* Fix O(2^n) query parser regression for deeply-nested queries
The top-level `ast()` parser used `alt((boolean_expr, single_leaf))` at
every group level. When the group contained a single leaf with no
trailing operand, `boolean_expr` would parse `occur_leaf` (recursing
into the inner group), fail at `multispace1`, backtrack, and then
`single_leaf` would re-parse `occur_leaf` from scratch. Every nesting
level doubled the work, giving O(2^n) time for queries like
`(((((title:test)))))`.
Parse `occur_leaf` once and peek ahead for a trailing operand instead
of backtracking. This keeps parsing O(n) and also avoids the duplicate
parse for simple single-leaf queries.
Fixes#2498.
Measured on the issue reproducer (release build):
depth before after
20 0.87 s <1 us
25 28.23 s <1 us
60 (years) ~5 us
Non-pathological queries are unaffected or slightly faster:
query before after
hello 650 ns 308 ns
a AND b AND c 1380 ns 1364 ns
title:rust AND (...) 3426 ns 3460 ns
All 53 existing grammar tests and 56 query_parser tests pass. Adds a
regression test at depth 60 that would not complete under the old
parser.
* Add ignored benchmark for nested query parsing at depth 20/21
Matches the depths from issue #2498 which reported 0.87 s / 1.72 s
under the regression. With the fix these parse in single-digit
microseconds. Runs via:
cargo test -p tantivy-query-grammar --release bench_deeply_nested \
-- --ignored --nocapture
* Propagate Err::Failure and Err::Incomplete from operand parser
`alt((boolean_expr, single_leaf))` only retried on `Err::Error` and
propagated `Err::Failure` and `Err::Incomplete`. The replacement was
catching all three with `Err(_)`, which would silently fall back to
a single leaf if any cut point were ever added to `operand_leaf` or
its descendants. Match specifically on `Err::Error` to preserve the
original `alt` semantics.
* Replace inline bench with binggan bench in benches/
Move the nested-query benchmark out of the query-grammar test module
and into a proper binggan benchmark at benches/query_parser_nested.rs,
registered as a harnessless bench in Cargo.toml. Keeps the correctness
regression test (depth 60) in place.
Run with: cargo bench --bench query_parser_nested
* Fix rustfmt import ordering in query_parser_nested bench
