der: have PemReader decode to Cow::Owned (#2094)

Adds an associated `CAN_READ_SLICE` boolean flag to the `Reader` trait.

When false, `Cow` decodes the owned type rather than the borrow one.

This makes it possible to use `Cow` with `PemReader`, where it will
automatically decode the owned type.

Author: tarcieri

der/src/decode.rs

@@ -212,14 +212,20 @@ where
 }
 
 #[cfg(feature = "alloc")]
-impl<'a, T> DecodeValue<'a> for Cow<'a, T>
+impl<'a, T, E> DecodeValue<'a> for Cow<'a, T>
 where
     T: ToOwned + ?Sized,
-    &'a T: DecodeValue<'a>,
+    &'a T: DecodeValue<'a, Error = E>,
+    T::Owned: for<'b> DecodeValue<'b, Error = E>,
+    E: From<Error> + 'static,
 {
-    type Error = <&'a T as DecodeValue<'a>>::Error;
+    type Error = E;
 
     fn decode_value<R: Reader<'a>>(reader: &mut R, header: Header) -> Result<Self, Self::Error> {
-        Ok(Cow::Borrowed(<&'a T>::decode_value(reader, header)?))
+        if R::CAN_READ_SLICE {
+            <&'a T>::decode_value(reader, header).map(Cow::Borrowed)
+        } else {
+            T::Owned::decode_value(reader, header).map(Cow::Owned)
+        }
     }
 }

der/src/reader.rs

@@ -20,6 +20,9 @@ use crate::length::indefinite::read_eoc;
 
 /// Reader trait which reads DER-encoded input.
 pub trait Reader<'r>: Clone {
+    /// Does this reader support the `read_slice` method? (i.e. can it borrow from his input?)
+    const CAN_READ_SLICE: bool;
+
     /// Get the [`EncodingRules`] which should be applied when decoding the input.
     fn encoding_rules(&self) -> EncodingRules;
 

der/src/reader/pem.rs

@@ -43,6 +43,8 @@ impl<'i> PemReader<'i> {
 
 #[cfg(feature = "pem")]
 impl<'i> Reader<'i> for PemReader<'i> {
+    const CAN_READ_SLICE: bool = false;
+
     fn encoding_rules(&self) -> EncodingRules {
         self.encoding_rules
     }

der/src/reader/slice.rs

@@ -71,6 +71,8 @@ impl<'a> SliceReader<'a> {
 }
 
 impl<'a> Reader<'a> for SliceReader<'a> {
+    const CAN_READ_SLICE: bool = true;
+
     fn encoding_rules(&self) -> EncodingRules {
         self.encoding_rules
     }