fix/ast-function-patching: two blockers before merge (verbatim splice, line-ending rewrite)

[Johin2]

Review of fix/ast-function-patching (commit 0c9b7c0, "resolve patch targets via AST instead of first-match").

The rewrite is the right direction and is a clear net improvement: libcst was never installed or declared in pyproject.toml, so the old _try_libcst path was dead code and only _textual_replace ever ran. I verified three constructs where main silently produced invalid Python and the new code correctly refuses instead (version guards, try/except ImportError fallbacks, @overload groups).

Two issues should be fixed before merge. Both have strict-xfail tests already on the branch pinning the expected behaviour, so each fix is turnkey: make the test pass and drop the marker.

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1. The whole repaired source is spliced in, not just the target function

src/self_heal/_patch.py — _ast_replace locates repaired_fn to inspect its decorators, but the text it writes is block = dedented, i.e. the entire repaired source including any other top-level statements.

propose.py's system prompt tells the model to return exactly one function definition, but nothing enforces it in code, so a non-compliant response corrupts the target scope.

apply_function_patch(
    src, "Foo.bar", "",
    "def helper():\n    return 42\n\n\ndef bar(self):\n    return helper()\n",
)

on class Foo:\n def bar(self):\n return 1 produces:

class Foo:
    def helper():      # <-- now Foo.helper, not a module-level function
        return 42

    def bar(self):
        return helper()   # NameError at runtime

This is syntactically valid, so the final ast.parse(new_text) guard passes and the CLI prints ✓ APPLIED for a patch that breaks the module. An LLM prepending import re hits the same path and writes a class-body import.

Fix: splice only repaired_fn's own line span out of dedented, or raise PatchError when repaired_tree.body contains anything besides the target def.

Pinned by: tests/test_patch.py::test_extra_statements_in_repair_do_not_leak_into_the_target_scope (accepts either fix — refusing outright is fine).

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2. Line endings are rewritten across the whole file

src/self_heal/_patch.py:53,69 — Path.read_text / Path.write_text both use newline=None, so newlines are normalised in both directions.

in:  b'def a():\n    return 1\n\n\ndef b():\n    return 2\n'
out: b'def a():\r\n    return 1\r\n\r\n\r\ndef b():\r\n    return 3\r\n'

Every line changes, so git diff shows the entire file instead of the one function. That defeats the point of the git-dirty guard and the reviewability the AST approach was meant to buy. On POSIX the same bug runs the other way: a CRLF file is rewritten to LF.

This is pre-existing (the calls are unchanged by the commit), but the branch newly makes byte-fidelity the stated goal, so it belongs here.

Fix: read and write with newline='', preserving the file's detected dominant ending. _ast_replace also hardcodes + "\n" when joining the replacement block.

Pinned by: tests/test_patch.py::test_line_endings_are_preserved (checks LF and CRLF in one test on purpose — the LF case fails on Windows and the CRLF case on POSIX, so a single test stays a deterministic xfail on both).

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Already fixed on the branch

For the record, these came out of the same review and are done:

• @overload stubs no longer make the implementation ambiguous — the sole non-stub definition resolves.
• Property getter/setter collisions now report the colliding line numbers and decorators instead of advising "pass a qualified name", which could not work for a getter/setter pair.
• Comments between decorators (or between a decorator and def) are no longer silently deleted when the repair echoes the same decorators back.
• Stale libcst references removed from README.md, pytest_plugin.py, site/app/page.tsx, and a test name.
• Two false module-docstring claims corrected ("byte-identical"; "falls back only when the file cannot be parsed").
• self-heal heal FILE::ClassName.method_name now works, so _patch.py's qualified-name disambiguation is actually reachable. The pytest marker still splits target on its last dot and so remains module-level only; that limitation is now documented in the plugin docstring.
• Tests added for the final ast.parse guard, comment preservation, overload resolution, the property-collision message, and CLI target resolution; the two collision tests now execute the patched module instead of substring-matching.

Known limitations (working as intended, not filed as bugs)

• Tab-indented files cannot be patched: indentation is rebuilt as " " * col_offset. Fails safe — the re-parse guard rejects it and the file is left untouched.
• PEP 614 parenthesized decorators (@(\n d\n)) orphan the @( line. Also fails safe via the same guard.
• Version guards, try/except ImportError fallbacks, and property accessor pairs are unpatchable by design; the ambiguity error names what collided.