gopls/internal/analysis/modernize: fix bug in mapsloop

A loop body of m[k] += v was spuriously matched because I
forgot to check the assignment operator.

+ test

Updates golang/go#70815

Change-Id: If74dcbb0ba920ebd475b1d0bd9191c9b44661a1a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/642076
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: adonovan

gopls/internal/analysis/modernize/maps.go

@@ -179,7 +179,11 @@ func mapsloop(pass *analysis.Pass) {
 
 			if rng.Tok == token.DEFINE && rng.Key != nil && rng.Value != nil && len(rng.Body.List) == 1 {
 				// Have: for k, v := range x { S }
-				if assign, ok := rng.Body.List[0].(*ast.AssignStmt); ok && len(assign.Lhs) == 1 {
+				if assign, ok := rng.Body.List[0].(*ast.AssignStmt); ok &&
+					assign.Tok == token.ASSIGN &&
+					len(assign.Lhs) == 1 {
+					// Have: for k, v := range x { lhs = rhs }
+
 					if index, ok := assign.Lhs[0].(*ast.IndexExpr); ok &&
 						equalSyntax(rng.Key, index.Index) &&
 						equalSyntax(rng.Value, assign.Rhs[0]) {

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

@@ -138,3 +138,11 @@ func nopeBodyNotASingleton(src map[int]string) {
 		println() // nope: other things in the loop body
 	}
 }
+
+// Regression test for https://github.com/golang/go/issues/70815#issuecomment-2581999787.
+func nopeAssignmentHasIncrementOperator(src map[int]int) {
+	dst := make(map[int]int)
+	for k, v := range src {
+		dst[k] += v
+	}
+}

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

@@ -110,3 +110,11 @@ func nopeBodyNotASingleton(src map[int]string) {
 		println() // nope: other things in the loop body
 	}
 }
+
+// Regression test for https://github.com/golang/go/issues/70815#issuecomment-2581999787.
+func nopeAssignmentHasIncrementOperator(src map[int]int) {
+	dst := make(map[int]int)
+	for k, v := range src {
+		dst[k] += v
+	}
+}