Fix ParserSyntaxError line number off-by-one error

[hippi345]

Fix ParserSyntaxError line number off-by-one error

Summary

Fixes #1472

The raw_line property in ParserSyntaxError is documented and treated as 1-based, but was being computed by adding 1 to an already 1-based line number from the parser. This caused several related bugs:

1. Incorrect line numbers shown to users in error messages
2. Tab adjustment logic using the wrong line to calculate column offsets, causing columns to be incorrectly clamped even when there are no tabs
3. EOF clamping logic erroneously triggering on the last line of input, causing errors on the last line to be reported at column 1

Changes

Rust changes (native/libcst/src/parser/errors.rs):

• Removed the +1 offset when passing line to raw_line (line 59)
• Fixed EOF clamping check from line + 1 > lines.len() to line > lines.len() (line 51)
• Fixed EOF clamping assignment from line = lines.len() - 1 to line = lines.len() (line 52)

Python changes (libcst/tests/test_exceptions.py):

• Added regression test test_parser_error_line_numbers() that verifies:
  • Errors on the first line of multi-line input report raw_line=1
  • Errors on the last line report the correct line number
  • Both raw_line/raw_column and editor_line/editor_column are correct
  • Error messages display the correct location
• Added regression test test_parser_error_tab_adjustment_on_correct_line() that verifies:
  • Tab adjustment uses the correct line to calculate column offsets
  • Tabs on other lines don't affect the error column position

Testing

All tests pass:

• ✅ Rust tests: cargo test in native/ (all tests pass)
• ✅ Python tests: uv run poe test (2163 tests pass, 14 skipped, 2 expected failures)
• ✅ New regression tests verify both line numbers and caret positions

Before fix:

import libcst as cst

# Error on line 1
try:
    cst.parse_module("x = (1 2)\ny = 3\nz = 4\n")
except cst.ParserSyntaxError as e:
    print(f"raw_line={e.raw_line}, raw_column={e.raw_column}")
    # Output: raw_line=2, raw_column=8  � Wrong!
    print(str(e).splitlines()[0])
    # Output: Syntax Error @ 2:6.  � Wrong line and column!

# Error on line 2
try:
    cst.parse_module("x = 1\ny = (1 2)\n")
except cst.ParserSyntaxError as e:
    print(f"raw_line={e.raw_line}, raw_column={e.raw_column}")
    # Output: raw_line=2, raw_column=0  � Column clamped to 0!
    print(str(e).splitlines()[0])
    # Output: Syntax Error @ 2:1.  � Wrong column!

After fix:

import libcst as cst

# Error on line 1
try:
    cst.parse_module("x = (1 2)\ny = 3\nz = 4\n")
except cst.ParserSyntaxError as e:
    print(f"raw_line={e.raw_line}, raw_column={e.raw_column}")
    # Output: raw_line=1, raw_column=8  ✅ Correct!
    print(str(e).splitlines()[0])
    # Output: Syntax Error @ 1:9.  ✅ Correct!

# Error on line 2
try:
    cst.parse_module("x = 1\ny = (1 2)\n")
except cst.ParserSyntaxError as e:
    print(f"raw_line={e.raw_line}, raw_column={e.raw_column}")
    # Output: raw_line=2, raw_column=8  ✅ Correct!
    print(str(e).splitlines()[0])
    # Output: Syntax Error @ 2:9.  ✅ Correct!

Scope

This fix is scoped to the issue and only modifies:

• The line number calculation in the Rust parser error conversion
• The EOF clamping logic to work correctly with 1-based line numbers
• Test coverage to prevent regression

The fix maintains backward compatibility with the Python API - raw_line and editor_line remain 1-based as documented, but now report the correct values.

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