der: add Error::(set_)source; remove Clone + Copy (#1328)

When the `std` feature is enabled, adds support for propagating an
arbitrary error source which can be set using `Error::set_source`.

The main use case is passing back an arbitrary error value in the
`ErrorKind::Value` use case, to handle passing back what specifically
was wrong with a given value in a DER-encoded document.

This entails removing the `Clone` + `Copy` bounds since the error is
stored in a `Box`, which can't be `Copy` and can't reflect a `Clone`
bound. Fortunately, nothing in this repo ever seems to clone an `Error`
so it's not really an issue.

Also adds `Error::source` impls to the downstream error types in
`pkcs1`, `pkcs5`, `pkcs8`, `sec1`, and `spki`.

der/src/error.rs

@@ -11,17 +11,24 @@ use crate::asn1::ObjectIdentifier;
 #[cfg(feature = "pem")]
 use crate::pem;
 
+#[cfg(feature = "std")]
+use alloc::boxed::Box;
+
 /// Result type.
 pub type Result<T> = core::result::Result<T, Error>;
 
 /// Error type.
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Debug)]
 pub struct Error {
     /// Kind of error.
     kind: ErrorKind,
 
     /// Position inside of message where error occurred.
     position: Option<Length>,
+
+    /// Source of the error.
+    #[cfg(feature = "std")]
+    source: Option<Box<dyn std::error::Error + Send + Sync + 'static>>,
 }
 
 impl Error {
@@ -30,6 +37,8 @@ impl Error {
         Error {
             kind,
             position: Some(position),
+            #[cfg(feature = "std")]
+            source: None,
         }
     }
 
@@ -48,15 +57,27 @@ impl Error {
     }
 
     /// Get the [`ErrorKind`] which occurred.
-    pub fn kind(self) -> ErrorKind {
+    pub fn kind(&self) -> ErrorKind {
         self.kind
     }
 
     /// Get the position inside of the message where the error occurred.
-    pub fn position(self) -> Option<Length> {
+    pub fn position(&self) -> Option<Length> {
         self.position
     }
 
+    /// Set the source of this error. Useful for e.g. propagating an additional error with
+    /// [`ErrorKind::Value`].
+    ///
+    /// Overwrites any previously set source.
+    #[cfg(feature = "std")]
+    pub fn set_source<E>(&mut self, source: E)
+    where
+        E: std::error::Error + Send + Sync + 'static,
+    {
+        self.source = Some(Box::new(source));
+    }
+
     /// For errors occurring inside of a nested message, extend the position
     /// count by the location where the nested message occurs.
     pub(crate) fn nested(self, nested_position: Length) -> Self {
@@ -66,13 +87,12 @@ impl Error {
         Self {
             kind: self.kind,
             position,
+            #[cfg(feature = "std")]
+            source: self.source,
         }
     }
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for Error {}
-
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{}", self.kind)?;
@@ -85,11 +105,20 @@ impl fmt::Display for Error {
     }
 }
 
+impl Eq for Error {}
+impl PartialEq for Error {
+    fn eq(&self, other: &Self) -> bool {
+        self.kind == other.kind && self.position == other.position
+    }
+}
+
 impl From<ErrorKind> for Error {
     fn from(kind: ErrorKind) -> Error {
         Error {
             kind,
             position: None,
+            #[cfg(feature = "std")]
+            source: None,
         }
     }
 }
@@ -101,10 +130,12 @@ impl From<Infallible> for Error {
 }
 
 impl From<TryFromIntError> for Error {
-    fn from(_: TryFromIntError) -> Error {
+    fn from(_err: TryFromIntError) -> Error {
         Error {
             kind: ErrorKind::Overflow,
             position: None,
+            #[cfg(feature = "std")]
+            source: Some(Box::new(_err)),
         }
     }
 }
@@ -114,6 +145,8 @@ impl From<Utf8Error> for Error {
         Error {
             kind: ErrorKind::Utf8(err),
             position: None,
+            #[cfg(feature = "std")]
+            source: Some(Box::new(err)),
         }
     }
 }
@@ -158,6 +191,15 @@ impl From<time::error::ComponentRange> for Error {
     }
 }
 
