Re-escape strings concatenated across quote styles

[xiehuanyi]

Summary

Appending a literal String to a basic String reuses raw TOML text with incompatible escaping: a literal backslash path can turn into a tab when the result is parsed. The reverse direction preserves escape spellings as literal text instead of their decoded value. Re-escape cross-style operands for the left string type, retaining the raw spelling for matching types and escaping bare CRs for multiline basic output.

Validation: 9 regression failures and 9 passing controls before the change; all 1202 tests pass afterward, including the TOML conformance corpus. All pre-commit hooks pass. Mypy 1.19.1 reports the same six existing diagnostics on pristine master and the patched tree, with no added diagnostics.

Agent Drafting Metadata

• Agent: Codex
• Model: GPT-6.1 Sol
• Notes: Investigated the raw-spelling mismatch, independently reproduced the round-trip failures, added the quote-style matrix and a CR control, and ran the full suite and checks.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Wkhanex]

Cross-style concatenation still emits invalid TOML when the left operand is literal and the right value cannot fit that literal style. Both operands can be valid individually:

a = tomlkit.string("left", literal=True)
b = tomlkit.string("newline" + chr(10))
c = a + b
tomllib.loads("x=" + c.as_string())  # TOMLDecodeError

The resulting Python string ends in a newline, but as_string emits a single-quoted literal containing that newline. I also reproduced invalid literal output for a basic-string value containing apostrophes, and invalid multiline-literal output for bare carriage returns/triple apostrophes. I used Python 3.12.14 against commit 715989b.

The branch uses no escaping when the left style has no escaped_sequences, and _new bypasses String.from_raw validation. Could it reject unrepresentable literal content consistently with from_raw, or deliberately switch to an escapable style? My 108-case style/content matrix used stdlib tomllib as the serialization oracle. This is a remaining edge case in the change’s cross-style scope; I did not run the full pytest suite.