Auto merge of #12821 - UK992:mach-update, r=aneeshusa

Create `mach fetch` command

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12775.

r? @larsbergstrom

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python/servo/devenv_commands.py

@@ -21,6 +21,12 @@ from mach.decorators import (
 
 from servo.command_base import CommandBase, cd, call
 
+CARGO_PATHS = [
+    path.join('components', 'servo'),
+    path.join('ports', 'cef'),
+    path.join('ports', 'geckolib'),
+]
+
 
 @CommandProvider
 class MachCommands(CommandBase):
@@ -81,11 +87,7 @@ class MachCommands(CommandBase):
             print("flag or update all packages with --all-packages (-a) flag")
             sys.exit(1)
 
-        cargo_paths = [path.join('components', 'servo'),
-                       path.join('ports', 'cef'),
-                       path.join('ports', 'geckolib')]
-
-        for cargo_path in cargo_paths:
+        for cargo_path in CARGO_PATHS:
             with cd(cargo_path):
                 print(cargo_path)
                 call(["cargo", "update"] + params,
@@ -153,6 +155,19 @@ class MachCommands(CommandBase):
             ["git"] + ["grep"] + params + ['--'] + grep_paths + [':(exclude)*.min.js'],
             env=self.build_env())
 
+    @Command('fetch',
+             description='Fetch Rust, Cargo and Cargo dependencies',
+             category='devenv')
+    def fetch(self):
+        # Fetch Rust and Cargo
+        self.ensure_bootstrapped()
+
+        # Fetch Cargo dependencies
+        for cargo_path in CARGO_PATHS:
+            with cd(cargo_path):
+                print(cargo_path)
+                call(["cargo", "fetch"], env=self.build_env())
+
     @Command('wpt-upgrade',
              description='upgrade wptrunner.',
              category='devenv')