+#[cfg(feature = "std")]
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        self.source
+            .as_ref()
+            .map(|source| source.as_ref() as &(dyn std::error::Error + 'static))
+    }
+}
+
 /// Error type.
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
 #[non_exhaustive]

pkcs1/src/error.rs

@@ -9,7 +9,7 @@ use der::pem;
 pub type Result<T> = core::result::Result<T, Error>;
 
 /// Error type
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Debug, Eq, PartialEq)]
 #[non_exhaustive]
 pub enum Error {
     /// ASN.1 DER-related errors.
@@ -92,4 +92,13 @@ impl From<pkcs8::spki::Error> for Error {
 }
 
 #[cfg(feature = "std")]
-impl std::error::Error for Error {}
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Error::Asn1(err) => Some(err),
+            #[cfg(feature = "pkcs8")]
+            Error::Pkcs8(err) => Some(err),
+            _ => None,
+        }
+    }
+}

pkcs5/Cargo.toml

@@ -33,6 +33,8 @@ hex-literal = "0.4"
 
 [features]
 alloc = []
+std = ["alloc"]
+
 3des = ["dep:des", "pbes2"]
 des-insecure = ["dep:des", "pbes2"]
 getrandom = ["rand_core/getrandom"]

pkcs5/src/error.rs

@@ -54,3 +54,6 @@ impl fmt::Display for Error {
         }
     }
 }
+
+#[cfg(feature = "std")]
+impl std::error::Error for Error {}

pkcs5/src/lib.rs

@@ -29,6 +29,9 @@
 #[cfg(all(feature = "alloc", feature = "pbes2"))]
 extern crate alloc;
 
+#[cfg(feature = "std")]
+extern crate std;
+
 mod error;
 
 pub mod pbes1;

pkcs8/Cargo.toml

@@ -30,7 +30,7 @@ tempfile = "3"
 
 [features]
 alloc = ["der/alloc", "der/zeroize", "spki/alloc"]
-std = ["alloc", "der/std", "spki/std"]
+std = ["alloc", "der/std", "pkcs5?/std", "spki/std"]
 
 3des = ["encryption", "pkcs5/3des"]
 des-insecure = ["encryption", "pkcs5/des-insecure"]

pkcs8/src/error.rs

@@ -9,7 +9,7 @@ use der::pem;
 pub type Result<T> = core::result::Result<T, Error>;
 
 /// Error type
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Debug, Eq, PartialEq)]
 #[non_exhaustive]
 pub enum Error {
     /// ASN.1 DER-related errors.
@@ -48,7 +48,17 @@ impl fmt::Display for Error {
 }
 
 #[cfg(feature = "std")]
-impl std::error::Error for Error {}
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Error::Asn1(err) => Some(err),
+            #[cfg(feature = "pkcs5")]
+            Error::EncryptedPrivateKey(err) => Some(err),
+            Error::PublicKey(err) => Some(err),
+            _ => None,
+        }
+    }
+}
 
 impl From<der::Error> for Error {
     fn from(err: der::Error) -> Error {

pkcs8/tests/encrypted_private_key.rs

@@ -3,11 +3,14 @@
 #![cfg(feature = "pkcs5")]
 
 use hex_literal::hex;
-use pkcs8::{pkcs5::pbes2, EncryptedPrivateKeyInfo, PrivateKeyInfo};
+use pkcs8::{pkcs5::pbes2, EncryptedPrivateKeyInfo};
 
 #[cfg(feature = "alloc")]
 use der::Encode;
 
+#[cfg(feature = "encryption")]
+use pkcs8::PrivateKeyInfo;
+
 #[cfg(feature = "pem")]
 use der::EncodePem;
 

sec1/src/error.rs

@@ -9,7 +9,7 @@ use der::pem;
 pub type Result<T> = core::result::Result<T, Error>;
 
 /// Error type
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Debug, Eq, PartialEq)]
 #[non_exhaustive]
 pub enum Error {
     /// ASN.1 DER-related errors.

spki/src/error.rs

@@ -10,7 +10,7 @@ pub type Result<T> = core::result::Result<T, Error>;
 use der::pem;
 
 /// Error type
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Debug, Eq, PartialEq)]
 #[non_exhaustive]
 pub enum Error {
     /// Algorithm parameters are missing.
@@ -65,4 +65,11 @@ impl From<pem::Error> for Error {
 }
 
 #[cfg(feature = "std")]
-impl std::error::Error for Error {}
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Error::Asn1(err) => Some(err),
+            _ => None,
+        }
+    }
+}