Auto merge of #15390 - servo:opt-in-incremental, r=nox

Option configuration to enable incremental compilation.

https://internals.rust-lang.org/t/incremental-compilation-beta/4721

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

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python/servo/command_base.py

@@ -270,6 +270,7 @@ class CommandBase(object):
         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", "")
@@ -427,6 +428,8 @@ class CommandBase(object):
             env["PATH"] = "%s%s%s" % (os.pathsep.join(extra_path), os.pathsep, env["PATH"])
 
         env["CARGO_HOME"] = self.config["tools"]["cargo-home-dir"]
+        if self.config["build"]["incremental"]:
+            env["CARGO_INCREMENTAL"] = "1"
 
         if extra_lib:
             if sys.platform == "darwin":

servobuild.example

@@ -38,15 +38,22 @@ rustc-with-gold = true
 # or Set "mode = release" or use `mach build --release` for optimized build.
 # Defaults to prompting before building
 #mode = "dev"
+
 # 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
 #ccache = "/usr/local/bin/ccache"
+
 # Any optional flags that will be added to $RUSTFLAGS
 #rustflags = ""
 
+# Enable rustc’s incremental compilation
+#incremental = false
+
 # Android information
 [android]
 # Defaults to the value of $ANDROID_SDK, $ANDROID_NDK, $ANDROID_TOOLCHAIN, $ANDROID_PLATFORM respectively