Add source collision test, rust_bindgen doc.

Bug: 159064919
Test: Soong tests pass (and fail if error is modified).
Change-Id: Id4f76e53e11e46ae495f9613a2699c598e7daea4

rust/Android.bp

@@ -27,6 +27,7 @@ bootstrap_go_package {
     testSrcs: [
         "binary_test.go",
         "bindgen_test.go",
+        "builder_test.go",
         "clippy_test.go",
         "compiler_test.go",
         "coverage_test.go",

rust/bindgen.go

@@ -147,6 +147,9 @@ func (b *bindgenDecorator) sourceProviderProps() []interface{} {
 		&b.Properties)
 }
 
+// rust_bindgen generates Rust FFI bindings to C libraries using bindgen given a wrapper header as the primary input.
+// Bindgen has a number of flags to control the generated source, and additional flags can be passed to clang to ensure
+// the header and generated source is appropriately handled.
 func RustBindgenFactory() android.Module {
 	module, _ := NewRustBindgen(android.HostAndDeviceSupported)
 	return module.Init()

rust/builder.go

@@ -219,15 +219,16 @@ func transformSrctoCrate(ctx ModuleContext, main android.Path, deps PathDeps, fl
 	}
 
 	if len(deps.SrcDeps) > 0 {
-		moduleGenDir := android.PathForModuleOut(ctx, "out/")
+		genSubDir := "out/"
+		moduleGenDir := android.PathForModuleOut(ctx, genSubDir)
 		var outputs android.WritablePaths
 
 		for _, genSrc := range deps.SrcDeps {
-			if android.SuffixInList(outputs.Strings(), "out/"+genSrc.Base()) {
+			if android.SuffixInList(outputs.Strings(), genSubDir+genSrc.Base()) {
 				ctx.PropertyErrorf("srcs",
 					"multiple source providers generate the same filename output: "+genSrc.Base())
 			}
-			outputs = append(outputs, android.PathForModuleOut(ctx, "out/"+genSrc.Base()))
+			outputs = append(outputs, android.PathForModuleOut(ctx, genSubDir+genSrc.Base()))
 		}
 
 		ctx.Build(pctx, android.BuildParams{

rust/builder_test.go

@@ -0,0 +1,40 @@
+// Copyright 2020 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 "testing"
+
+func TestSourceProviderCollision(t *testing.T) {
+	testRustError(t, "multiple source providers generate the same filename output: bindings.rs", `
+		rust_binary {
+			name: "source_collider",
+			srcs: [
+				"foo.rs",
+				":libbindings1",
+				":libbindings2",
+			],
+		}
+		rust_bindgen {
+			name: "libbindings1",
+			stem: "bindings",
+			wrapper_src: "src/any.h",
+		}
+		rust_bindgen {
+			name: "libbindings2",
+			stem: "bindings",
+			wrapper_src: "src/any.h",
+		}
+	`)
+}