Rustdoc support.

Adds `m rustdoc` which generates documentation for all Rust libraries
to $OUT_DIR/soong/rustdoc.

Follow up work:

* Generate an index page that lists all modules.
* Preserve the artifacts so we can have an always-up-to-date go link.

Test: m rustdoc
Bug: None
Change-Id: Id2d6b9cbab5b02e36b575567563d7cc7606b9401

rust/doc.go

@@ -0,0 +1,43 @@
+// Copyright 2021 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package rust
+
+import (
+	"android/soong/android"
+)
+
+func init() {
+	android.RegisterSingletonType("rustdoc", RustdocSingleton)
+}
+
+func RustdocSingleton() android.Singleton {
+	return &rustdocSingleton{}
+}
+
+type rustdocSingleton struct{}
+
+func (n *rustdocSingleton) GenerateBuildActions(ctx android.SingletonContext) {
+	ctx.VisitAllModules(func(module android.Module) {
+		if !module.Enabled() {
+			return
+		}
+
+		if m, ok := module.(*Module); ok {
+			if m.docTimestampFile.Valid() {
+				ctx.Phony("rustdoc", m.docTimestampFile.Path())
+			}
+		}
+	})
+}