der: make Reader cloneable (#1883)

Alternative to #1882 which makes arbitrary peeking possible by directly
cloning the `Reader`, rather than providing a facade which clones it for
you.

This also makes `Reader::peek_into` a provided method.

Author: tarcieri

der/src/reader.rs

@@ -16,19 +16,13 @@ use crate::{
 use alloc::vec::Vec;
 
 /// Reader trait which reads DER-encoded input.
-pub trait Reader<'r>: Sized {
+pub trait Reader<'r>: Clone {
     /// Get the [`EncodingRules`] which should be applied when decoding the input.
     fn encoding_rules(&self) -> EncodingRules;
 
     /// Get the length of the input.
     fn input_len(&self) -> Length;
 
-    /// Peek at the decoded PEM without updating the internal state, writing into the provided
-    /// output buffer.
-    ///
-    /// Attempts to fill the entire buffer, returning an error if there is not enough data.
-    fn peek_into(&self, buf: &mut [u8]) -> crate::Result<()>;
-
     /// Get the position within the buffer.
     fn position(&self) -> Length;
 
@@ -121,6 +115,16 @@ pub trait Reader<'r>: Sized {
         self.peek_into(&mut byte).ok().map(|_| byte[0])
     }
 
+    /// Peek at the decoded data without updating the internal state, writing into the provided
+    /// output buffer.
+    ///
+    /// Attempts to fill the entire buffer, returning an error if there is not enough data.
+    fn peek_into(&self, buf: &mut [u8]) -> Result<(), Error> {
+        let mut reader = self.clone();
+        reader.read_into(buf)?;
+        Ok(())
+    }
+
     /// Peek forward in the input data, attempting to decode a [`Header`] from
     /// the data at the current position in the decoder.
     ///

der/src/reader/pem.rs

@@ -51,11 +51,6 @@ impl<'i> Reader<'i> for PemReader<'i> {
         self.position.input_len()
     }
 
-    fn peek_into(&self, buf: &mut [u8]) -> Result<()> {
-        self.clone().read_into(buf)?;
-        Ok(())
-    }
-
     fn position(&self) -> Length {
         self.position.current()
     }

der/src/reader/slice.rs

@@ -84,11 +84,6 @@ impl<'a> Reader<'a> for SliceReader<'a> {
         self.bytes.len()
     }
 
-    fn peek_into(&self, buf: &mut [u8]) -> crate::Result<()> {
-        self.clone().read_into(buf)?;
-        Ok(())
-    }
-
     fn position(&self) -> Length {
         self.position
     }