Pre-existing bug surfaced by CodeRabbit on PR #567 (v0.0.129). Not in scope for #475 PR 2 (which addressed seven specific Major findings); filed separately for follow-up.

Reproduction

public fn main(@Unit -> @Unit)
  requires(true) ensures(true) effects(<IO>)
{
  match url_parse(":foo") {
    Ok(@UrlParts) -> IO.print(url_join(@UrlParts.0)),
    Err(@String) -> IO.print("ERR")
  }
}

Expected: either round-trips as :foo or url_parse rejects the input as not RFC 3986-compliant (a URL scheme must start with ALPHA per § 3.1).

Actual: url_parse returns Ok with scheme="" (empty), path="foo"; url_join then drops the colon (because its scheme delimiter emit is gated on s_len > 0 in both explicit-mode and legacy-mode branches) and produces foo.

Root cause

In vera/wasm/calls_encoding.py _translate_url_parse:

• The colon scan loop sets colon_pos = 0 for input :foo (colon found at position 0)
• s_len = colon_pos = 0 (empty scheme)
• The flag word at offset 44 captures has_auth / has_query / has_frag but not whether a colon was present

In _translate_url_join, the scheme delimiter emit branch checks s_len > 0. With s_len = 0 no : is written.

Fix options

1. RFC-compliant rejection: url_parse rejects colon_pos == 0 (returns Err). Behavior change — programs that previously got Ok for :foo would get Err. Simpler.
2. Add has_colon flag: extend the offset-44 packed-flag word with a fourth bit (the explicit-mode sentinel currently lives at bit 3; has_colon would go at bit 4) so url_join can emit : whenever url_parse saw one, independent of s_len. Preserves current Ok/Err behavior of url_parse; only fixes round-trip.

Option 2 is more conservative; option 1 is more correct vs RFC 3986.

Verified pre-existing

Reproduced on main at 448924c (vera/wasm/calls_encoding.py decomposition commit) — same foo output.

References

• CodeRabbit comment on PR v0.0.129 — WASM call translator major correctness fixes (#475 PR 2 of 2) #567 (calls_encoding.py 1970-1978)
• RFC 3986 § 3.1 (Scheme): scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )