compiler: do not emit code for unused imported functions

Our current algorithm marks function as used if it is called
at least ones, even if the callee function is itself unused.
This commit implements more clever traversal to collect usage
information more precisely.

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/compiler/function_call_test.go

@@ -324,3 +324,30 @@ func TestFunctionUnusedParameters(t *testing.T) {
 	}`
 	eval(t, src, big.NewInt(101))
 }
+
+func TestUnusedFunctions(t *testing.T) {
+	t.Run("only variable", func(t *testing.T) {
+		src := `package foo
+		import "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/nestedcall"
+		func Main() int {
+			return nestedcall.X
+		}`
+
+		b, err := compiler.Compile("foo", strings.NewReader(src))
+		require.NoError(t, err)
+		require.Equal(t, 3, len(b)) // PUSHINT8 (42) + RET
+		eval(t, src, big.NewInt(42))
+	})
+	t.Run("imported function", func(t *testing.T) {
+		// Check that import map is set correctly during package traversal.
+		src := `package foo
+		import inner "github.com/nspcc-dev/neo-go/pkg/compiler/testdata/nestedcall"
+		func Main() int {
+			return inner.N()
+		}`
+
+		_, err := compiler.Compile("foo", strings.NewReader(src))
+		require.NoError(t, err)
+		eval(t, src, big.NewInt(65))
+	})
+}