Reinstate HashMap.mangle().

This was removed when the Robin Hood hash map came along, but it is a
useful thing to have.

The comment is taken directly from what was there before (e.g. in 0.9)
but with appropriate language changes (like `StrBuf` instead of `~str`).
Added substring searching benchmarks.

test str::bench::bench_contains_bad_naive                   ... bench:      1309 ns/iter (+/- 36)
test str::bench::bench_contains_equal                       ... bench:       137 ns/iter (+/- 2)
test str::bench::bench_contains_short_long                  ... bench:      5473 ns/iter (+/- 14)
test str::bench::bench_contains_short_short                 ... bench:        57 ns/iter (+/- 6)
Tidy up the HashMap.mangle example.
Rename HashMap.mangle to find_with_or_insert_with.

This also entails swapping the order of the find and insert callbacks so
that their order matches the order of the terms in the method name.
Use assertions in find_with_or_insert_with example

This lets us test it automatically and also makes it more obvious what
the expected result is.
Change str::from_utf8_owned() to return Result

This allows the original vector to be recovered in the event that it is
not valid UTF-8.

[breaking-change]
Change StrBuf::from_utf8() to return Result

This allows the original vector to be recovered in the event that it is
not UTF-8.

[breaking-change]
Switched to the two-way algorithm for string searching

test str::bench::bench_contains_bad_naive                   ... bench:       300 ns/iter (+/- 12)     from 1309 ns/iter (+/- 36)
test str::bench::bench_contains_equal                       ... bench:       154 ns/iter (+/- 7)      from  137 ns/iter (+/- 2)
test str::bench::bench_contains_short_long                  ... bench:      2998 ns/iter (+/- 74)     from 5473 ns/iter (+/- 14)
test str::bench::bench_contains_short_short                 ... bench:        65 ns/iter (+/- 2)      from   57 ns/iter (+/- 6)
debuginfo: Represent SIMD types as tuple structs instead of as fixed-sized arrays
Add compiler flag to configure output coloring

This adds the flag --color, which allows the user to force coloring or
turn it off. The default behavior stays the same as before (colorize, if
output goes to tty).
Why this is beneficial is explained in issue #12881.

Please note that this commit doesn't include any regression tests. I
thought about how I'd write a test for this and it doesn't seem to be
worth the effort to me for a UI change like this.

Fixes #12881.
auto merge of #14040 : hannobraun/rust/force-color-output, r=alexcrichton

This pull request fixes #12881.

Two caveats:

1. As explained in the commit message, this doesn't include a regression test. If this is unacceptable, please let me know, I'll see what I can do.
1. I'm getting some test failures on make check, all from debuginfo. I suspect this is due to #13680 and  not related to my changes (I have GDB 7.7). This is the list of failed tests:

> [debuginfo-gdb] debuginfo/basic-types-globals.rs
> [debuginfo-gdb] debuginfo/basic-types-mut-globals.rs
> [debuginfo-gdb] debuginfo/basic-types.rs
> [debuginfo-gdb] debuginfo/borrowed-basic.rs
> [debuginfo-gdb] debuginfo/borrowed-managed-basic.rs
> [debuginfo-gdb] debuginfo/borrowed-struct.rs
> [debuginfo-gdb] debuginfo/borrowed-unique-basic.rs
> [debuginfo-gdb] debuginfo/box.rs
> [debuginfo-gdb] debuginfo/by-value-non-immediate-argument.rs
> [debuginfo-gdb] debuginfo/by-value-self-argument-in-trait-impl.rs
> [debuginfo-gdb] debuginfo/closure-in-generic-function.rs
> [debuginfo-gdb] debuginfo/evec-in-struct.rs
> [debuginfo-gdb] debuginfo/function-arg-initialization.rs
> [debuginfo-gdb] debuginfo/function-prologue-stepping-no-split-stack.rs
> [debuginfo-gdb] debuginfo/generic-function.rs
> [debuginfo-gdb] debuginfo/generic-functions-nested.rs
> [debuginfo-gdb] debuginfo/generic-method-on-generic-struct.rs
> [debuginfo-gdb] debuginfo/generic-static-method-on-struct-and-enum.rs
> [debuginfo-gdb] debuginfo/generic-struct.rs
> [debuginfo-gdb] debuginfo/lexical-scope-in-stack-closure.rs
> [debuginfo-gdb] debuginfo/lexical-scope-in-unique-closure.rs
> [debuginfo-gdb] debuginfo/method-on-generic-struct.rs
> [debuginfo-gdb] debuginfo/method-on-tuple-struct.rs
> [debuginfo-gdb] debuginfo/name-shadowing-and-scope-nesting.rs
> [debuginfo-gdb] debuginfo/recursive-struct.rs
> [debuginfo-gdb] debuginfo/self-in-generic-default-method.rs
> [debuginfo-gdb] debuginfo/shadowed-argument.rs
> [debuginfo-gdb] debuginfo/shadowed-variable.rs
> [debuginfo-gdb] debuginfo/simd.rs
> [debuginfo-gdb] debuginfo/simple-lexical-scope.rs
> [debuginfo-gdb] debuginfo/simple-struct.rs
> [debuginfo-gdb] debuginfo/simple-tuple.rs
> [debuginfo-gdb] debuginfo/static-method-on-struct-and-enum.rs
> [debuginfo-gdb] debuginfo/tuple-struct.rs
> [debuginfo-gdb] debuginfo/var-captured-in-nested-closure.rs
> [debuginfo-gdb] debuginfo/var-captured-in-sendable-closure.rs
> [debuginfo-gdb] debuginfo/var-captured-in-stack-closure.rs

