Disable LLVM assertions by default, on supported platforms.

But keep them on linux-dev CI.

python/servo/command_base.py

@@ -23,7 +23,7 @@ from mach.registrar import Registrar
 import toml
 
 from servo.packages import WINDOWS_MSVC as msvc_deps
-from servo.util import host_triple
+from servo.util import host_triple, host_platform
 
 BIN_SUFFIX = ".exe" if sys.platform == "win32" else ""
 
@@ -263,10 +263,15 @@ class CommandBase(object):
                 context.sharedir, "cargo", self.cargo_build_id())
         self.config["tools"].setdefault("rustc-with-gold", get_env_bool("SERVO_RUSTC_WITH_GOLD", True))
 
+        # https://github.com/rust-lang/rust/pull/39754
+        platforms_with_rustc_alt_builds = ["unknown-linux-gnu", "apple-darwin", "pc-windows-msvc"]
+        llvm_assertions_default = ("SERVO_RUSTC_LLVM_ASSERTIONS" in os.environ
+                                   or host_platform() not in platforms_with_rustc_alt_builds)
+
         self.config.setdefault("build", {})
         self.config["build"].setdefault("android", False)
         self.config["build"].setdefault("mode", "")
-        self.config["build"].setdefault("llvm-assertions", True)
+        self.config["build"].setdefault("llvm-assertions", llvm_assertions_default)
         self.config["build"].setdefault("debug-mozjs", False)
         self.config["build"].setdefault("ccache", "")
         self.config["build"].setdefault("rustflags", "")

python/servo/util.py

@@ -20,7 +20,7 @@ import zipfile
 import urllib2
 
 
-def host_triple():
+def host_platform():
     os_type = platform.system().lower()
     if os_type == "linux":
         os_type = "unknown-linux-gnu"
@@ -42,7 +42,11 @@ def host_triple():
         os_type = "unknown-freebsd"
     else:
         os_type = "unknown"
+    return os_type
 
+
+def host_triple():
+    os_type = host_platform()
     cpu_type = platform.machine().lower()
     if os_type.endswith("-msvc"):
         # vcvars*.bat should set it properly