for an
extra helper thread (not just the timer).

This is a breaking change in terms of the io::Process API. It is now possible
for wait() to fail, and subsequently wait_with_output(). These two functions now
return IoResult<T> due to the fact that they can time out.

Additionally, the wait_with_output() function has moved from taking `&mut self`
to taking `self`. If a timeout occurs while waiting with output, the semantics
are undesirable in almost all cases if attempting to re-wait on the process.
Equivalent functionality can still be achieved by dealing with the output
handles manually.

[breaking-change]

cc #13523

LLVM internally uses `uint64_t` for array size, but the corresponding
C API (`LLVMArrayType`) uses `unsigned int` so ths value is truncated.
Therefore rustc generates wrong type for fixed-sized large vector e.g.
`[0 x i8]` for `[0u8, ..(1 << 32)]`.

This patch adds `LLVMRustArrayType` function for `uint64_t` support.

When printing doc comments, always put a newline after them in a macro
invocation to ensure that a line-doc-comment doesn't consume remaining tokens on
the line.

Now that the #[deriving] attribute is removed, the raw_pointers_deriving lint
was broken. This commit restores the lint by preserving lint attributes
across #[deriving] to the implementations and using #[automatically_derived] as
the trigger for activating the lint.

Previously, literals "1i" were printed as "1". This fixes the
numeric-method-autoexport test for pretty printing.

When expanding, an extra unsafe block is generated which is currently not
handled well.

The pretty printer handles inlines comments quite badly

This uses the trick of replacing the << token with a < token to parse closure
types correctly.

Closes #13324

Integers are always parsed as a u64 in libsyntax, but they're stored as i64. The
parser and pretty printer both printed an i64 instead of u64, sometimes
introducing an extra negative sign.