From 6f03ef8001904020c2de379b73cedadf577ee9d4 Mon Sep 17 00:00:00 2001
From: dishmaker <141624503+dishmaker@users.noreply.github.com>
Date: Thu, 10 Jul 2025 13:38:27 +0200
Subject: [PATCH] der: add `DecodeValueExt` trait with fn `from_der_value`

---
 der/src/decode.rs | 22 ++++++++++++++++++++--
 der/src/lib.rs    |  2 +-
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/der/src/decode.rs b/der/src/decode.rs
index 0ba035a44..4c717fd52 100644
--- a/der/src/decode.rs
+++ b/der/src/decode.rs
@@ -1,13 +1,16 @@
 //! Trait definition for [`Decode`].
 
-use crate::{EncodingRules, Error, FixedTag, Header, Reader, SliceReader, reader::read_value};
+use crate::{
+    AnyRef, Choice, EncodingRules, Error, FixedTag, Header, Reader, SliceReader, Tag,
+    reader::read_value,
+};
 use core::marker::PhantomData;
 
 #[cfg(feature = "pem")]
 use crate::{PemReader, pem::PemLabel};
 
 #[cfg(doc)]
-use crate::{ErrorKind, Length, Tag};
+use crate::{ErrorKind, Length};
 
 #[cfg(feature = "alloc")]
 use alloc::boxed::Box;
@@ -143,3 +146,18 @@ where
         Ok(Box::new(T::decode_value(reader, header)?))
     }
 }
+
+/// Extension trait for [`DecodeValue`], simplifying slice decoding.
+pub trait DecodeValueExt<'a>: DecodeValue<'a> {
+    /// Parse `Self` from the provided DER-encoded value bytes, i.e. without the tag-length.
+    ///
+    /// Returns [`ErrorKind::TrailingData`] if message is incomplete.
+    fn from_der_value(tag: Tag, value: &'a [u8]) -> Result<Self, <Self as DecodeValue<'a>>::Error>
+    where
+        Self: Choice<'a>,
+    {
+        AnyRef::new(tag, value)?.decode_as::<Self>()
+    }
+}
+
+impl<'a, T> DecodeValueExt<'a> for T where T: DecodeValue<'a> {}
diff --git a/der/src/lib.rs b/der/src/lib.rs
index d867ef928..67ad5853d 100644
--- a/der/src/lib.rs
+++ b/der/src/lib.rs
@@ -364,7 +364,7 @@ pub use crate::{
     asn1::bit_string::allowed_len_bit_string::AllowedLenBitString,
     asn1::{AnyRef, Choice, Sequence},
     datetime::DateTime,
-    decode::{Decode, DecodeOwned, DecodeValue},
+    decode::{Decode, DecodeOwned, DecodeValue, DecodeValueExt},
     encode::{Encode, EncodeValue},
     encode_ref::{EncodeRef, EncodeValueRef},
     encoding_rules::EncodingRules,