Add ./mach build --with-layout-2020

… with corresponding `layout` and `layout_thread` crates,
which for now do nothing.

components/servo/Cargo.toml

@@ -20,6 +20,8 @@ energy-profiling = ["profile_traits/energy-profiling"]
 profilemozjs = ["script/profilemozjs"]
 googlevr = ["webvr/googlevr"]
 js_backtrace = ["script/js_backtrace"]
+layout-2013 = ["layout_thread_2013"]
+layout-2020 = ["layout_thread_2020"]
 max_log_level = ["log/release_max_level_info"]
 native-bluetooth = ["bluetooth/native-bluetooth"]
 no_wgl = ["canvas/no_wgl"]
@@ -53,7 +55,8 @@ gfx = {path = "../gfx"}
 gleam = "0.6"
 ipc-channel = "0.11"
 keyboard-types = "0.4"
-layout_thread = {path = "../layout_thread"}
+layout_thread_2013 = {path = "../layout_thread", optional = true}
+layout_thread_2020 = {path = "../layout_thread_2020", optional = true}
 log = "0.4"
 msg = {path = "../msg"}
 net = {path = "../net"}

components/servo/build.rs

@@ -0,0 +1,20 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+fn main() {
+    let layout_2013 = std::env::var_os("CARGO_FEATURE_LAYOUT_2013").is_some();
+    let layout_2020 = std::env::var_os("CARGO_FEATURE_LAYOUT_2020").is_some();
+
+    if !(layout_2013 || layout_2020) {
+        error("Must enable one of the `layout-2013` or `layout-2020` features.")
+    }
+    if layout_2013 && layout_2020 {
+        error("Must not enable both of the `layout-2013` or `layout-2020` features.")
+    }
+}
+
+fn error(message: &str) {
+    print!("\n\n    Error: {}\n\n", message);
+    std::process::exit(1)
+}