rust: Emit android vndk cfg flag.

Pass an "android_vndk" cfg flag that indicates this code
targets the vndk. This can be useful in instances where code
might need to behave differently.

This also includes a fix to make sure our vendor Soong tests
are correctly configured.

Bug: 179907868
Test: Soong tests pass.
Test: Example module emits new cfg flags.
Change-Id: I01cdf91f6f9d42cd8a759266d5170479664bf4bc

rust/image_test.go

@@ -15,6 +15,7 @@
 package rust
 
 import (
+	"strings"
 	"testing"
 
 	"android/soong/android"
@@ -23,7 +24,7 @@ import (
 
 // Test that cc modules can link against vendor_available rust_ffi_static libraries.
 func TestVendorLinkage(t *testing.T) {
-	ctx := testRust(t, `
+	ctx := testRustVndk(t, `
 			cc_binary {
 				name: "fizz_vendor",
 				static_libs: ["libfoo_vendor"],
@@ -37,16 +38,34 @@ func TestVendorLinkage(t *testing.T) {
 			}
 		`)
 
-	vendorBinary := ctx.ModuleForTests("fizz_vendor", "android_arm64_armv8-a").Module().(*cc.Module)
+	vendorBinary := ctx.ModuleForTests("fizz_vendor", "android_vendor.VER_arm64_armv8-a").Module().(*cc.Module)
 
 	if !android.InList("libfoo_vendor", vendorBinary.Properties.AndroidMkStaticLibs) {
 		t.Errorf("vendorBinary should have a dependency on libfoo_vendor")
 	}
 }
 
+// Test that variants which use the vndk emit the appropriate cfg flag.
+func TestImageVndkCfgFlag(t *testing.T) {
+	ctx := testRustVndk(t, `
+			rust_ffi_static {
+				name: "libfoo",
+				crate_name: "foo",
+				srcs: ["foo.rs"],
+				vendor_available: true,
+			}
+		`)
+
+	vendor := ctx.ModuleForTests("libfoo", "android_vendor.VER_arm64_armv8-a_static").Rule("rustc")
+
+	if !strings.Contains(vendor.Args["rustcFlags"], "--cfg 'android_vndk'") {
+		t.Errorf("missing \"--cfg 'android_vndk'\" for libfoo vendor variant, rustcFlags: %#v", vendor.Args["rustcFlags"])
+	}
+}
+
 // Test that cc modules can link against vendor_ramdisk_available rust_ffi_static libraries.
 func TestVendorRamdiskLinkage(t *testing.T) {
-	ctx := testRust(t, `
+	ctx := testRustVndk(t, `
 			cc_library_static {
 				name: "libcc_vendor_ramdisk",
 				static_libs: ["libfoo_vendor_ramdisk"],