gopls/internal/analysis/modernize: rangeint: avoid offering wrong fix

This change adds additional checking to ensure that rangeint
won't offer a fix in cases where RHS of 'i < limit' depends on loop var.

Given the code snippet below, this change will no longer offer a wrong fix as it did before.

	var n, kd int
	for j := 0; j < min(n-j, kd+1); j++ {
	}

- offered fix before(build error 'undefined: j')

	var n, kd int
	for j := range min(n-j, kd+1){
	}

Fixes golang/go#72726

Change-Id: I78c5457406258c44dd2fa861aa43d9ddb9c707fc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/656975
Reviewed-by: Alan Donovan <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: xieyuschen

gopls/internal/analysis/modernize/rangeint.go

@@ -62,7 +62,15 @@ func rangeint(pass *analysis.Pass) {
 					compare.Op == token.LSS &&
 					equalSyntax(compare.X, init.Lhs[0]) {
 					// Have: for i = 0; i < limit; ... {}
+
 					limit := compare.Y
+					curLimit, _ := curLoop.FindNode(limit)
+					// Don't offer a fix if the limit expression depends on the loop index.
+					for cur := range curLimit.Preorder((*ast.Ident)(nil)) {
+						if cur.Node().(*ast.Ident).Name == index.Name {
+							continue nextLoop
+						}
+					}
 
 					// Skip loops up to b.N in benchmarks; see [bloop].
 					if sel, ok := limit.(*ast.SelectorExpr); ok &&

gopls/internal/analysis/modernize/testdata/src/rangeint/rangeint.go

@@ -77,3 +77,24 @@ func issue71847d() {
 	for i := 0; i < limit2; i++ { // want "for loop can be modernized using range over int"
 	}
 }
+
+func issue72726() {
+	var n, kd int
+	for i := 0; i < n; i++ { // want "for loop can be modernized using range over int"
+		// nope: j will be invisible once it's refactored to 'for j := range min(n-j, kd+1)'
+		for j := 0; j < min(n-j, kd+1); j++ { // nope
+			_, _ = i, j
+		}
+	}
+
+	for i := 0; i < i; i++ { // nope
+	}
+
+	var i int
+	for i = 0; i < i/2; i++ { // nope
+	}
+
+	var arr []int
+	for i = 0; i < arr[i]; i++ { // nope
+	}
+}

gopls/internal/analysis/modernize/testdata/src/rangeint/rangeint.go.golden

@@ -77,3 +77,24 @@ func issue71847d() {
 	for range int(limit2) { // want "for loop can be modernized using range over int"
 	}
 }
+
+func issue72726() {
+	var n, kd int
+	for i := range n { // want "for loop can be modernized using range over int"
+		// nope: j will be invisible once it's refactored to 'for j := range min(n-j, kd+1)'
+		for j := 0; j < min(n-j, kd+1); j++ { // nope
+			_, _ = i, j
+		}
+	}
+
+	for i := 0; i < i; i++ { // nope
+	}
+
+	var i int
+	for i = 0; i < i/2; i++ { // nope
+	}
+
+	var arr []int
+	for i = 0; i < arr[i]; i++ { // nope
+	}
+}