Nested f-strings fail when an inner interpolation contains a brace-bearing string literal
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Assessment
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Newbie friendliness
- 76/100
Research direction
Start in src/lexer.c around the outer interpolation boundary scanner at lines 407–422, then inspect tests/test_fstrings.eigs lines 46–51 for existing coverage. Run the reproducer and controls first. Done means nested f-strings containing brace-bearing ordinary strings parse and print the expected output, while escaped quotes, dictionaries, direct-string cases, and malformed inputs retain their stated behavior.
Written by the indexing model from the issue text.
Description
A nested f-string fails to parse when one of its interpolation expressions contains a string literal with a closing brace. The same brace-bearing string works in a single f-string, and nesting works when that brace is removed.
Reproduce
print of f"outer:{f"inner:{"a}b"}"}"
Actual: exit 1, empty stdout. Stderr reports unterminated f-string expression, unterminated string, unterminated f-string, then unexpected '}' after statement (four parse errors).
Expected: exit 0 and outer:inner:a}b\n.
Paired control: change only a}b to abc:
print of f"outer:{f"inner:{"abc"}"}"
This exits 0 and prints outer:inner:abc\n.
The direct brace-bearing-string control also succeeds:
print of f"inner:{"a}b"}"
It exits 0 and prints inner:a}b\n. The defect therefore concerns composition of supported forms, not a proposed new interpolation syntax.
Cause
The outer interpolation boundary scanner skips from a quote to the next unescaped quote, treating the nested f-string as an ordinary string. The quote beginning the nested interpolation's "a}b" literal ends that skip. Its literal } is then exposed to the outer brace-depth counter, which closes the outer expression prematurely.
Boundary scanning must understand nested f-string lexical structure instead of treating every quoted span as an ordinary string.
Relationship to #334/#344
This is a remaining composition case in the class addressed by closed #334 and fixed in #344, rather than a duplicate of their original direct reproducer. The original direct f"{"a}b"}" case passes on current main. tests/test_fstrings.eigs:46–51 tests brace-bearing ordinary strings and simple nested f-strings separately; it does not combine them.
Regression acceptance
- The reproducer prints exactly
outer:inner:a}b\nand both controls keep passing. - Cover opening and closing braces in ordinary strings inside nested f-strings.
- Retain escaped quotes/backslashes, nested dictionary expressions, and the existing direct-string and simple-nesting cases.
- Verify actual output for valid cases and keep malformed nested strings failing with diagnostics.
This does not duplicate #1244's LSP source-coordinate issue; here a valid program cannot execute at all.
Verification
Confirmed on a fresh default release build of b91768e23c5a874a64e76e4af9ab291e6aa49983, with inherited EIGS_* variables cleared. Each reproducer and paired control was executed; no runtime or test source was changed. This is a current correctness defect, not a proposed v1 language restriction.
- Dominant language
- C
- Stars
- 3
- Forks
- 7
- Avg merge
- 3h 56m
- Merged PRs (30d)
- 102
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