Fixes #6801

What was happening

When a layout constraint asks for newlines to be stripped between two tokens (for example spacing_within = single:inline on an expression), the reflow code could delete a newline that sat right in front of a comment. The result was that the token before the comment got glued straight onto the -- marker:

SELECT * FROM t WHERE a AND
-- comment
b

came back as

SELECT * FROM t WHERE a AND-- comment
b

which quietly changes what the statement means, since everything after -- is now commented out on that line.

Root cause

In determine_constraints (src/sqlfluff/utils/reflow/respace.py), the guard that keeps newlines around comments only looked at the previous block. If the previous block was a comment it kept the newline, but if the next block was the comment it did nothing, so an inherited strip_newlines=True went ahead and removed the newline in front of it.

The fix

Check the next block for comment segments as well as the previous one. If either side of the point is a comment, we leave the newline alone. A newline that sits immediately before a comment now survives the respace.

Before / after

• Before: a AND\n-- comment\nb -> a AND-- comment\nb (meaning changed)
• After: a AND\n-- comment\nb -> unchanged

Tests

Added test_reflow__respace_does_not_strip_newline_before_comment in test/utils/reflow/respace_test.py, which sets the single:inline expression constraint and asserts the newline before the comment is preserved. The existing reflow suite (test/utils/reflow/) still passes.

Summary by cubic

Stop stripping a newline that sits directly before a comment during respace. Prevents code being glued to the -- marker and changing SQL meaning. Fixes #6801.

• Bug Fixes
  • In determine_constraints, also check the next block for comment segments; if previous or next is a comment, do not strip newlines.
  • Added regression test test_reflow__respace_does_not_strip_newline_before_comment; existing reflow tests still pass.

^{Written for commit 56e06b5. Summary will update on new commits.}