cmd/fix: x/tools/go/analysis/passes/modernize: slicescontains hoists needle expression, changing side effect count #77564
Description
The slicescontains modernizer rewrites a for/range loop with an equality check into slices.ContainsFunc (or slices.Contains), but this changes how many times the needle expression is evaluated. In the original loop, the needle is evaluated once per iteration (until break). With slices.ContainsFunc, the needle is hoisted and evaluated once before iteration.
Before (go fix):
package main
import "fmt"
func main() {
n := 0
f := func() int { n++; return 3 }
s := []int{1, 2, 3}
found := false
for _, elem := range s {
if elem == f() {
found = true
break
}
}
_ = found
fmt.Println(n)
}Output: 3 (f() called once per iteration: elem=1 vs 1, elem=2 vs 2, elem=3 vs 3 — match, break)
After (GOTOOLCHAIN=go1.26.0 go fix):
package main
import (
"fmt"
"slices"
)
func main() {
n := 0
f := func() int { n++; return 3 }
s := []int{1, 2, 3}
found := slices.Contains(s, f())
_ = found
fmt.Println(n)
}Output: 1 (f() called once before iteration, needle=3 used for all comparisons)
The modernizer should not apply the transformation when the needle expression has side effects.
CC @adonovan