I can provide the full output on request.
mk: Don't run benchmarks with `make check`

The current suite of benchmarks for the standard distribution take a significant
amount of time to run, but it's unclear whether we're gaining any benefit from
running them. Some specific pain points:

* No one is looking at the data generated by the benchmarks. We have no graphs
  or analysis of what's happening, so all the data is largely being cast into
  the void.

* No benchmark has ever uncovered a bug, they have always run successfully.

* Benchmarks not only take a significant amount of time to run, but also take a
  significant amount of time to compile. I don't think we should mitigate this
  for now because it's useful to ensure that they do indeed still compile.

This commit disables --bench test runs by default as part of `make check`,
flipping the NO_BENCH environment variable to a PLEASE_BENCH variable which will
manually enable benchmarking. If and when a dedicated bot is set up for
benchmarking, this flag can be enabled on that bot.
std: Modify TempDir to not fail on drop. Closes #12628

After discussion with Alex, we think the proper policy is for dtors
to not fail. This is consistent with C++. BufferedWriter already
does this, so this patch modifies TempDir to not fail in the dtor,
adding a `close` method for handling errors on destruction.
rustdoc: functions in ffi blocks are unsafe

Fixes #14188
Make Vec.truncate() resilient against failure in Drop

If a vector element fails in Drop, Vec needs to make sure it doesn't try
to re-drop any elements it already dropped.
Make `from_bits` in `bitflags!` safe; add `from_bits_truncate`

Previously, the `from_bits` function in the `std::bitflags::bitflags`
macro was marked as unsafe, as it did not check that the bits being
converted actually corresponded to flags.

This patch changes the function to check against the full set of
possible flags and return an `Option` which is `None` if a non-flag bit
is set. It also adds a `from_bits_truncate` function which simply zeros
any bits not corresponding to a flag.

This addresses the concern raised in https://github.com/mozilla/rust/pull/13897
Register new snapshots
use sched_yield on linux and freebsd

pthread_yield is non-standard, while sched_yield is POSIX

The Linux documentation recommends using the standard function. This is
the only feature we're currently using that's present in glibc but not
in musl.
Add a crate for missing stubs from libcore

The core library in theory has 0 dependencies, but in practice it has some in
order for it to be efficient. These dependencies are in the form of the basic
memory operations provided by libc traditionally, such as memset, memcmp, etc.
These functions are trivial to implement and themselves have 0 dependencies.

This commit adds a new crate, librlibc, which will serve the purpose of
providing these dependencies. The crate is never linked to by default, but is
available to be linked to by downstream consumers. Normally these functions are
provided by the system libc, but in other freestanding contexts a libc may not
be available. In these cases, librlibc will suffice for enabling execution with
libcore.

cc #10116
shootout-mandlebrot: calculate two bits of the result per inner loop, +10-15%