Auto merge of #9611 - danlrobertson:i9557, r=larsbergstrom

Download extra stdlib only when required: #9557

Split [`ensure_bootstrap`](https://github.com/danlrobertson/servo/blob/i9557/python/servo/command_base.py#L397-L422) into two phases including a phase checking the compiler, and a phase checking for target libraries. E.g.

```
    # should download the stdlib for "i686-unknown-linux-gnu", "arm-linux-androideabi"
    # and the hosts target
    ./mach build -d --target i686-unknown-linux-gnu --android
    # should only download the stdlib for the hosts target
    ./mach build -d
```

Let me know if I missed anything! There are a few parts of this patch in its current state that I'm not a huge fan of, but I couldn't think of a better way in the moment.

Still new to working on servo, so any comments or critiques are welcome!

Fix #9557

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1 files changed, 7 insertions, 4 deletions

diff --git a/python/servo/command_base.py b/python/servo/command_base.py
index 450403e903b..d43c3ab4eda 100644
--- a/python/servo/command_base.py
+++ b/python/servo/command_base.py
@@ -394,18 +394,21 @@ class CommandBase(object):
     def android_build_dir(self, dev):
         return path.join(self.get_target_dir(), "arm-linux-androideabi", "debug" if dev else "release")
 
-    def ensure_bootstrapped(self):
+    def ensure_bootstrapped(self, targets=[]):
         if self.context.bootstrapped:
             return
 
         Registrar.dispatch("update-submodules", context=self.context)
 
         if not self.config["tools"]["system-rust"] and \
-           not path.exists(path.join(
-                self.config["tools"]["rust-root"], "rustc", "bin", "rustc" + BIN_SUFFIX)):
+           (not path.exists(path.join(
+                            self.config["tools"]["rust-root"], "rustc", "bin", "rustc" + BIN_SUFFIX)) or
+                not all([path.exists(path.join(
+                        self.config["tools"]["rust-root"], "rustc", "lib", "rustlib", x
+                        )) for x in targets])):
             print("looking for rustc at %s" % path.join(
                 self.config["tools"]["rust-root"], "rustc", "bin", "rustc" + BIN_SUFFIX))
-            Registrar.dispatch("bootstrap-rust", context=self.context)
+            Registrar.dispatch("bootstrap-rust", context=self.context, target=targets)
         if not self.config["tools"]["system-cargo"] and \
            not path.exists(path.join(
                 self.config["tools"]["cargo-root"], "cargo", "bin", "cargo" + BIN_SUFFIX)):