Auto merge of #15564 - servo:no-gods-no-masters-no-assertions, r=Ms2ger

Disable LLVM assertions by default, on supported platforms

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https://github.com/servo/servo/pull/15559#issuecomment-280003926

> With an empty incremental compilation cache (or, presumably, with incremental compilation disabled), LLVM assertions add 16% to the compilation time in debug mode, 53% (!) in release mode.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15548 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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1 files changed, 7 insertions, 2 deletions

diff --git a/python/servo/command_base.py b/python/servo/command_base.py
index 5ad351b3411..34ce2bfc682 100644
--- a/python/servo/command_base.py
+++ b/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", "")