refactor: combine check-tsconfig with parse-tsconfig

- `checkTsConfig` is literally only used by `parseTsConfig`
  - I first just moved the function over, but it's a two-liner, so thought it made more sense to just in-line it

- merge the unit tests as well
  - we already check the non-error case in `parse-tsconfig.spec`, so only add the error case

- `check-tsconfig` was longer in the past and more files were being created then for separation, so this may have made more sense then, but now it is unnecessary
  - c.f. 7332077

CONTRIBUTING.md

@@ -66,7 +66,7 @@ It can also be useful to review some issues and have a "goal" in mind (especiall
 Once you have some understanding of the codebase's main workflow, you can start to dive deeper into pieces that require more domain knowledge.<br />
 A useful resource as you dive deeper are the [unit tests](__tests__/). They're good to look through as you dig into a module to understand how it's used.
 
-1. From here, you can start to read more of the modules that integrate with the TypeScript API, such as [`host`](src/host.ts), [`parse-tsconfig`](src/parse-tsconfig.ts), [`check-tsconfig`](src/check-tsconfig.ts), and maybe how TS is imported in [`tsproxy`](src/tsproxy.ts) and [`tslib`](src/tslib.ts)
+1. From here, you can start to read more of the modules that integrate with the TypeScript API, such as [`host`](src/host.ts) and [`parse-tsconfig`](src/parse-tsconfig.ts), and maybe how TS is imported in [`tsproxy`](src/tsproxy.ts) and [`tslib`](src/tslib.ts)
     - A _very_ useful reference here is the [TypeScript Wiki](https://github.com/microsoft/TypeScript/wiki), which has two main articles that are the basis for most Compiler integrations:
       - [Using the Compiler API](https://github.com/microsoft/TypeScript/wiki/Using-the-Compiler-API)
       - [Using the Language Service API](https://github.com/microsoft/TypeScript/wiki/Using-the-Language-Service-API)

__tests__/parse-tsconfig.spec.ts

@@ -14,6 +14,13 @@ test("parseTsConfig", () => {
 	expect(() => parseTsConfig(makeContext(), defaultOpts)).not.toThrow();
 });
 
+test("parseTsConfig - incompatible module", () => {
+	expect(() => parseTsConfig(makeContext(), {
+		...defaultOpts,
+		tsconfigOverride: { compilerOptions: { module: "none" } },
+	})).toThrow("Incompatible tsconfig option. Module resolves to 'None'. This is incompatible with Rollup, please use");
+});
+
 test("parseTsConfig - tsconfig errors", () => {
 	const context = makeContext();
 

src/parse-tsconfig.ts

@@ -7,7 +7,6 @@ import { printDiagnostics } from "./print-diagnostics";
 import { convertDiagnostic } from "./tscache";
 import { getOptionsOverrides } from "./get-options-overrides";
 import { IOptions } from "./ioptions";
-import { checkTsConfig } from "./check-tsconfig";
 
 export function parseTsConfig(context: IContext, pluginOptions: IOptions)
 {
@@ -45,7 +44,10 @@ export function parseTsConfig(context: IContext, pluginOptions: IOptions)
 	const compilerOptionsOverride = getOptionsOverrides(pluginOptions, preParsedTsConfig);
 	const parsedTsConfig = tsModule.parseJsonConfigFileContent(mergedConfig, tsModule.sys, baseDir, compilerOptionsOverride, configFileName);
 
-	checkTsConfig(parsedTsConfig);
+	const module = parsedTsConfig.options.module!;
+	if (module !== tsModule.ModuleKind.ES2015 && module !== tsModule.ModuleKind.ES2020 && module !== tsModule.ModuleKind.ESNext)
+		throw new Error(`rpt2: Incompatible tsconfig option. Module resolves to '${tsModule.ModuleKind[module]}'. This is incompatible with Rollup, please use 'module: "ES2015"', 'module: "ES2020"', or 'module: "ESNext"'.`);
+
 	printDiagnostics(context, convertDiagnostic("config", parsedTsConfig.errors), pretty);
 
 	context.debug(`built-in options overrides: ${JSON.stringify(compilerOptionsOverride, undefined, 4)}`);