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ready · opened 2026-11-03 · updated 2026-11-03 · 0 comments
Found while writing the grammars chapter of the guide (d2, guide/grammars.md).
1. An uncaught grammar entry reaches rustc instead of NK2605
grammar P { entry rule n -> i64 = x:dec[i64](digit+) -> { x } }
fn main() { let v = P::n("1"); println(f"{v}") }Expected: NK2605. Actual: rustc says “The ? operator can only be used in a function that returns Result”.
2. A cut inside recover(p, sync) ends the whole parse
entry rule block -> Vec[i64?] = "{" => xs:recover(stmt, ";")* "}" -> { xs }
rule stmt -> i64 = "let" => n:dec[i64](digit+) ";" -> { n }With the input "{ let 1; let x; let 3; }", the parse fails at x. Without the inner => it gives 1, null, 3. The recovered failure’s message also never reaches the program, although Part II 10.6 and 10.8 say “p’s value or the report”.
3. A repeated dec[T](digit+) in a lowercase rule
entry rule i -> Vec[i64] = xs:dec[i64](digit+)* eof -> { xs }On "1 2" this errors “expected a digit” instead of ending the repetition. The same inside an uppercase rule gives 2 items. Also, an overflowing dec reports “expected a digit” at the next character, not that the number does not fit.
4. Left recursion works but is unspecified
expr = a:expr "-" b:NUM | n:NUM gives 10-3-2 = 5 today. Part II does not specify it, and the comment in examples/calc says it would never terminate.
NK2605 for an entry called outside catch / throws, with a test in tests/compiler/.recover commits only within p; the report reaches the program as 10.6/10.8 say; tests.dec in a lowercase rule, and an overflow names the overflow; tests.