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/rust.go

@@ -58,6 +58,7 @@ type Flags struct {
 	RustFlags       []string // Flags that apply to rust
 	LinkFlags       []string // Flags that apply to linker
 	ClippyFlags     []string // Flags that apply to clippy-driver, during the linting
+	RustdocFlags    []string // Flags that apply to rustdoc
 	Toolchain       config.Toolchain
 	Coverage        bool
 	Clippy          bool
@@ -124,6 +125,7 @@ type Module struct {
 	// as a library. The stripped output which is used for installation can be found via
 	// compiler.strippedOutputFile if it exists.
 	unstrippedOutputFile android.OptionalPath
+	docTimestampFile     android.OptionalPath
 
 	hideApexVariantFromMake bool
 }
@@ -355,10 +357,12 @@ type compiler interface {
 	compile(ctx ModuleContext, flags Flags, deps PathDeps) android.Path
 	compilerDeps(ctx DepsContext, deps Deps) Deps
 	crateName() string
+	rustdoc(ctx ModuleContext, flags Flags, deps PathDeps) android.OptionalPath
 
 	// Output directory in which source-generated code from dependencies is
 	// copied. This is equivalent to Cargo's OUT_DIR variable.
 	CargoOutDir() android.OptionalPath
+
 	inData() bool
 	install(ctx ModuleContext)
 	relativeInstallPath() string
@@ -755,6 +759,8 @@ func (mod *Module) GenerateAndroidBuildActions(actx android.ModuleContext) {
 		mod.unstrippedOutputFile = android.OptionalPathForPath(unstrippedOutputFile)
 		bloaty.MeasureSizeForPaths(ctx, mod.compiler.strippedOutputFilePath(), mod.unstrippedOutputFile)
 
+		mod.docTimestampFile = mod.compiler.rustdoc(ctx, flags, deps)
+
 		apexInfo := actx.Provider(android.ApexInfoProvider).(android.ApexInfo)
 		if mod.installable(apexInfo) {
 			mod.compiler.install(ctx)