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fix: handle Unicode characters correctly in mutator byte offset calculation #2832

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Merged
smonicas merged 1 commit into from
Jan 9, 2026
Merged

fix: handle Unicode characters correctly in mutator byte offset calculation #2832

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3 changes: 2 additions & 1 deletion slither/formatters/utils/patches.py
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Original file line number Diff line number Diff line change
Expand Up @@ -31,7 +31,8 @@ def apply_patch(original_txt: bytes, patch: Dict, offset: int) -> Tuple[bytes, i
patched_txt += original_txt[int(patch["end"] + offset) :]

# Keep the diff of text added or sub, in case of multiple patches
patch_length_diff = len(patch["new_string"]) - (patch["end"] - patch["start"])
# Note: must use byte length since patch offsets are byte offsets from solc
patch_length_diff = len(patch["new_string"].encode("utf8")) - (patch["end"] - patch["start"])
Comment on lines +34 to +35
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@bohendo bohendo Jan 5, 2026

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Subtle, the bug here would only manifest if the new_string contains unicode which I don't think it does usually. But yea, this change is correct

return patched_txt, patch_length_diff + offset


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9 changes: 6 additions & 3 deletions slither/tools/mutator/utils/testing_generated_mutant.py
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Expand Up @@ -92,12 +92,15 @@ def test_patch(
function to verify whether each patch is caught by tests
returns: 0 (uncaught), 1 (caught), or 2 (compilation failure)
"""
with open(file, "r", encoding="utf8") as filepath:
with open(file, "rb") as filepath:
content = filepath.read()
# Perform the replacement based on the index values
replaced_content = content[: patch["start"]] + patch["new_string"] + content[patch["end"] :]
# Note: patch offsets are byte offsets from solc, so we must work with bytes
replaced_content = (
content[: patch["start"]] + patch["new_string"].encode("utf8") + content[patch["end"] :]
)
# Write the modified content back to the file
with open(file, "w", encoding="utf8") as filepath:
with open(file, "wb") as filepath:
Comment on lines +95 to +103
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@bohendo bohendo Jan 5, 2026

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Man. I fixed a related issue in a prev PR and I thought that had us good to go wrt unicode support. But looking at this now, idk how the prev fix actually fixed anything. This is the same byte-wise indexing-against-strings bug that I was wrestling w earlier, just in a different but still critical spot. Good job finding & fixing this.

filepath.write(replaced_content)

if compile_generated_mutant(file, mappings):
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