automation: install Rust in Linux environment

This will install Rust 1.31.1, 1.34.2, and whatever stable is at
the time the install runs. We install 1.31.1 as our minimum supported
Rust version (I think that's what we're currently targeting) and
1.34 because that's what Debian 10 is shipping.

Differential Revision: https://phab.mercurial-scm.org/D6715

contrib/automation/hgautomation/linux.py

@@ -65,6 +65,18 b' pyenv global ${PYENV2_VERSIONS} ${PYENV3'
 '''.lstrip().replace('\r\n', '\n')
 
 
+INSTALL_RUST = r'''
+RUSTUP_INIT_SHA256=a46fe67199b7bcbbde2dcbc23ae08db6f29883e260e23899a88b9073effc9076
+wget -O rustup-init --progress dot:mega https://static.rust-lang.org/rustup/archive/1.18.3/x86_64-unknown-linux-gnu/rustup-init
+echo "${RUSTUP_INIT_SHA256} rustup-init" | sha256sum --check -
+
+chmod +x rustup-init
+sudo -H -u hg -g hg ./rustup-init -y
+sudo -H -u hg -g hg /home/hg/.cargo/bin/rustup install 1.31.1 1.34.2
+sudo -H -u hg -g hg /home/hg/.cargo/bin/rustup component add clippy
+'''
+
+
 BOOTSTRAP_VIRTUALENV = r'''
 /usr/bin/virtualenv /hgdev/venv-bootstrap
 
@@ -286,6 +298,8 b' sudo mkdir /hgdev'
 # Will be normalized to hg:hg later.
 sudo chown `whoami` /hgdev
 
+{install_rust}
+
 cp requirements-py2.txt /hgdev/requirements-py2.txt
 cp requirements-py3.txt /hgdev/requirements-py3.txt
 
@@ -309,6 +323,7 b' EOF'
 
 sudo chown -R hg:hg /hgdev
 '''.lstrip().format(
+    install_rust=INSTALL_RUST,
     install_pythons=INSTALL_PYTHONS,
     bootstrap_virtualenv=BOOTSTRAP_VIRTUALENV
 ).replace('\r\n', '\n')