From b0bb1b7b39a6e68ef28b10df8ba33ee15bfc85ed Mon Sep 17 00:00:00 2001
From: Tony Arcieri <bascule@gmail.com>
Date: Fri, 25 Mar 2022 09:23:32 -0600
Subject: [PATCH] der: add `DecodeOwned` marker trait

Adds a marker trait for types which can be `der::Decode`d without
borrowing any data from the input.

It's useful for cases where borrowing from the input isn't possible. For
example, if we were to add on-the-fly PEM decoding as proposed in #525.
---
 der/src/decode.rs | 12 ++++++++++++
 der/src/lib.rs    |  2 +-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/der/src/decode.rs b/der/src/decode.rs
index 61ee2fe5c..a3e854fd0 100644
--- a/der/src/decode.rs
+++ b/der/src/decode.rs
@@ -35,6 +35,18 @@ where
     }
 }
 
+/// Marker trait for data structures that can be decoded from DER without
+/// borrowing any data from the decoder.
+///
+/// This is primarily useful for trait bounds on functions which require that
+/// no data is borrowed from the decoder, for example a PEM decoder which needs
+/// to first decode data from Base64.
+///
+/// This trait is inspired by the [`DeserializeOwned` trait from `serde`](https://docs.rs/serde/latest/serde/de/trait.DeserializeOwned.html).
+pub trait DecodeOwned: for<'a> Decode<'a> {}
+
+impl<T> DecodeOwned for T where T: for<'a> Decode<'a> {}
+
 /// Decode the value part of a Tag-Length-Value encoded field, sans the [`Tag`]
 /// and [`Length`].
 pub trait DecodeValue<'a>: Sized {
diff --git a/der/src/lib.rs b/der/src/lib.rs
index 212332ddd..12ac3877e 100644
--- a/der/src/lib.rs
+++ b/der/src/lib.rs
@@ -353,7 +353,7 @@ mod document;
 pub use crate::{
     asn1::{Any, Choice, Sequence},
     datetime::DateTime,
-    decode::{Decode, DecodeValue},
+    decode::{Decode, DecodeOwned, DecodeValue},
     decoder::Decoder,
     encode::{Encode, EncodeValue},
     encoder::Encoder,