Improve error message on parsing await expression

[zen0wu]

With the following file, esbuild will print this seemingly confusing error message.

function f() { // This should really be an async function
  await f();
}

❯ npx esbuild test.ts
 > test.ts:2:8: error: Expected ";" but found "f"
    2 │   await f();
      ╵         ^

1 error

This is pretty hard to understand especially when this function definition is non-trivial. Not sure how much this factor into speed consideration since improving the error message here might involve changing how the parser works? But it would be nice to get it fixed.

Thanks!

[evanw]

Sure. This seems simple enough. Here's the implementation from V8:

  void ExpectSemicolon() {
    // Check for automatic semicolon insertion according to
    // the rules given in ECMA-262, section 7.9, page 21.
    Token::Value tok = peek();
    if (V8_LIKELY(tok == Token::SEMICOLON)) {
      Next();
      return;
    }
    if (V8_LIKELY(scanner()->HasLineTerminatorBeforeNext() ||
                  Token::IsAutoSemicolon(tok))) {
      return;
    }

    if (scanner()->current_token() == Token::AWAIT && !is_async_function()) {
      ReportMessageAt(scanner()->location(),
                      flags().allow_harmony_top_level_await()
                          ? MessageTemplate::kAwaitNotInAsyncContext
                          : MessageTemplate::kAwaitNotInAsyncFunction);
      return;
    }

    ReportUnexpectedToken(Next());
  }

So basically: if you hit a syntax error and the previous token was an await, complain about the await instead.

[evanw]

The error message now looks like this:

❯ npx esbuild test.ts
 > example.js:2:2: error: "await" can only be used inside an "async" function
    2 │   await f();
      ╵   ~~~~~
   example.js:1:0: note: Consider adding the "async" keyword here
    1 │ function f() { // This should really be an async function
      │ ^
      ╵ async

1 error

[zen0wu]

Thanks for such speedy turnaround 🙌🏽 ! I have so much more faith in esbuild :D