fix(codegen): reject invalid indent options (#25807)

## Summary

 - throw a TypeError when indent is empty, contains characters other than spaces or tabs, or is not a string
 - preserve the existing default tab indentation and unchanged printIndent hot path
 - update API documentation and validation coverage

packages/codegen/src-js/print/options.ts

@@ -44,7 +44,8 @@ export interface Position {
 export interface Options {
   /**
    * String to use for indentation, defaults to `"\t"`.
-   * Must consist only of spaces and/or tabs - anything else falls back to a tab.
+   * Must be a non-empty string consisting only of spaces and/or tabs.
+   * Throws a `TypeError` otherwise.
    */
   indent?: string;
 

packages/codegen/src-js/state.ts

@@ -28,10 +28,7 @@ let indentString = "\t";
  */
 const indents = [""];
 
-/**
- * The `indent` option must be made up of only spaces and tabs, and not empty string.
- * Anything else falls back to a tab.
- */
+/** The `indent` option must be a non-empty string made up of only spaces and tabs. */
 const INDENT_REGEX = /^[ \t]+$/;
 
 /** Upper bound for the process-wide indentation cache. */
@@ -105,8 +102,13 @@ export class State {
     // The `indent` option is validated here, not in the printer, and changing of it discards
     // the cache grown for the old `indentString`.
     // That should be rare - most users have an indent style they prefer, and use it consistently.
-    let { indent } = options;
-    if (typeof indent !== "string" || !INDENT_REGEX.test(indent)) indent = "\t";
+    const { indent: indentOption } = options;
+    let indent = indentOption;
+    if (indent === undefined) {
+      indent = "\t";
+    } else if (typeof indent !== "string" || !INDENT_REGEX.test(indent)) {
+      throw new TypeError("`indent` must be a non-empty string containing only spaces and tabs");
+    }
     if (indent !== indentString) {
       indentString = indent;
       indents.length = 1;

packages/codegen/test/print.test.ts

@@ -107,6 +107,22 @@ function checkCases(cases: Case[]): void {
 
 // --- Tests ----------------------------------------------------------------------------------
 
+describe("indent", () => {
+  const ast = e(id("x"));
+
+  test.each(["", "x", "\n", "\r\n", " x "])("rejects %j", (indent) => {
+    expect(() => printSync(ast, { indent })).toThrowError(
+      new TypeError("`indent` must be a non-empty string containing only spaces and tabs"),
+    );
+  });
+
+  test.each([4, null, {}])("rejects non-string value %j", (indent) => {
+    expect(() => printSync(ast, { indent: indent as unknown as string })).toThrowError(
+      new TypeError("`indent` must be a non-empty string containing only spaces and tabs"),
+    );
+  });
+});
+
 describe("starting indent level", () => {
   const ast = e(id("x"));
 

packages/codegen/test/source-maps.test.ts

@@ -329,31 +329,9 @@ for (const fixture of FIXTURES) {
 // --- Indentation ----------------------------------------------------------------------------
 
 // `printIndent` goes through the same write path as everything else, so the `indent` option and
-// the mappings have to agree: indentation is honoured (or falls back to a tab), and no mapping
-// ever points into the middle of an indent run.
-//
-// The invalid values are typed `unknown` because they are what an untyped JS caller can pass -
-// that they fall back to a tab rather than corrupting the output is the point of testing them.
-const INDENT_CASES: unknown[] = [
-  // Valid - used as given
-  undefined,
-  "\t",
-  "  ",
-  "    ",
-  "\t\t",
-  " \t",
-  "\t ",
-  // Invalid - must fall back to a tab
-  "",
-  "x",
-  "\n",
-  "\r\n",
-  " x ",
-  "   ",
-  4,
-  null,
-  {},
-];
+// the mappings have to agree: indentation is honoured, and no mapping ever points into the middle
+// of an indent run.
+const INDENT_CASES = [undefined, "\t", "  ", "    ", "\t\t", " \t", "\t "];
 
 describe("indent option", () => {
   test.each(
@@ -362,11 +340,11 @@ describe("indent option", () => {
     const program = parseWithLocs("indent.js", INLINE_JS);
     const collector = new Collector("indent.js");
     const out = printSync(program, {
-      indent: indent as string | undefined,
+      indent,
       sourceMap: collector,
     }).code;
 
-    const expectedIndent = typeof indent === "string" && /^[ \t]+$/.test(indent) ? indent : "\t";
+    const expectedIndent = indent ?? "\t";
     const lines = out.split("\n");
     const indented = lines.filter((line) => line.startsWith("\t") || line.startsWith(" "));
     expect(indented.length).toBeGreaterThan(0);