cpp output of an AIDL file is together with the headers

For an AIDL file, five files are generated for the CPP backend: cpp
output, depfile, and three headers. Previously, the cpp output and the
dep file were created at <module_out>/gen/<abs_path_to_input_aidl>.cpp,
while the headers were at <module_out>/gen/aidl/<package_name>/*.h.

This not only looks inconsistent, but more critically makes it difficult
for the aidl compiler to infer the path to the headers that the build
system registered as implicit outputs. Inferring the implicit outputs
by the aidl compiler is needed because otherwise users will see the
error message from sbox just saying that some of the expected files are
not created. This can happen when the input AIDL file is put directly
into the srcs property without specifying the base directory, e.g.
some/subdir/android/foo/IFoo.aidl where the pacakge is actually
android.foo.

In order to make it easy for developers to fix such an error,
I0f23b6027ba3a4755cc2901f4a7f7fc70bffd0ef introduces a check in the aidl
compiler which is triggered earlier than the sbox error. The compiler
now enforces that the cpp output is at
<out_dir>/<package_name>/<type>.app. When the check fails, it suggests
to fix that by correctly feeding the AIDL file via filegroup and the
path property.

This change in Soong is required to satisfy aidl compiler when the base
directory is correctly set. The cpp output is now at
<module_out>/gen/aidl/<pacakge_name>/<type>.cpp.

Bug: 184586092
Test: aidl_unittests
Change-Id: I172180a40bded4f6c08679a2d862b086998be1e1
diff --git a/cc/gen.go b/cc/gen.go
index 83c019c..b152e02 100644
--- a/cc/gen.go
+++ b/cc/gen.go
@@ -111,9 +111,7 @@
 	return ret
 }
 
-func genAidl(ctx android.ModuleContext, rule *android.RuleBuilder, aidlFile android.Path,
-	outFile, depFile android.ModuleGenPath, aidlFlags string) android.Paths {
-
+func genAidl(ctx android.ModuleContext, rule *android.RuleBuilder, aidlFile android.Path, aidlFlags string) (cppFile android.OutputPath, headerFiles android.Paths) {
 	aidlPackage := strings.TrimSuffix(aidlFile.Rel(), aidlFile.Base())
 	baseName := strings.TrimSuffix(aidlFile.Base(), aidlFile.Ext())
 	shortName := baseName
@@ -126,6 +124,8 @@
 	}
 
 	outDir := android.PathForModuleGen(ctx, "aidl")
+	cppFile = outDir.Join(ctx, aidlPackage, baseName+".cpp")
+	depFile := outDir.Join(ctx, aidlPackage, baseName+".cpp.d")
 	headerI := outDir.Join(ctx, aidlPackage, baseName+".h")
 	headerBn := outDir.Join(ctx, aidlPackage, "Bn"+shortName+".h")
 	headerBp := outDir.Join(ctx, aidlPackage, "Bp"+shortName+".h")
@@ -142,14 +142,14 @@
 		Flag(aidlFlags).
 		Input(aidlFile).
 		OutputDir().
-		Output(outFile).
+		Output(cppFile).
 		ImplicitOutputs(android.WritablePaths{
 			headerI,
 			headerBn,
 			headerBp,
 		})
 
-	return android.Paths{
+	return cppFile, android.Paths{
 		headerI,
 		headerBn,
 		headerBp,
@@ -293,10 +293,9 @@
 				aidlRule = android.NewRuleBuilder(pctx, ctx).Sbox(android.PathForModuleGen(ctx, "aidl"),
 					android.PathForModuleGen(ctx, "aidl.sbox.textproto"))
 			}
-			cppFile := android.GenPathWithExt(ctx, "aidl", srcFile, "cpp")
-			depFile := android.GenPathWithExt(ctx, "aidl", srcFile, "cpp.d")
+			cppFile, aidlHeaders := genAidl(ctx, aidlRule, srcFile, buildFlags.aidlFlags)
 			srcFiles[i] = cppFile
-			aidlHeaders := genAidl(ctx, aidlRule, srcFile, cppFile, depFile, buildFlags.aidlFlags)
+
 			info.aidlHeaders = append(info.aidlHeaders, aidlHeaders...)
 			// Use the generated headers as order only deps to ensure that they are up to date when
 			// needed.