compiler: support variable shadowing

Closes #1131.

pkg/compiler/codegen.go

@@ -499,6 +499,9 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		return nil
 
 	case *ast.IfStmt:
+		c.scope.vars.newScope()
+		defer c.scope.vars.dropScope()
+
 		lIf := c.newLabel()
 		lElse := c.newLabel()
 		lElseEnd := c.newLabel()
@@ -553,6 +556,8 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 				}
 			}
 
+			c.scope.vars.newScope()
+
 			c.setLabel(lStart)
 			last := len(cc.Body) - 1
 			for j, stmt := range cc.Body {
@@ -564,6 +569,8 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 			}
 			emit.Jmp(c.prog.BinWriter, opcode.JMPL, switchEnd)
 			c.setLabel(lEnd)
+
+			c.scope.vars.dropScope()
 		}
 
 		c.setLabel(switchEnd)
@@ -884,6 +891,9 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		return nil
 
 	case *ast.ForStmt:
+		c.scope.vars.newScope()
+		defer c.scope.vars.dropScope()
+
 		fstart, label := c.generateLabel(labelStart)
 		fend := c.newNamedLabel(labelEnd, label)
 		fpost := c.newNamedLabel(labelPost, label)
@@ -926,6 +936,9 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		return nil
 
 	case *ast.RangeStmt:
+		c.scope.vars.newScope()
+		defer c.scope.vars.dropScope()
+
 		start, label := c.generateLabel(labelStart)
 		end := c.newNamedLabel(labelEnd, label)
 		post := c.newNamedLabel(labelPost, label)

pkg/compiler/global_test.go

@@ -1,6 +1,7 @@
 package compiler_test
 
 import (
+	"fmt"
 	"math/big"
 	"testing"
 )
@@ -55,3 +56,29 @@ func TestMultiDeclarationLocalCompound(t *testing.T) {
 	}`
 	eval(t, src, big.NewInt(6))
 }
+
+func TestShadow(t *testing.T) {
+	srcTmpl := `package foo
+	func Main() int {
+		x := 1
+		y := 10
+		%s
+			x += 1  // increase old local
+			x := 30 // introduce new local
+			y += x  // make sure is means something
+		}
+		return x+y
+	}`
+
+	runCase := func(b string) func(t *testing.T) {
+		return func(t *testing.T) {
+			src := fmt.Sprintf(srcTmpl, b)
+			eval(t, src, big.NewInt(42))
+		}
+	}
+
+	t.Run("If", runCase("if true {"))
+	t.Run("For", runCase("for i := 0; i < 1; i++ {"))
+	t.Run("Range", runCase("for range []int{1} {"))
+	t.Run("Switch", runCase("switch true {\ncase false: x += 2\ncase true:"))
+}