[billythekidz]

os.rename(old, new) deleted new before checking that old exists. With a missing source it returned nil, error but the destination file was already gone.

local f = io.open("keep.txt", "w"); f:write("data"); f:close()
print(os.rename("missing.txt", "keep.txt"))  -- nil + error, and keep.txt was deleted

FileSystem.Rename now checks the source first, then deletes the destination, then moves. os.rename with a missing source returns nil, "...: No such file or directory" and leaves the destination alone, as in C Lua.

This is also why tests-lua/files.lua needed the PatchFilesLuaSource patch ("UTF-16 text-mode" block): the failing assertion at files.lua:129 was this bug, not an encoding issue. The patch is removed and files.lua runs unmodified.

Verification

• New tests FileSystem_Rename_MissingSource_KeepsDestination and OsRename_MissingSource_ReturnsNilAndKeepsDestination fail on main and pass with the change.
• dotnet test tests/Lua.Tests/Lua.Tests.csproj -c Release --filter "TestCategory!=ExpectedFailure": 612 passed, 0 failed (610 on main + 2 new).
• dotnet csharpier check . is clean.
• Run on macOS (arm64), .NET 8 runtime only.

[Copilot]

🟢 Approval recommended

The targeted fix addresses the reported data loss, with regression coverage for both entry points and no unresolved findings.

No open issues 🎉

What changed in this PR

Fixes os.rename deleting an existing destination when the source is missing, preserving destination data on this failure path.

Changes:

• Checks source existence before deleting the destination and removes the redundant post-move deletion.
• Adds regression tests for destination preservation through both filesystem and Lua APIs.
• Restores the previously skipped block of files.lua assertions.

File	Description
tests/​Lua.Tests/​LuaTests.cs	Removes the harness override that skipped the affected assertions.
tests/​Lua.Tests/​IOTests.cs	Adds missing-source rename regression tests for both entry points.
src/​Lua/​IO/​FileSystem.cs	Validates source existence before destination deletion and removes redundant cleanup.

_{Review effort: Balanced}

[akeit0]

Thank you for finding and fixing this data-loss bug, adding regression tests, and restoring the files.lua assertions! Your original commit has been merged through #368 with its authorship preserved.

During review we added follow-up hardening for other rename failures, including a locked source on Windows and directory-to-file attempts. The implementation now replaces the destination through filesystem operations instead of deleting it before the move, with a compatible path for .NET Standard 2.1 and coverage for directory renames. The combined changes passed local tests and Linux CI.