Add a soong wrapper for R8's tracereferences

Add a new traceReferences rule that wraps the base R8 tracereferences
command. This allows generating a set of keep rules after tracing
references from a given set of sources onto a given target jar.

This will be used to keep hidden/unannotated members of optimzed
libraries that may be referenced by other bundled libraries on the
platform.

Bug: 212737576
Test: m
Flag: EXEMPT basic soong rule addition
Change-Id: I92cbe8839ce68056311c7c38de181f2e5f7cb775

3 files changed, 56 insertions, 0 deletions

diff --git a/java/Android.bp b/java/Android.bp
index 911af8336..99d9c38a6 100644
--- a/java/Android.bp
+++ b/java/Android.bp
@@ -78,6 +78,7 @@ bootstrap_go_package {
         "system_modules.go",
         "systemserver_classpath_fragment.go",
         "testing.go",
+        "tracereferences.go",
         "tradefed.go",
     ],
     testSrcs: [
diff --git a/java/config/config.go b/java/config/config.go
index 71025de6a..fdb8d7886 100644
--- a/java/config/config.go
+++ b/java/config/config.go
@@ -173,6 +173,7 @@ func init() {
 	pctx.HostBinToolVariable("R8Cmd", "r8")
 	pctx.HostBinToolVariable("ExtractR8RulesCmd", "extract-r8-rules")
 	pctx.HostBinToolVariable("ResourceShrinkerCmd", "resourceshrinker")
+	pctx.HostBinToolVariable("TraceReferencesCmd", "tracereferences")
 	pctx.HostBinToolVariable("HiddenAPICmd", "hiddenapi")
 	pctx.HostBinToolVariable("ExtractApksCmd", "extract_apks")
 	pctx.VariableFunc("TurbineJar", func(ctx android.PackageVarContext) string {
diff --git a/java/tracereferences.go b/java/tracereferences.go
new file mode 100644
index 000000000..342b6a630
--- /dev/null
+++ b/java/tracereferences.go
@@ -0,0 +1,54 @@
+// Copyright 2025 Google Inc. All rights reserved.
+//
+// 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 java
+
+import (
+	"android/soong/android"
+
+	"github.com/google/blueprint"
+)
+
+var traceReferences = pctx.AndroidStaticRule("traceReferences",
+	blueprint.RuleParams{
+		Command: `${config.TraceReferencesCmd} ` +
+			// Note that we suppress missing def errors, as we're only interested
+			// in the direct deps between the sources and target.
+			`--map-diagnostics:MissingDefinitionsDiagnostic error none ` +
+			`--keep-rules ` +
+			`--output ${out} ` +
+			`--target ${in} ` +
+			// `--source` and `--lib` are already prepended to each
+			// jar reference in the sources and libs joined string args.
+			`${sources} ` +
+			`${libs}`,
+		CommandDeps: []string{"${config.TraceReferencesCmd}"},
+	}, "sources", "libs")
+
+// Generates keep rules in output corresponding to any references from sources
+// (a list of jars) onto target (the referenced jar) that are not included in
+// libs (a list of external jars).
+func TraceReferences(ctx android.ModuleContext, sources android.Paths, target android.Path, libs android.Paths,
+	output android.WritablePath) {
+	ctx.Build(pctx, android.BuildParams{
+		Rule:      traceReferences,
+		Input:     target,
+		Output:    output,
+		Implicits: append(sources, libs...),
+		Args: map[string]string{
+			"sources": android.JoinWithPrefix(sources.Strings(), "--source "),
+			"libs":    android.JoinWithPrefix(libs.Strings(), "--lib "),
+		},
+	})
+}