feat: support delta-only (changed lines) matching in builtin pattern hooks (deny-pattern, require-pattern)

[udaya2899]

Summary

Add an option (e.g. --delta-only / --check-delta) to builtin pattern hooks (deny-pattern and require-pattern) so that patterns are evaluated only against newly added or modified lines in the Git diff, rather than scanning the full file content from disk.

Motivation

In large codebases and monorepos, teams frequently introduce rules to ban deprecated APIs, imports, or patterns in new code (ratcheted migrations).

Currently, deny-pattern scans the entire file from disk whenever a file is staged or selected:

1. If a developer makes a small edit in a legacy file containing pre-existing violations on untouched lines, deny-pattern fails the commit.
2. This forces developers to either fix out-of-scope legacy code, add broad exclusions, skip hooks, or maintain custom repo: local diff-parsing scripts.

Having native --delta-only support in repo: builtin would allow zero-setup, blazingly fast ratcheting checks directly within prek.

Proposed Configuration

repos:
  - repo: builtin
    hooks:
      - id: deny-pattern
        name: Ban deprecated helper in new/modified lines
        args: [--delta-only, 'deprecated_call\(']
        types: [python]

Proposed Behavior

• When --delta-only is enabled:
  • Local commit / Staged run (prek run): Inspects staged diff hunks (e.g. git diff --cached -U0) and evaluates patterns only on added lines (+ lines in diff hunks).
  • Ref-range run (prek run --from-ref <REF> [--to-ref <REF>]): Inspects diff hunks across the specified revision range (e.g. git diff -U0 <FROM_REF>...<TO_REF>).
  • Output: Reports <file>:<line>: <matched_text> for matches found within added lines and exits with code 1. Pre-existing matches on untouched lines are ignored.
• When --delta-only is omitted: Default behavior is preserved (scans full file from disk).
• Flag interaction:
  • Incompatible with whole-file multiline matching (-m / --multiline), or should operate on contiguous added hunk blocks.

References

• crates/prek/src/hooks/builtin_hooks/pattern.rs
• add a native rustgrep builtin for one-off hooks #2355 (Initial rustgrep / deny-pattern builtin request)

[j178]

I haven’t thought this through carefully yet, but my initial impression is that it would be difficult to implement. I’ll take a closer look later.

[udaya2899]

Created #2677 as a proof-of-concept. Half of the PR's diff is tests, so please don't be overtly alarmed by the size. Feel free to also reject the PR if you don't like the approach.