introduce and expose BorrowedPayload

Co-authored-by: Jorge Aparicio <jorge.aparicio@ferrous-systems.com>

rustls/src/crypto/cipher.rs

@@ -5,6 +5,7 @@ use std::error::Error as StdError;
 
 use crate::enums::{ContentType, ProtocolVersion};
 use crate::error::Error;
+pub use crate::msgs::base::BorrowedPayload;
 use crate::msgs::codec;
 pub use crate::msgs::message::{BorrowedPlainMessage, OpaqueMessage, PlainMessage};
 use crate::suites::ConnectionTrafficSecrets;

rustls/src/msgs/base.rs

@@ -4,10 +4,13 @@ use crate::msgs::codec::{Codec, Reader};
 
 use alloc::vec::Vec;
 use core::fmt;
+use core::ops::{Deref, DerefMut};
 
 use pki_types::CertificateDer;
 use zeroize::Zeroize;
 
+use super::codec::ReaderMut;
+
 /// An externally length'd payload
 #[derive(Clone, Eq, PartialEq)]
 pub struct Payload(pub Vec<u8>);
@@ -36,6 +39,30 @@ impl Payload {
     }
 }
 
+/// Non-owning version of [`Payload`]
+pub struct BorrowedPayload<'a>(&'a mut [u8]);
+
+impl Deref for BorrowedPayload<'_> {
+    type Target = [u8];
+
+    fn deref(&self) -> &Self::Target {
+        self.0
+    }
+}
+
+impl<'a> DerefMut for BorrowedPayload<'a> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.0
+    }
+}
+
+impl<'a> BorrowedPayload<'a> {
+    #[allow(dead_code)] // TODO(@cpu): remove in "introduce and expose BorrowedOpaqueMessage"
+    pub(crate) fn read(r: &mut ReaderMut<'a>) -> Self {
+        Self(r.rest())
+    }
+}
+
 impl<'a> Codec for CertificateDer<'a> {
     fn encode(&self, bytes: &mut Vec<u8>) {
         codec::u24(self.as_ref().len() as u32).encode(bytes);