compiler: support initializing struct fields from a variable

While initializing a struct, it is a top item on ALTSTACK.
This means that if we need to load a local variable,
DUPFROMALTSTACK won't longer push an array of locals on stack
but rather a currently initializing struct.

Closes #656.

pkg/compiler/codegen.go

@@ -872,7 +872,6 @@ func (c *codegen) convertStruct(lit *ast.CompositeLit) {
 	emit.Opcode(c.prog.BinWriter, opcode.NOP)
 	emit.Int(c.prog.BinWriter, int64(strct.NumFields()))
 	emit.Opcode(c.prog.BinWriter, opcode.NEWSTRUCT)
-	emit.Opcode(c.prog.BinWriter, opcode.TOALTSTACK)
 
 	// We need to locally store all the fields, even if they are not initialized.
 	// We will initialize all fields to their "zero" value.
@@ -886,9 +885,14 @@ func (c *codegen) convertStruct(lit *ast.CompositeLit) {
 			fieldName := f.Key.(*ast.Ident).Name
 
 			if sField.Name() == fieldName {
-				ast.Walk(c, f.Value)
+				emit.Opcode(c.prog.BinWriter, opcode.DUP)
+
 				pos := indexOfStruct(strct, fieldName)
-				c.emitStoreLocal(pos)
+				emit.Int(c.prog.BinWriter, int64(pos))
+
+				ast.Walk(c, f.Value)
+
+				emit.Opcode(c.prog.BinWriter, opcode.SETITEM)
 				fieldAdded = true
 				break
 			}
@@ -902,10 +906,12 @@ func (c *codegen) convertStruct(lit *ast.CompositeLit) {
 			c.prog.Err = err
 			return
 		}
+
+		emit.Opcode(c.prog.BinWriter, opcode.DUP)
+		emit.Int(c.prog.BinWriter, int64(i))
 		c.emitLoadConst(typeAndVal)
-		c.emitStoreLocal(i)
+		emit.Opcode(c.prog.BinWriter, opcode.SETITEM)
 	}
-	emit.Opcode(c.prog.BinWriter, opcode.FROMALTSTACK)
 }
 
 func (c *codegen) convertToken(tok token.Token) {

pkg/compiler/struct_test.go

@@ -93,6 +93,71 @@ var structTestCases = []testCase{
 		`,
 		big.NewInt(12),
 	},
+	{
+		"initialize struct field from variable",
+		`
+		package foo
+		type token struct {
+			x int
+			y int
+		}
+
+		func Main() int {
+			x := 10
+			t := token {
+				x: x,
+				y: 4,
+			}
+			y := t.x + t.y
+			return y
+		}`,
+		big.NewInt(14),
+	},
+	{
+		"assign a variable to a struct field",
+		`
+		package foo
+		type token struct {
+			x int
+			y int
+		}
+
+		func Main() int {
+			ten := 10
+			t := token {
+				x: 2,
+				y: 4,
+			}
+			t.x = ten
+			y := t.y + t.x
+			return y
+		}`,
+		big.NewInt(14),
+	},
+	{
+		"assign a struct field to a struct field",
+		`
+		package foo
+		type token struct {
+			x int
+			y int
+		}
+
+		func Main() int {
+			t1 := token {
+				x: 2,
+				y: 4,
+			}
+			t2 := token {
+				x: 3,
+				y: 5,
+			}
+			t1.x = t2.y
+			y := t1.x + t2.x
+			return y
+		}`,
+		big.NewInt(8),
+	},
 	{
 		"initialize same struct twice",
 		`