fix(grep): align preview highlight stripping with Rust query parser (#343)

* fix(grep): align preview highlight stripping with Rust query parser

The Lua heuristic in highlight_grep_matches used a simple prefix check
(^[*!/] or ^.) to strip constraints. This diverged from the Rust
GrepConfig parser in several ways:

- Multi-word queries like 'foo bar *.rs' only highlighted 'foo'
- Constraint prefixes like type:rust were not stripped
- Tokens starting with '.' were incorrectly treated as constraints
- Escaped constraint tokens (e.g. \*.config) were not handled

Replace the heuristic with _is_grep_constraint() that matches the
Rust parser's actual GrepConfig rules: extensions (*.rs), path segments
(/src/), exclusions (!test), type filters (type:rust), and path-oriented
globs. Use all text parts joined with space for highlighting, matching
grep_text() on the Rust side.

Fixes #331

* refactor(grep): expose parsed query to lua via Rust function

Replace the Lua-side constraint detection (_is_grep_constraint) with a
new parse_grep_query() function that delegates to the Rust GrepConfig
parser. This keeps the Rust parser as the single source of truth for
query parsing, avoiding drift when new token types are added.

The new function is exposed to Lua as fff.parse_grep_query(query) and
returns a table with the grep_text field (the search text with all
constraints stripped).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mvanhorn

crates/fff-nvim/src/lib.rs

@@ -5,8 +5,9 @@ use fff::frecency::FrecencyTracker;
 use fff::path_utils::expand_tilde;
 use fff::query_tracker::QueryTracker;
 use fff::{
-    DbHealthChecker, Error, FFFMode, FileSearchConfig, FuzzySearchOptions, PaginationArgs,
-    QueryParser, Score, SearchResult, SharedFrecency, SharedPicker, SharedQueryTracker,
+    DbHealthChecker, Error, FFFMode, FileSearchConfig, FuzzySearchOptions, GrepConfig,
+    PaginationArgs, QueryParser, Score, SearchResult, SharedFrecency, SharedPicker,
+    SharedQueryTracker,
 };
 use mimalloc::MiMalloc;
 use mlua::prelude::*;
@@ -582,6 +583,18 @@ pub fn get_historical_grep_query(_: &Lua, offset: usize) -> LuaResult<Option<Str
         .into_lua_result()
 }
 
+/// Parse a grep query string and return its text portion (with constraints stripped).
+///
+/// Uses the Rust `GrepConfig` parser as the single source of truth, so Lua
+/// code never needs to re-implement constraint detection.
+pub fn parse_grep_query(lua: &Lua, query: String) -> LuaResult<LuaTable> {
+    let parser = QueryParser::new(GrepConfig);
+    let parsed = parser.parse(&query);
+    let table = lua.create_table()?;
+    table.set("grep_text", parsed.grep_text())?;
+    Ok(table)
+}
+
 pub fn wait_for_initial_scan(_: &Lua, timeout_ms: Option<u64>) -> LuaResult<bool> {
     // Extract the scan signal Arc WITHOUT holding the read lock, so the
     // scan thread can acquire the write lock to store its results.
@@ -822,6 +835,7 @@ fn create_exports(lua: &Lua) -> LuaResult<LuaTable> {
     exports.set("health_check", lua.create_function(health_check)?)?;
     exports.set("shorten_path", lua.create_function(shorten_path)?)?;
     exports.set("hex_dump", lua.create_function(hex_dump::hex_dump)?)?;
+    exports.set("parse_grep_query", lua.create_function(parse_grep_query)?)?;
 
     Ok(exports)
 }

lua/fff/fuzzy.lua

@@ -46,6 +46,7 @@ M.get_git_root = rust_module.get_git_root
 
 -- Grep functions
 M.live_grep = rust_module.live_grep
+M.parse_grep_query = rust_module.parse_grep_query
 
 -- Utility functions
 M.health_check = rust_module.health_check

lua/fff/location_utils.lua

@@ -178,16 +178,13 @@ function M.highlight_grep_matches(bufnr, location, namespace)
 
   local query = location.grep_query
 
-  -- Extract the actual search text from the grep query (strip file constraints like *.rs /src/)
-  -- The query parser uses space-separated tokens; the first non-constraint token is the pattern.
-  -- Simple heuristic: strip tokens that look like constraints (start with *, /, or !)
-  local search_text = query
-  local parts = vim.split(query, '%s+')
-  local text_parts = {}
-  for _, part in ipairs(parts) do
-    if part ~= '' and not part:match('^[%*!/]') and not part:match('^%.') then table.insert(text_parts, part) end
-  end
-  if #text_parts > 0 then search_text = text_parts[1] end
+  -- Use the Rust GrepConfig parser as the single source of truth for
+  -- stripping constraint tokens. This avoids duplicating constraint
+  -- detection in Lua, which would break whenever a new token type is added.
+  local fuzzy = require('fff.fuzzy')
+  local parsed = fuzzy.parse_grep_query(query)
+  local search_text = parsed.grep_text
+  if search_text == '' then search_text = query end
 
   if not search_text or search_text == '' then return nil end