compiler: do not emit code for unnamed unused variables

If variable is unnamed and does not contain function call then it's
treated as unused and code generation may be omitted for it
initialization/declaration.

pkg/compiler/global_test.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/nspcc-dev/neo-go/pkg/compiler"
 	"github.com/nspcc-dev/neo-go/pkg/vm"
+	"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
 	"github.com/stretchr/testify/require"
 )
 
@@ -52,6 +53,30 @@ func TestUnusedGlobal(t *testing.T) {
 				}`
 			eval(t, src, big.NewInt(1))
 		})
+		t.Run("used", func(t *testing.T) {
+			src := `package foo
+				var _, A = f()
+				func Main() int {
+					return A
+				}
+				func f() (int, int) {
+					return 5, 6
+				}`
+			eval(t, src, big.NewInt(6))
+		})
+	})
+	t.Run("unused without function call", func(t *testing.T) {
+		src := `package foo
+				var _ = 1
+				var (
+					_ = 2 + 3
+					_, _ = 3 + 4, 5
+				)
+				func Main() int {
+					return 1
+				}`
+		prog := eval(t, src, big.NewInt(1))
+		require.Equal(t, 2, len(prog)) // PUSH1 + RET
 	})
 }
 
@@ -324,3 +349,30 @@ func TestUnderscoreVarsDontUseSlots(t *testing.T) {
 	src := buf.String()
 	eval(t, src, big.NewInt(count))
 }
+
+func TestUnderscoreGlobalVarDontEmitCode(t *testing.T) {
+	src := `package foo
+		var _ int
+		var _ = 1
+		var (
+			A = 2
+			_ = A + 3
+			_, B, _ = 4, 5, 6
+			_, C, _ = f(A, B)
+		)
+		var D = 7 // unused but named, so the code is expected
+		func Main() int {
+			return 1
+		}
+		func f(a, b int) (int, int, int) {
+			return 8, 9, 10
+		}`
+	eval(t, src, big.NewInt(1), []interface{}{opcode.INITSSLOT, []byte{4}}, // sslot for A, B, C, D
+		opcode.PUSH2, opcode.STSFLD0, // store A
+		opcode.PUSH5, opcode.STSFLD1, // store B
+		opcode.LDSFLD0, opcode.LDSFLD1, opcode.SWAP, []interface{}{opcode.CALL, []byte{10}}, // evaluate f
+		opcode.DROP, opcode.STSFLD2, opcode.DROP, // store C
+		opcode.PUSH7, opcode.STSFLD3, opcode.RET, // store D
+		opcode.PUSH1, opcode.RET, // Main
+		[]interface{}{opcode.INITSLOT, []byte{0, 2}}, opcode.PUSH10, opcode.PUSH9, opcode.PUSH8, opcode.RET) // f
+}