Add Soong test for device proc-macro deps.

Ensure that devices can include proc_macros, which are host-only
and may include host-only dependencies.

Bug: 141491501
Test: Soong tests pass.
Test: Test fails as expected when removing CL 1126496
Change-Id: I3ae7ab80283cd1fd4b800a533cb3205b3c108d45

rust/rust_test.go

@@ -18,6 +18,7 @@ import (
 	"io/ioutil"
 	"os"
 	"runtime"
+	"strings"
 	"testing"
 
 	"android/soong/android"
@@ -175,3 +176,28 @@ func TestDepsTracking(t *testing.T) {
 	}
 
 }
+
+// Test to make sure proc_macros use host variants when building device modules.
+func TestProcMacroDeviceDeps(t *testing.T) {
+	ctx := testRust(t, `
+		rust_library_host_rlib {
+			name: "libbar",
+			srcs: ["foo.rs"],
+		}
+		rust_proc_macro {
+			name: "libpm",
+			rlibs: ["libbar"],
+			srcs: ["foo.rs"],
+		}
+		rust_binary {
+			name: "fizz-buzz",
+			proc_macros: ["libpm"],
+			srcs: ["foo.rs"],
+		}
+	`)
+	rustc := ctx.ModuleForTests("libpm", "linux_glibc_x86_64").Rule("rustc")
+
+	if !strings.Contains(rustc.Args["libFlags"], "libbar/linux_glibc_x86_64") {
+		t.Errorf("Proc_macro is not using host variant of dependent modules.")
+	}
+}