browser run: compile errors report no line, column, or source #353

Summary

A program that fails to compile inside browser run is reported without any line, column, or source context, and is prefixed with a message that points at async plumbing rather than at the program. A one-character quoting mistake becomes an unactionable failure.

Repro

webcmd --session <id> browser run --stdin --no-snapshot-diff <<'JS'
const a = 1;
const s = 'x'y';
return a;
JS

Actual:

✖  BROWSER_RUN_SYNTAX_ERROR: QuickJS promise rejected: expecting ';'
✖  Fix the browser-run JavaScript syntax and retry.

No position. On a real program this is unactionable — the case that surfaced this had a 2,361-character line and reported only invalid number literal.

Cause

src/browser/run/runner.ts compiles the program with new AsyncFunction(source) inside the Promise.race:

value = await Promise.race([new AsyncFunction(source)(), cancellation]);

Two consequences:

1. A compile failure surfaces as a rejected promise, so quickjs-host.ts prefixes it with QuickJS promise rejected:. Readers chase heredocs, await, and shell quoting instead of the program.
2. The position is discarded. QuickJS does supply it — probing the sandbox directly returns lineNumber, columnNumber, and a stack of at <input>:4:14 — but normalizeExecutionError uses only .message.

The hint (Fix the browser-run JavaScript syntax and retry) restates the error code and names no cause.

Impact

Measured in the internal task eval: agents given this error blamed shell interpolation, encoded data URLs, arrow syntax, and top-level await in turn. Two scenarios abandoned the supplied page entirely and rebuilt a substitute rather than fix a single quote. The underlying mistake in every case was the same — an unescaped ' inside a single-quoted string holding a URL or HTML.

Expected

✖  BROWSER_RUN_SYNTAX_ERROR: invalid number literal at line 1, column 1215
  tput%3E%3Cscript%3Econst%20form%3Ddocument.querySelector('%23registration')%2Cdr
                                                             ^
✖  Your program failed to compile; the browser was never touched. A common cause
   is an unescaped quote when embedding a URL or HTML in a string literal — use
   double quotes or a template literal for values that contain apostrophes.

Fix

Compile before racing, and turn a compile failure into a BROWSER_RUN_SYNTAX_ERROR carrying:

• the caller's own line and column — new AsyncFunction(body) prepends two lines of wrapper, so subtract them
• a windowed excerpt with a caret, so a 2,000-character line stays readable
• a hint naming the quoting cause

Also skip the host stage prefix in quickjs-host.ts#toError for errors that already carry a BROWSER_RUN_ code.

Note

src/browser/run/runner.test.ts imports MemorySnapshotBaselineStore twice, so the file fails to load and its 74 tests have never run. They pass once the duplicate is removed.