slicesbackward: omit unused value variable in range clause

[Jah-yee]

Good day,

When converting a backward loop to use slices.Backward, the analyzer was incorrectly including the value variable (v) in the generated code even when v was never used in the loop body.

Problem:

For example, when the original loop was:

for i := len(s) - 1; i >= 0; i-- {
    println(i)
}

The generated code was:

for i, v := range slices.Backward(s) {
    println(i)
}

This caused a "declared and not used" compile error because v was never referenced.

Fix:

The fix checks if the slice value is actually used before including the value variable in the range clause. If the value is not used, only the index is included:

for i := range slices.Backward(s) {
    println(i)
}

The key change is in the header generation logic: when otherUses > 0 (meaning the index is used for non-indexing purposes), we now check whether len(sliceIndexes) > 0 (meaning the slice value is actually used). If the slice value is not used, we drop the v variable from the range clause.

Files changed:

• go/analysis/passes/modernize/slicesbackward.go - the fix
• go/analysis/passes/modernize/testdata/src/slicesbackward/slicesbackward.go.golden - updated test expectation

Tests: All existing tests pass, including the new test case indexNoSliceAccess which verifies this scenario.

Fixes golang/go#78629

Thank you for your attention. If there are any issues or suggestions, please leave a comment and I will address them promptly.

Warmly,
RoomWithOutRoof

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