Produce valid file URLs for import.meta.url on Windows in Turbopack

[sokra]

What?

import.meta.url evaluated in a Turbopack-compiled module on Windows produced
an invalid file URI: the path portion contained backslashes (\) and was not
URL-encoded, e.g.

file://C:\Users\me\project\apps\web\app\page.tsx

This affects any code that hands the value to URL, fileURLToPath, source
maps, dev tools, or anything else that expects a valid file URI.

This PR makes import.meta.url return a correct, percent-encoded file URI on
all platforms, e.g. file:///C:/Users/me/project/apps/web/app/page.tsx.

Why?

The existing codegen in references/esm/meta.rs emitted

`file://${__turbopack_context__.P("<rel/path>")}`

where __turbopack_context__.P is the runtime helper resolveAbsolutePath.
On the Node-side runtime (shared-node/node-externals-utils.ts) that helper
returns path.join(ABSOLUTE_ROOT, modulePath), which is OS-native — so on
Windows it returns a backslash-separated path with no percent-encoding. The
result was then concatenated into a file://... template literal, producing
an invalid URI.

The initial hint that turbopack-core/src/source_map/utils.rs was the culprit
turned out to be a false lead: that code path uses
FileSystemPath::try_join, which already normalizes \ to / on Windows.
The bug is in the codegen + runtime helper for import.meta.url, not in
source-map handling.

How?

• Introduce a new runtime function __turbopack_resolve_file_url__
  (shortcut F) whose contract is to return a complete file:// URI for a
  given relative module path.
  • Node implementation (shared-node/node-externals-utils.ts): uses
    url.pathToFileURL(resolveAbsolutePath(modulePath)).href. pathToFileURL
    handles drive letters (file:///C:/...), normalizes slashes, and
    percent-encodes path segments.
  • Browser implementation (browser/runtime/base/runtime-base.ts): returns
    `file:///ROOT/${modulePath ?? ''}` — the browser runtime
    intentionally does not expose the real filesystem path, so it returns the
    same stable placeholder as before, just with a valid file:/// prefix
    instead of having callers concatenate one.
• Change the import.meta.url codegen in references/esm/meta.rs to call
  __turbopack_resolve_file_url__($formatted) directly instead of
  concatenating a file:// prefix to the absolute path. The pre-existing
  encode_path is kept only to keep the embedded JS string literal safe
  (the runtime helper is responsible for the URI-level encoding).
• Update TurbopackBaseContext and add a ResolveFileUrl type alias in
  runtime-types.d.ts so the new shortcut is typed.
• Register the new shortcut in TURBOPACK_RUNTIME_FUNCTION_SHORTCUTS so user
  code can also reference __turbopack_resolve_file_url__ (mirroring the
  existing __turbopack_resolve_absolute_path__). P is left in place and
  unchanged because it's still used elsewhere.
• Regenerate the affected Turbopack snapshot fixtures (import-meta/*,
  comptime/typeof, runtime/default_*_runtime, workers/*, debug-ids/*,
  etc.). The functional change in the generated code is uniform:
  `file://${__turbopack_context__.P(<rel>)}` →
  __turbopack_context__.F(<rel>).
• Drop the // TODO: These file URIs are wrong on turbopack+windows comment
  and the .replaceAll('\\', '/') workarounds from
  test/e2e/app-dir/non-root-project-monorepo/non-root-project-monorepo.test.ts,
  so the assertion now verifies that the URI is correctly slashed and encoded
  on all platforms.

Verification

• cargo clippy --workspace --all-targets — clean
• cargo fmt --check — clean
• cargo test -p turbopack-tests --test snapshot — 87/87 pass
• cargo test -p turbopack-tests --test execution — 219/219 pass (1 ignored)
• pnpm test-dev-turbo test/e2e/app-dir/non-root-project-monorepo/non-root-project-monorepo.test.ts — 9/9 pass
• pnpm test-start-turbo test/e2e/app-dir/non-root-project-monorepo/non-root-project-monorepo.test.ts — 6/6 pass
• pnpm test-dev-webpack test/e2e/app-dir/non-root-project-monorepo/non-root-project-monorepo.test.ts — 9/9 pass (verifying webpack path still works)

Windows itself can't be exercised in the sandbox, but the new code path no
longer depends on path.sep or any OS-native string manipulation for the
URI: url.pathToFileURL is the canonical Node API for producing a file URI
from any OS-native path, and the test assertion has been tightened so that a
regression on Windows would fail in CI rather than be papered over.

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