Flag an error on empty rust module srcs

It's an error if a rust module's srcs is empty. Prior to this change
the output was a panic that complains loudly about a slice error and
nothing else. This change doesn't stop the panic, but at least adds
a bit of context so devs who make a simple mistake can more easily
fix it.

Test: SOONG_GEN_RUST_PROJECT=1 m nothing
Change-Id: Id7d8465d533413c3000699661222a53a7c8678f3

rust/compiler.go

@@ -449,6 +449,10 @@ func (compiler *baseCompiler) relativeInstallPath() string {
 
 // Returns the Path for the main source file along with Paths for generated source files from modules listed in srcs.
 func srcPathFromModuleSrcs(ctx ModuleContext, srcs []string) (android.Path, android.Paths) {
+	if len(srcs) == 0 {
+		ctx.PropertyErrorf("srcs", "srcs must not be empty")
+	}
+
 	// The srcs can contain strings with prefix ":".
 	// They are dependent modules of this module, with android.SourceDepTag.
 	// They are not the main source file compiled by rustc.

rust/compiler_test.go

@@ -98,6 +98,41 @@ func TestEnforceSingleSourceFile(t *testing.T) {
 		}`)
 }
 
+// Test that we reject _no_ source files.
+func TestEnforceMissingSourceFiles(t *testing.T) {
+
+	singleSrcError := "srcs must not be empty"
+
+	// Test libraries
+	testRustError(t, singleSrcError, `
+		rust_library_host {
+			name: "foo-bar-library",
+			crate_name: "foo",
+		}`)
+
+	// Test binaries
+	testRustError(t, singleSrcError, `
+		rust_binary_host {
+			name: "foo-bar-binary",
+			crate_name: "foo",
+		}`)
+
+	// Test proc_macros
+	testRustError(t, singleSrcError, `
+		rust_proc_macro {
+			name: "foo-bar-proc-macro",
+			crate_name: "foo",
+		}`)
+
+	// Test prebuilts
+	testRustError(t, singleSrcError, `
+		rust_prebuilt_dylib {
+			name: "foo-bar-prebuilt",
+			crate_name: "foo",
+		  host_supported: true,
+		}`)
+}
+
 // Test environment vars for Cargo compat are set.
 func TestCargoCompat(t *testing.T) {
 	ctx := testRust(t, `