der: Custom error types in derive macros. (#1560)

* der: Custom error types in derive macros.

`Sequence`, `Enumerated` and `Choice` macros now support `#[asn1(error = Ty)]` attribute that provides a custom error type for `Decode`/`DecodeValue` implementations.

This addresses #1559.

* der_derive: use an ErrorType to store the error attribute

* der_derive: apply errortype to enumerated

followup on turbocool3r#1

* der: Add documentation for the `#[asn1(error)]` attribute.

---------

Co-authored-by: Arthur Gautier <arthur.gautier@arista.com>

Author: turbocool3r

der/tests/derive.rs

@@ -11,10 +11,29 @@
 // TODO: fix needless_question_mark in the derive crate
 #![allow(clippy::bool_assert_comparison, clippy::needless_question_mark)]
 
+#[derive(Debug)]
+#[allow(dead_code)]
+pub struct CustomError(der::Error);
+
+impl From<der::Error> for CustomError {
+    fn from(value: der::Error) -> Self {
+        Self(value)
+    }
+}
+
+impl From<std::convert::Infallible> for CustomError {
+    fn from(_value: std::convert::Infallible) -> Self {
+        unreachable!()
+    }
+}
+
 /// Custom derive test cases for the `Choice` macro.
 mod choice {
+    use super::CustomError;
+
     /// `Choice` with `EXPLICIT` tagging.
     mod explicit {
+        use super::CustomError;
         use der::{
             asn1::{GeneralizedTime, UtcTime},
             Choice, Decode, Encode, SliceWriter,
@@ -50,6 +69,13 @@ mod choice {
             }
         }
 
+        #[derive(Choice)]
+        #[asn1(error = CustomError)]
+        pub enum WithCustomError {
+            #[asn1(type = "GeneralizedTime")]
+            Foo(GeneralizedTime),
+        }
+
         const UTC_TIMESTAMP_DER: &[u8] = &hex!("17 0d 39 31 30 35 30 36 32 33 34 35 34 30 5a");
         const GENERAL_TIMESTAMP_DER: &[u8] =
             &hex!("18 0f 31 39 39 31 30 35 30 36 32 33 34 35 34 30 5a");
@@ -61,6 +87,10 @@ mod choice {
 
             let general_time = Time::from_der(GENERAL_TIMESTAMP_DER).unwrap();
             assert_eq!(general_time.to_unix_duration().as_secs(), 673573540);
+
+            let WithCustomError::Foo(with_custom_error) =
+                WithCustomError::from_der(GENERAL_TIMESTAMP_DER).unwrap();
+            assert_eq!(with_custom_error.to_unix_duration().as_secs(), 673573540);
         }
 
         #[test]
@@ -154,6 +184,7 @@ mod choice {
 
 /// Custom derive test cases for the `Enumerated` macro.
 mod enumerated {
+    use super::CustomError;
     use der::{Decode, Encode, Enumerated, SliceWriter};
     use hex_literal::hex;
 
@@ -176,13 +207,24 @@ mod enumerated {
     const UNSPECIFIED_DER: &[u8] = &hex!("0a 01 00");
     const KEY_COMPROMISE_DER: &[u8] = &hex!("0a 01 01");
 
+    #[derive(Enumerated, Copy, Clone, Eq, PartialEq, Debug)]
+    #[asn1(error = CustomError)]
+    #[repr(u32)]
+    pub enum EnumWithCustomError {
+        Unspecified = 0,
+        Specified = 1,
+    }
+
     #[test]
     fn decode() {
         let unspecified = CrlReason::from_der(UNSPECIFIED_DER).unwrap();
         assert_eq!(CrlReason::Unspecified, unspecified);
 
         let key_compromise = CrlReason::from_der(KEY_COMPROMISE_DER).unwrap();
         assert_eq!(CrlReason::KeyCompromise, key_compromise);
+
+        let custom_error_enum = EnumWithCustomError::from_der(UNSPECIFIED_DER).unwrap();
+        assert_eq!(custom_error_enum, EnumWithCustomError::Unspecified);
     }
 
