--- a/servo/etc/ci/buildbot_steps.yml
+++ b/servo/etc/ci/buildbot_steps.yml
@@ -9,19 +9,19 @@ mac-rel-wpt1:
   - bash ./etc/ci/manifest_changed.sh
 
 mac-rel-wpt2:
   - ./mach build --release
   - ./mach test-wpt --release --processes 8 --total-chunks 2 --this-chunk 2 --log-raw test-wpt.log --log-errorsummary wpt-errorsummary.log --always-succeed
   - ./mach filter-intermittents wpt-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-wpt-errorsummary.log --use-tracker
 
 mac-dev-unit:
-  - ./mach build --dev
-  - ./mach test-unit
-  - ./mach build-cef
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --dev
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach test-unit
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build-cef
   - ./mach build-geckolib
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
 
 mac-rel-css:
   - ./mach build --release
   - ./mach test-css --release --processes 4 --log-raw test-css.log --log-errorsummary css-errorsummary.log --always-succeed
   - ./mach filter-intermittents css-errorsummary.log --log-intermittents intermittents.log --log-filteredsummary filtered-css-errorsummary.log --use-tracker
@@ -41,20 +41,20 @@ linux-rel-intermittent:
 
 mac-rel-intermittent:
   - ./mach build --release
   - ./etc/ci/check_intermittents.sh --log-raw intermittents.log
 
 linux-dev:
   - ./mach test-tidy --no-progress --all
   - ./mach test-tidy --no-progress --self-test
-  - ./mach build --dev
-  - ./mach test-compiletest
-  - ./mach test-unit
-  - ./mach build-cef
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --dev
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach test-compiletest
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach test-unit
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build-cef
   - ./mach build-geckolib
   - ./mach test-stylo
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
   - bash ./etc/ci/check_no_panic.sh
 
 linux-rel-wpt:
   - ./mach build --release --with-debug-assertions
@@ -74,34 +74,34 @@ linux-rel-css:
   - bash ./etc/ci/manifest_changed.sh
 
 linux-nightly:
   - ./mach build --release
   - ./mach package --release
   - ./etc/ci/upload_nightly.sh linux
 
 android:
-  - ./mach build --android --dev
-  - ./mach package --android --dev
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --android --dev
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach package --android --dev
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
   - python ./etc/ci/check_dynamic_symbols.py
 
 android-nightly:
-  - ./mach build --android --release
-  - ./mach package --android --release
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --android --release
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach package --android --release
   - ./etc/ci/upload_nightly.sh android
 
 arm32:
-  - ./mach build --rel --target=arm-unknown-linux-gnueabihf
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --rel --target=arm-unknown-linux-gnueabihf
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
 
 arm64:
-  - ./mach build --rel --target=aarch64-unknown-linux-gnu
+  - env SERVO_RUSTC_LLVM_ASSERTIONS=1 ./mach build --rel --target=aarch64-unknown-linux-gnu
   - bash ./etc/ci/lockfile_changed.sh
   - bash ./etc/ci/manifest_changed.sh
 
 windows-gnu-dev:
   - ./mach build --dev
   - ./mach test-unit
   - ./mach build-geckolib
 

--- a/servo/python/servo/command_base.py
+++ b/servo/python/servo/command_base.py
@@ -18,17 +18,17 @@ import subprocess
 from subprocess import PIPE
 import sys
 import tarfile
 
 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 ""
 
 
 @contextlib.contextmanager
 def cd(new_path):
     """Context manager for changing the current working directory"""
     previous_path = os.getcwd()
@@ -258,20 +258,25 @@ class CommandBase(object):
         self.config["tools"].setdefault("system-cargo", False)
         self.config["tools"].setdefault("rust-root", "")
         self.config["tools"].setdefault("cargo-root", "")
         if not self.config["tools"]["system-cargo"]:
             self.config["tools"]["cargo-root"] = path.join(
                 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", "")
         self.config["build"].setdefault("incremental", False)
 
         self.config.setdefault("android", {})
         self.config["android"].setdefault("sdk", "")
         self.config["android"].setdefault("ndk", "")

--- a/servo/python/servo/util.py
+++ b/servo/python/servo/util.py
@@ -15,17 +15,17 @@ import platform
 import sys
 from socket import error as socket_error
 import StringIO
 import tarfile
 import zipfile
 import urllib2
 
 
-def host_triple():
+def host_platform():
     os_type = platform.system().lower()
     if os_type == "linux":
         os_type = "unknown-linux-gnu"
     elif os_type == "darwin":
         os_type = "apple-darwin"
     elif os_type == "android":
         os_type = "linux-androideabi"
     elif os_type == "windows":
@@ -37,17 +37,21 @@ def host_triple():
         else:
             os_type = "unknown"
     elif os_type.startswith("mingw64_nt-") or os_type.startswith("cygwin_nt-"):
         os_type = "pc-windows-gnu"
     elif os_type == "freebsd":
         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
         platform_env = os.environ.get("PLATFORM")
         if platform_env == "X86":
             cpu_type = "i686"
         elif platform_env == "X64":
             cpu_type = "x86_64"

--- a/servo/servobuild.example
+++ b/servo/servobuild.example
@@ -35,17 +35,17 @@ rustc-with-gold = true
 
 [build]
 # Set "mode = dev" or use `mach build --dev` to build the project with warning.
 # or Set "mode = release" or use `mach build --release` for optimized build.
 # Defaults to prompting before building
 #mode = "dev"
 
 # Whether to enable LLVM assertions in rustc.
-#llvm-assertions = true
+#llvm-assertions = false
 
 # Set "android = true" or use `mach build --android` to build the Android app.
 android = false
 
 # Set "debug-mozjs" or use `mach build --debug-mozjs` to build a debug spidermonkey.
 debug-mozjs = false
 
 # Set to the path to your ccache binary to enable caching of compiler outputs