SA4006: Fix for compound operators, increment/decrement

In the case of compound operators such as +=, -=, etc we need to use the
left-hand side to get the value.

For IncDec we need a different codepath from AssignStmt altogether.

Fixes #1329

staticcheck/sa4006/sa4006.go

@@ -3,6 +3,7 @@ package sa4006
 import (
 	"fmt"
 	"go/ast"
+	"go/token"
 
 	"honnef.co/go/tools/analysis/code"
 	"honnef.co/go/tools/analysis/facts/generated"
@@ -31,7 +32,7 @@ var SCAnalyzer = lint.InitializeAnalyzer(&lint.Analyzer{
 
 var Analyzer = SCAnalyzer.Analyzer
 
-func run(pass *analysis.Pass) (interface{}, error) {
+func run(pass *analysis.Pass) (any, error) {
 	for _, fn := range pass.ResultOf[buildir.Analyzer].(*buildir.IR).SrcFuncs {
 		if irutil.IsExample(fn) {
 			continue
@@ -99,6 +100,22 @@ func run(pass *analysis.Pass) (interface{}, error) {
 		}
 
 		ast.Inspect(node, func(node ast.Node) bool {
+			inc, ok := node.(*ast.IncDecStmt)
+			if ok {
+				val, _ := fn.ValueForExpr(inc.X)
+				if val == nil {
+					return true
+				}
+				if _, ok := val.(*ir.Const); ok {
+					// a zero-valued constant, for example in 'foo := []string(nil)'
+					return true
+				}
+				if !hasUse(val, nil) {
+					report.Report(pass, inc, fmt.Sprintf("this value of %s is never used", inc.X))
+				}
+				return true
+			}
+
 			assign, ok := node.(*ast.AssignStmt)
 			if !ok {
 				return true
@@ -137,7 +154,12 @@ func run(pass *analysis.Pass) (interface{}, error) {
 				}
 				val, _ := fn.ValueForExpr(rhs)
 				if val == nil {
-					continue
+					if assign.Tok != token.ASSIGN { // +=, *=, etc.
+						val, _ = fn.ValueForExpr(lhs)
+					}
+					if val == nil {
+						continue
+					}
 				}
 
 				if _, ok := val.(*ir.Const); ok {

staticcheck/sa4006/testdata/src/example.com/CheckUnreadVariableValues/CheckUnreadVariableValues.go

@@ -142,3 +142,43 @@ func fn10() {
 	}
 	fmt.Println(slice)
 }
+
+func issue1329() {
+	{
+		n := 1
+		n += 1 //@ diag(`this value of n is never used`)
+	}
+	{
+		n := 1
+		n ^= 1 //@ diag(`this value of n is never used`)
+	}
+	{
+		n := ""
+		n += "" //@ diag(`this value of n is never used`)
+	}
+	{
+		n := 1
+		n++ //@ diag(`this value of n is never used`)
+	}
+
+	{
+		n := 1
+		n += 1
+		fmt.Println(n)
+	}
+	{
+		n := 1
+		n ^= 1
+		fmt.Println(n)
+	}
+	{
+		n := ""
+		n += ""
+		fmt.Println(n)
+	}
+	{
+		n := 1
+		n++
+		fmt.Println(n)
+	}
+}