I tried to execute the bootstrap script from the start in a just installed OS, and got this error:

Collecting MozPhab
  Downloading https://files.pythonhosted.org/packages/51/f5/7b58d0b701f91772051f3e676fe8484d5383d72092c9f6ca656a4bbc6eea/MozPhab-0.1.99.tar.gz (79kB)
    100% |████████████████████████████████| 81kB 1.9MB/s 
Requirement already satisfied, skipping upgrade: distro in /usr/lib/python3/dist-packages (from MozPhab) (1.3.0)
Collecting glean-sdk>=36.0.0 (from MozPhab)
  Cache entry deserialization failed, entry ignored
  Downloading https://files.pythonhosted.org/packages/23/5e/365c5c105b783d8ce0891c26f6b7b693fe098c9c05396f4d31515abd9091/glean-sdk-38.0.1.tar.gz (1.5MB)
    100% |████████████████████████████████| 1.5MB 1.0MB/s 
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-3o8m0fbc/glean-sdk/setup.py", line 16, in <module>
        from setup import *
      File "/tmp/pip-install-3o8m0fbc/glean-sdk/glean-core/python/setup.py", line 147, in <module>
        target = get_rustc_info()["host"]
      File "/tmp/pip-install-3o8m0fbc/glean-sdk/glean-core/python/setup.py", line 133, in get_rustc_info
        output = subprocess.check_output(["rustc", "--version", "--verbose"])
      File "/usr/lib/python3.7/subprocess.py", line 395, in check_output
        **kwargs).stdout
      File "/usr/lib/python3.7/subprocess.py", line 472, in run
        with Popen(*popenargs, **kwargs) as process:
      File "/usr/lib/python3.7/subprocess.py", line 775, in __init__
        restore_signals, start_new_session)
      File "/usr/lib/python3.7/subprocess.py", line 1522, in _execute_child
        raise child_exception_type(errno_num, err_msg, err_filename)
    FileNotFoundError: [Errno 2] No such file or directory: 'rustc': 'rustc'
    
    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-3o8m0fbc/glean-sdk/
Error running mach:

    ['install-moz-phab']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.
You can invoke |./mach busted| to check if this issue is already on file. If it
isn't, please use |./mach busted file install-moz-phab| to report it. If |./mach busted| is
misbehaving, you can also inspect the dependencies of bug 1543241.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

subprocess.CalledProcessError: Command '['/usr/bin/pip3', 'show', '-f', 'MozPhab']' returned non-zero exit status 1.

  File "/home/julien/travail/git/mozilla-central/tools/phabricator/mach_commands.py", line 103, in install_moz_phab
    [pip3, "show", "-f", "MozPhab"], universal_newlines=True
  File "/usr/lib/python3.7/subprocess.py", line 395, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.7/subprocess.py", line 487, in run
    output=stdout, stderr=stderr)
Error running mach:

    ['bootstrap', '--application-choice', 'browser_artifact_mode']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.
You can invoke |./mach busted| to check if this issue is already on file. If it
isn't, please use |./mach busted file bootstrap| to report it. If |./mach busted| is
misbehaving, you can also inspect the dependencies of bug 1543241.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

subprocess.CalledProcessError: Command '('/usr/bin/python3', '/home/julien/travail/git/mozilla-central/mach', 'install-moz-phab')' returned non-zero exit status 1.

  File "/home/julien/travail/git/mozilla-central/python/mozboot/mozboot/mach_commands.py", line 55, in bootstrap
    bootstrapper.bootstrap(self.settings)
  File "/home/julien/travail/git/mozilla-central/python/mozboot/mozboot/bootstrap.py", line 389, in bootstrap
    self.check_code_submission(checkout_root)
  File "/home/julien/travail/git/mozilla-central/python/mozboot/mozboot/bootstrap.py", line 287, in check_code_submission
    subprocess.check_call((sys.executable, mach_binary, "install-moz-phab"))
  File "/usr/lib/python3.7/subprocess.py", line 347, in check_call
    raise CalledProcessError(retcode, cmd)

This is when running the bootstrap script for artifact mode, where rustc and friends aren't installed.