Let Rust plugin fetch dependencies in pull (canonical#908)

Pulling the dependencies in the pull stage allows
building Rust snaps on Launchpad builders,
which are offline during the build stage

LP: #1647847

integration_tests/snaps/simple-rust/Cargo.toml

@@ -4,3 +4,4 @@ version = "0.1.0"
 authors = ["Marius Gripsgard <mariogrip@ubuntu.com>"]
 
 [dependencies]
+log = "*"

integration_tests/snaps/simple-rust/src/main.rs

@@ -1,3 +1,5 @@
+extern crate log;
+
 fn main() {
     println!("There is rust on snaps!");
 }

snapcraft/plugins/rust.py

@@ -54,6 +54,14 @@ def schema(cls):
 
     def __init__(self, name, options, project):
         super().__init__(name, options, project)
+        self.build_packages.extend([
+            'gcc',
+            'binutils',
+            'libc6-dev',
+            'git',
+            'curl',
+            'file',
+        ])
         self._rustpath = os.path.join(self.partdir, "rust")
         self._rustc = os.path.join(self._rustpath, "bin", "rustc")
         self._rustdoc = os.path.join(self._rustpath, "bin", "rustdoc")
@@ -78,6 +86,7 @@ def _build_env(self):
     def pull(self):
         super().pull()
         self._fetch_rust()
+        self._fetch_deps()
 
     def clean_pull(self):
         super().clean_pull()
@@ -104,3 +113,6 @@ def _fetch_rust(self):
         self.run(["%s" % self._rustup,
                   "--prefix=%s" % self._rustpath,
                   "--disable-sudo", "--save"])
+
+    def _fetch_deps(self):
+        self.run([self._cargo, "fetch"])

snapcraft/tests/test_plugin_rust.py

@@ -86,14 +86,10 @@ def test_pull(self, run_mock, script_mock):
 
         plugin.pull()
 
-        self.assertEqual(1, run_mock.call_count)
-        run_mock.assert_has_calls([
-            mock.call(["%s" % plugin._rustup,
-                       "--prefix=%s" % plugin._rustpath,
-                       "--disable-sudo", "--save"])
-        ])
+        self.assertEqual(2, run_mock.call_count)
 
         rustdir = os.path.join(plugin.partdir, 'rust')
-        run_mock.assert_called_once_with([
+        run_mock.assert_has_calls([mock.call([
             os.path.join(rustdir, 'rustup.sh'), '--prefix={}'.format(rustdir),
-            '--disable-sudo', '--save'])
+            '--disable-sudo', '--save']),
+            mock.call([plugin._cargo, "fetch"])])