der: fix append in Encode::encode_to_vec (#1760)

* der: fix append in `Encode::encode_to_vec`

* der: test encode_to_vec with append

Author: dishmaker

der/src/encode.rs

@@ -4,7 +4,7 @@ use crate::{Header, Length, Result, SliceWriter, Tagged, Writer};
 use core::marker::PhantomData;
 
 #[cfg(feature = "alloc")]
-use {alloc::boxed::Box, alloc::vec::Vec, core::iter};
+use {alloc::boxed::Box, alloc::vec::Vec};
 
 #[cfg(feature = "pem")]
 use {
@@ -40,12 +40,11 @@ pub trait Encode {
     #[cfg(feature = "alloc")]
     fn encode_to_vec(&self, buf: &mut Vec<u8>) -> Result<Length> {
         let expected_len = usize::try_from(self.encoded_len()?)?;
-        buf.reserve(expected_len);
-        buf.extend(iter::repeat(0).take(expected_len));
+        let initial_len = buf.len();
+        buf.resize(initial_len + expected_len, 0u8);
 
-        let mut writer = SliceWriter::new(buf);
-        self.encode(&mut writer)?;
-        let actual_len = writer.finish()?.len();
+        let buf_slice = &mut buf[initial_len..];
+        let actual_len = self.encode_to_slice(buf_slice)?.len();
 
         if expected_len != actual_len {
             return Err(ErrorKind::Incomplete {

der/tests/derive.rs

@@ -271,7 +271,7 @@ mod sequence {
     // ```
     //
     // [RFC 5280 Section 5.2.5]: https://datatracker.ietf.org/doc/html/rfc5280#section-5.2.5
-    #[derive(Sequence)]
+    #[derive(Sequence, Default)]
     pub struct IssuingDistributionPointExample {
         // Omit distributionPoint and only_some_reasons because corresponding structs are not
         // available here and are not germane to the example
@@ -601,6 +601,28 @@ mod sequence {
         assert_eq!(idp.only_contains_attribute_certs, true);
     }
 
+    #[test]
+    fn idp_encode_twice() {
+        let mut vec_buf = Vec::new();
+
+        IssuingDistributionPointExample {
+            only_contains_user_certs: true,
+            ..Default::default()
+        }
+        .encode_to_vec(&mut vec_buf)
+        .unwrap();
+
+        // encode to the same vec by appending
+        IssuingDistributionPointExample {
+            only_contains_cacerts: true,
+            ..Default::default()
+        }
+        .encode_to_vec(&mut vec_buf)
+        .unwrap();
+
+        assert_eq!(vec_buf, hex!("30038101FF 30038201FF"));
+    }
+
     // demonstrates default field that is not context specific
     #[test]
     fn extension_test() {