fix: keep DuckDB array-slice colon dense inside [ ]

[gaoflow]

DuckDB's array-slice operator formats with a stray space: foo[:5] comes out as foo[: 5] and baz[1:5] as baz[1: 5].

Root cause

Commit f9575c0 ("Support ':' as struct key-value separator") removed ':' from DuckDB's alwaysDenseOperators, so struct literals like {'a': 1} keep the space after the colon. As a side effect the array-slice colon, which travels the same generic operator-spacing path, also became spaced. The identical test still passes for PostgreSQL (which keeps ':' dense and has no struct literals), and the DuckDB copy was skipped with a // TODO: This currently conflicts with ":"-operator in struct literals.

The colon is genuinely context-dependent in DuckDB — dense inside [...] (array slice), spaced inside {...} (struct key) — but alwaysDenseOperators is a flat, dialect-global list with no notion of context, so neither setting is correct for both.

Fix

Make the ':' spacing decision depend on the nearest enclosing bracket rather than being dialect-global: dense inside [...], spaced otherwise. The bracket context flows through the recursive ExpressionFormatter (each parenthesis passes its own openParen down to its children), so a struct nested in a list — [{'a': 1}] — still gets the spaced colon because its nearest bracket is {.

Only the colon-followed-by-a-value case was visibly wrong (foo[:5], baz[1:5]); bar[1:] and zap[:] already rendered correctly because the trailing space was absorbed by the closing bracket.

Tests

• Un-skips the pre-existing supports array slice operator test (all four forms: foo[:5], bar[1:], baz[1:5], zap[:]).
• Adds a test pinning both directions in one statement — {'k': list[1:5]} — where the struct colon stays spaced while the slice colon is dense.

The four formats {} struct literal ... and formats prefix aliases tests double as an over-fix guard: making : dense unconditionally passes the slice test but breaks all of them. Full pnpm test is green across every dialect.

Note

alwaysDenseOperators has no context mechanism today, so I threaded the bracket context through the formatter rather than widening that config surface. If you'd prefer a different shape (e.g. recognising the slice colon at the grammar level), happy to adjust.

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between aa8efae and d248e73.

📒 Files selected for processing (2)

• src/formatter/ExpressionFormatter.ts
• test/duckdb.test.ts

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Improved formatting for array-slice expressions by removing unnecessary spacing around the slice colon.
  • Preserved standard spacing for colons in struct and list literals.

• Tests

  • Enabled coverage for array-slice formatting.
  • Added validation to distinguish array-slice colons from literal key/value colons.

Walkthrough

ExpressionFormatter now propagates enclosing bracket context to nested expressions, allowing array-slice colons to be formatted densely without changing struct or list literal colon spacing. DuckDB tests cover multiple slice forms and mixed literal expressions.

Changes

Array slice formatting

Layer / File(s)	Summary
Bracket-context propagation and colon spacing src/formatter/ExpressionFormatter.ts, test/duckdb.test.ts	Formatter instances carry enclosing bracket context through nested and inline expressions; colons inside square brackets use dense spacing, with DuckDB tests covering slice syntax and spaced literal colons.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• sql-formatter-org/sql-formatter#958: Updates operator-spacing behavior in the same ExpressionFormatter.formatOperator code path.

Suggested reviewers: nene, sarathfrancis90

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: keeping DuckDB array-slice colons dense inside brackets.
Description check	✅ Passed	The description directly explains the DuckDB colon-spacing fix, root cause, and tests, and matches the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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