Auto merge of #13387 - emilio:debug-assertions, r=aneeshusa,jdm,pcwalton

Honor SERVO_ENABLE_DEBUG_ASSERTIONS on the build machines.

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As part of #13127.

cc @aneeshusa

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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python/servo/build_commands.py

@@ -216,6 +216,12 @@ class MachCommands(CommandBase):
         build_start = time()
         env = self.build_env(target=target, is_build=True)
 
+        # TODO: If this ends up making it, we should probably add a
+        # --release-with-debug-assertions option or similar, so it's easier to
+        # build locally.
+        if env.get("SERVO_ENABLE_DEBUG_ASSERTIONS", None):
+            env["RUSTFLAGS"] = "-C debug_assertions"
+
         if android:
             # Build OpenSSL for android
             make_cmd = ["make"]