S1011: reject dynamic lhs

Closes: gh-1399

simple/lint.go

@@ -778,13 +778,13 @@ var checkLoopAppendQ = pattern.MustParse(`
 		x
 		[(AssignStmt [lhs] "=" [(CallExpr (Builtin "append") [lhs val])])])
 	(RangeStmt
-		idx@(Ident _)
+		idx@(Object _)
 		nil
 		_
 		x
 		[(AssignStmt [lhs] "=" [(CallExpr (Builtin "append") [lhs (IndexExpr x idx)])])])
 	(RangeStmt
-		idx@(Ident _)
+		idx@(Object _)
 		nil
 		_
 		x
@@ -800,8 +800,7 @@ func CheckLoopAppend(pass *analysis.Pass) (interface{}, error) {
 			return
 		}
 
-		val, ok := m.State["val"].(types.Object)
-		if ok && refersTo(pass, m.State["lhs"].(ast.Expr), val) {
+		if val, ok := m.State["val"].(types.Object); ok && refersTo(pass, m.State["lhs"].(ast.Expr), val) {
 			return
 		}
 
@@ -811,6 +810,26 @@ func CheckLoopAppend(pass *analysis.Pass) (interface{}, error) {
 			return
 		}
 
+		if idx, ok := m.State["idx"].(types.Object); ok && refersTo(pass, m.State["lhs"].(ast.Expr), idx) {
+			// The lhs mustn't refer to the index loop variable.
+			return
+		}
+
+		if code.MayHaveSideEffects(pass, m.State["lhs"].(ast.Expr), pure) {
+			// The lhs may be dynamic and return different values on each iteration. For example:
+			//
+			// 	func bar() map[int][]int { /* return one of several maps */ }
+			//
+			// 	func foo(x []int, y [][]int) {
+			// 		for i := range x {
+			// 			bar()[0] = append(bar()[0], x[i])
+			// 		}
+			// 	}
+			//
+			// The dynamic nature of the lhs might also affect the value of the index.
+			return
+		}
+
 		src := pass.TypesInfo.TypeOf(m.State["x"].(ast.Expr))
 		dst := pass.TypesInfo.TypeOf(m.State["lhs"].(ast.Expr))
 		if !types.Identical(src, dst) {

simple/testdata/src/example.com/CheckLoopAppend/loop-append.go

@@ -109,3 +109,25 @@ func fn7() {
 		x = append(x, fn6()[i])
 	}
 }
+
+func fn8() {
+	// The lhs isn't allowed to refer to i
+	var i int
+	var x []int
+	var y [][]int
+	for i = range x {
+		y[i] = append(y[i], x[i])
+	}
+	for i := range x {
+		y[i] = append(y[i], x[i])
+	}
+}
+
+func fn9() {
+	// The lhs isn't allowed to have side effects
+	bar := func() map[int][]int { return nil }
+	var x []int
+	for i := range x {
+		bar()[0] = append(bar()[0], x[i])
+	}
+}

simple/testdata/src/example.com/CheckLoopAppend/loop-append.go.golden

@@ -99,3 +99,25 @@ func fn7() {
 		x = append(x, fn6()[i])
 	}
 }
+
+func fn8() {
+	// The lhs isn't allowed to refer to i
+	var i int
+	var x []int
+	var y [][]int
+	for i = range x {
+		y[i] = append(y[i], x[i])
+	}
+	for i := range x {
+		y[i] = append(y[i], x[i])
+	}
+}
+
+func fn9() {
+	// The lhs isn't allowed to have side effects
+	bar := func() map[int][]int { return nil }
+	var x []int
+	for i := range x {
+		bar()[0] = append(bar()[0], x[i])
+	}
+}