der: add Decode::from_ber (#1389)

Support for decoding an input byte slice while specifying what encoding
rules should be used with the given reader.

der/src/decode.rs

@@ -1,6 +1,6 @@
 //! Trait definition for [`Decode`].
 
-use crate::{Error, FixedTag, Header, Reader, SliceReader};
+use crate::{EncodingRules, Error, FixedTag, Header, Reader, SliceReader};
 use core::marker::PhantomData;
 
 #[cfg(feature = "pem")]
@@ -23,6 +23,16 @@ pub trait Decode<'a>: Sized + 'a {
     /// Attempt to decode this message using the provided decoder.
     fn decode<R: Reader<'a>>(decoder: &mut R) -> Result<Self, Self::Error>;
 
+    /// Parse `Self` from the provided BER-encoded byte slice.
+    ///
+    /// Note that most usages should probably use [`Decode::from_der`]. This method allows some
+    /// BER productions which are not allowed under DER.
+    fn from_ber(bytes: &'a [u8]) -> Result<Self, Self::Error> {
+        let mut reader = SliceReader::new_with_encoding_rules(bytes, EncodingRules::Ber)?;
+        let result = Self::decode(&mut reader)?;
+        Ok(reader.finish(result)?)
+    }
+
     /// Parse `Self` from the provided DER-encoded byte slice.
     fn from_der(bytes: &'a [u8]) -> Result<Self, Self::Error> {
         let mut reader = SliceReader::new(bytes)?;

der/src/reader/slice.rs

@@ -21,9 +21,17 @@ pub struct SliceReader<'a> {
 impl<'a> SliceReader<'a> {
     /// Create a new slice reader for the given byte slice.
     pub fn new(bytes: &'a [u8]) -> Result<Self, Error> {
+        Self::new_with_encoding_rules(bytes, EncodingRules::default())
+    }
+
+    /// Create a new slice reader with the given encoding rules.
+    pub fn new_with_encoding_rules(
+        bytes: &'a [u8],
+        encoding_rules: EncodingRules,
+    ) -> Result<Self, Error> {
         Ok(Self {
             bytes: BytesRef::new(bytes)?,
-            encoding_rules: EncodingRules::default(),
+            encoding_rules,
             failed: false,
             position: Length::ZERO,
         })
@@ -196,7 +204,7 @@ mod tests {
         assert_eq!(
             ErrorKind::TrailingData {
                 decoded: 3u8.into(),
-                remaining: 1u8.into()
+                remaining: 1u8.into(),
             },
             err.kind()
         );