fix: return error for empty slices in In() to prevent invalid SQL generation

[1RB]

Fix: Prevent invalid SQL from empty slices in sqlx.In

Fixes #1

Problem

sqlx.In with an empty slice for an IN (?) placeholder generated invalid SQL like IN (), causing syntax errors in PostgreSQL and MySQL.

Solution

• Added ErrEmptySlice sentinel error
• In() now detects empty slices/arrays via reflection and returns ErrEmptySlice immediately
• Properly expands ? placeholders for multi-arg queries with mixed scalar and slice arguments
• Fixed expansion to replace ? with ?,?,? (no double parens — the query already has IN (?))

Test coverage (8 tests, all passing with -race)

✓ TestInWithEmptySlice — empty slice returns ErrEmptySlice
✓ TestInWithMultipleSlices — multiple slices expand correctly
✓ TestInWithMixedArgs — mixed scalars + slices work
✓ TestInWithSingleSlice — single slice expansion
✓ TestInWithEmptySliceFirst — empty first arg errors
✓ TestInWithOnlyScalars — scalars pass through unchanged
✓ TestInWithArray — arrays are expanded too
✓ TestInWithEmptyArray — empty array returns error

/attempt #1

[Copilot]

Pull request overview

This PR updates the sqlx.In helper to proactively reject empty slice/array arguments (to avoid generating invalid SQL like IN ()) and adds unit tests to cover common expansion scenarios.

Changes:

• Introduces ErrEmptySlice and returns it when any slice/array argument has length 0.
• Reworks In() to expand slice/array args into a flattened argument list and to expand ? placeholders accordingly.
• Adds a new sqlx_test.go suite covering empty slices/arrays, multiple slices, mixed scalar/slice args, and arrays; removes the previous main.go stub and adds go.mod.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
sqlx.go	Adds ErrEmptySlice and implements new In() placeholder/argument expansion logic.
sqlx_test.go	Adds unit tests validating slice/array expansion behavior and empty-slice erroring.
main.go	Removes unused example main program.
go.mod	Introduces a Go module definition for the repository.

Comments suppressed due to low confidence (1)

sqlx.go:51

• The second pass also calls reflect.ValueOf on args[argIndex] and will panic for nil arguments. Treat nil as a non-slice scalar placeholder and skip reflection in that case.

			v := reflect.ValueOf(args[argIndex])
			if v.Kind() == reflect.Slice || v.Kind() == reflect.Array {
				if v.Len() == 0 {

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