⚡ Bolt: Early exit in is_likely_code_text heuristic

.jules/bolt.md

@@ -17,3 +17,7 @@
 ## 2025-05-19 - File Discovery Allocations
 **Learning:** In `discover_importing_files`, `WalkBuilder` results were being converted to `PathBuf` via `.map(|e| e.into_path())` *before* filtering. This caused allocations for every single file in the workspace (including excluded files and directories).
 **Action:** Filter `ignore::DirEntry` directly using `entry.file_type()` and `entry.path()` before mapping to `PathBuf`. This avoids allocations for non-matching files.
+
+## 2024-06-10 - Early Exit in Code Text Heuristics
+**Learning:** In Rust, heuristic functions that count characters or bytes (e.g., `is_likely_code_text` in `crates/mill-services/src/services/reference_updater/detectors/generic.rs`) should be optimized with an early exit as soon as a failure/success threshold is met. This significantly improves performance on 'mismatch' cases by avoiding full scans of the sampled data.
+**Action:** When implementing threshold-based heuristic scans, add an early return within the loop to short-circuit as soon as the threshold guarantees the outcome. Ensure default return values (like `sample_len > 0`) correctly handle empty inputs to preserve original functionality.

crates/mill-services/src/services/reference_updater/detectors/generic.rs

@@ -519,12 +519,17 @@ fn is_likely_code_text(content: &str) -> bool {
     let sample = &content.as_bytes()[..sample_len];
     let mut non_printable = 0usize;
     for &b in sample {
-        let is_text = b == b'\n' || b == b'\r' || b == b'\t' || (b >= 0x20 && b <= 0x7e);
+        let is_text = b == b'\n' || b == b'\r' || b == b'\t' || (0x20..=0x7e).contains(&b);
         if !is_text {
             non_printable += 1;
+            // Early exit optimization: if non-printable characters exceed the threshold,
+            // we immediately know this is not likely text, avoiding a full 8KB scan.
+            if non_printable * 20 >= sample_len {
+                return false;
+            }
         }
     }
-    non_printable * 20 < sample_len
+    sample_len > 0
 }
 
 fn is_obviously_irrelevant_extension(ext: &str) -> bool {