This is a simple way to workaround the debugging issues caused by the rustc
wrapper used in the bootstrap process. Namely, it uses some obscure environment
variables and you can’t just copy the failed command and run it in the shell or
debugger to examine the failure more closely.

With `--on-fail` its possible to run an arbitrary command within exactly the
same environment under which rustc failed. Theres’s multiple ways to use this
new flag:

$ python x.py build --stage=1 --on-fail=env

    would print a list of environment variables and the failed command, so a
    few copy-pastes and you now can run the same rust in your shell outside the
    bootstrap system.

$ python x.py build --stage=1 --on-fail=bash

    Is a more useful variation of the command above in that it launches a whole
    shell with environment already in place! All that’s left to do is copy-paste
    the command just above the shell prompt!

Fixes rust-lang#38686
Fixes rust-lang#38221

* Update bootstrap to recognize the cputype 'sparcv9' (used on Solaris)
* Change to never use -fomit-frame-pointer on Solaris or for sparc
* Adds rust target sparcv9-sun-solaris

Fixes rust-lang#39901

The mx_handle_wait_* syscalls in Magenta were renamed to
mx_object_wait. The syscall is used in the Magenta/Fuchsia
implementation of std::process, to wait on child processes.

In addition, this patch enables the use of the system provided
libbacktrace library on Fuchsia targets. Symbolization is not yet
working, but at least it allows printing hex addresses in a backtrace
and makes building succeed when the backtrace feature is not disabled.

So I just encountered this error for the first time. It's unclear what
it means, why I encountered it, or how to fix it. But worst of all, it
has a random newline and weird indentation! This commit fixes that last
bit.

Fix variable name typo

Additional docs for Vec, String, and slice trait impls

r? @steveklabnik

…chton

[rustbuild] add a way to run command after failure

This is a simple way to workaround the debugging issues caused by the rustc
wrapper used in the bootstrap process. Namely, it uses some obscure environment
variables and you can't just copy the failed command and run it in the shell or
debugger to examine the failure more closely.

With `--on-fail` its possible to run an arbitrary command within exactly the
same environment under which rustc failed. Theres's multiple ways to use this
new flag:

$ python x.py build --stage=1 --on-fail=env

    would print a list of environment variables and the failed command, so a
    few copy-pastes and you now can run the same rust in your shell outside the
    bootstrap system.

$ python x.py build --stage=1 --on-fail=bash

    Is a more useful variation of the command above in that it launches a whole
    shell with environment already in place! All that's left to do is copy-paste
    the command just above the shell prompt!

Fixes rust-lang#38686
Fixes rust-lang#38221

Fix test caching on Windows/GNU

Addresses rust-lang#36385 (comment)

Previously the sysroot directory was purged on every build and mingw startup objects were rebuilt unconditionally and always triggered test reruns.
Now the sysroot directory is reused and mingw startup objects are rebuilt only when necessary, so test caching works.

…richton

add solaris sparcv9 support

Fixes rust-lang#39901

…chton

Follow rename of mx_handle_wait Magenta syscalls

The mx_handle_wait_* syscalls in Magenta were renamed to
mx_object_wait. The syscall is used in the Magenta/Fuchsia
implementation of std::process, to wait on child processes.

In addition, this patch enables the use of the system provided
libbacktrace library on Fuchsia targets. Symbolization is not yet
working, but at least it allows printing hex addresses in a backtrace
and makes building succeed when the backtrace feature is not disabled.

Fix indentation of error message

So I just encountered this error for the first time. It's unclear what
it means, why I encountered it, or how to fix it. But worst of all, it
has a random newline and weird indentation! This commit fixes that last
bit.

<img width="680" alt="screen shot 2017-02-18 at 4 35 36 pm" src="https://cloud.githubusercontent.com/assets/1529387/23097087/0ef5ac4a-f5f9-11e6-9132-18ce759f7092.png">

…sxcv

Add missing urls and examples into Barrier structs

r? @frewsxcv

Clarify thread::Builder::stack_size docs

Add two pieces of information:

* the size is in bytes
* the actual stack size might be bigger.

Also change the example to use a more realistic value of 32 kilobytes.

r? @steveklabnik

Update exception-safety.md

Fix variable name typo

Add missing urls in MutexGuard docs

r? @frewsxcv