     #[test]
@@ -202,6 +244,7 @@ mod enumerated {
 /// Custom derive test cases for the `Sequence` macro.
 #[cfg(feature = "oid")]
 mod sequence {
+    use super::CustomError;
     use core::marker::PhantomData;
     use der::{
         asn1::{AnyRef, ObjectIdentifier, SetOf},
@@ -383,6 +426,12 @@ mod sequence {
         pub typed_context_specific_optional: Option<&'a [u8]>,
     }
 
+    #[derive(Sequence)]
+    #[asn1(error = CustomError)]
+    pub struct TypeWithCustomError {
+        pub simple: bool,
+    }
+
     #[test]
     fn idp_test() {
         let idp = IssuingDistributionPointExample::from_der(&hex!("30038101FF")).unwrap();
@@ -444,6 +493,9 @@ mod sequence {
             PRIME256V1_OID,
             ObjectIdentifier::try_from(algorithm_identifier.parameters.unwrap()).unwrap()
         );
+
+        let t = TypeWithCustomError::from_der(&hex!("30030101FF")).unwrap();
+        assert!(t.simple);
     }
 
     #[test]

der_derive/src/attributes.rs

@@ -2,11 +2,33 @@
 
 use crate::{Asn1Type, Tag, TagMode, TagNumber};
 use proc_macro2::{Span, TokenStream};
-use quote::quote;
+use quote::{quote, ToTokens};
 use std::{fmt::Debug, str::FromStr};
 use syn::punctuated::Punctuated;
 use syn::{parse::Parse, parse::ParseStream, Attribute, Ident, LitStr, Path, Token};
 
+/// Error type used by the structure
+#[derive(Debug, Clone, Default, Eq, PartialEq)]
+pub(crate) enum ErrorType {
+    /// Represents the ::der::Error type
+    #[default]
+    Der,
+    /// Represents an error designed by Path
+    Custom(Path),
+}
+
+impl ToTokens for ErrorType {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        match self {
+            Self::Der => {
+                let err = quote! { ::der::Error };
+                err.to_tokens(tokens)
+            }
+            Self::Custom(path) => path.to_tokens(tokens),
+        }
+    }
+}
+
 /// Attribute name.
 pub(crate) const ATTR_NAME: &str = "asn1";
 
@@ -18,37 +40,47 @@ pub(crate) struct TypeAttrs {
     ///
     /// The default value is `EXPLICIT`.
     pub tag_mode: TagMode,
+    pub error: ErrorType,
 }
 
 impl TypeAttrs {
     /// Parse attributes from a struct field or enum variant.
     pub fn parse(attrs: &[Attribute]) -> syn::Result<Self> {
         let mut tag_mode = None;
+        let mut error = None;
 
-        let mut parsed_attrs = Vec::new();
-        AttrNameValue::from_attributes(attrs, &mut parsed_attrs)?;
-
-        for attr in parsed_attrs {
-            // `tag_mode = "..."` attribute
-            let mode = attr.parse_value("tag_mode")?.ok_or_else(|| {
-                syn::Error::new_spanned(
-                    &attr.name,
-                    "invalid `asn1` attribute (valid options are `tag_mode`)",
-                )
-            })?;
-
-            if tag_mode.is_some() {
-                return Err(syn::Error::new_spanned(
-                    &attr.name,
-                    "duplicate ASN.1 `tag_mode` attribute",
-                ));
+        attrs.iter().try_for_each(|attr| {
+            if !attr.path().is_ident(ATTR_NAME) {
+                return Ok(());
             }
 
-            tag_mode = Some(mode);
-        }
+            attr.parse_nested_meta(|meta| {
+                if meta.path.is_ident("tag_mode") {
+                    if tag_mode.is_some() {
+                        abort!(attr, "duplicate ASN.1 `tag_mode` attribute");
+                    }
+
+                    tag_mode = Some(meta.value()?.parse()?);
+                } else if meta.path.is_ident("error") {
+                    if error.is_some() {
+                        abort!(attr, "duplicate ASN.1 `error` attribute");
+                    }
+
+                    error = Some(ErrorType::Custom(meta.value()?.parse()?));
+                } else {
+                    return Err(syn::Error::new_spanned(
+                        attr,
+                        "invalid `asn1` attribute (valid options are `tag_mode` and `error`)",
+                    ));
+                }
+
+                Ok(())
+            })
+        })?;
 
         Ok(Self {
             tag_mode: tag_mode.unwrap_or_default(),
+            error: error.unwrap_or_default(),
         })
     }
 }

der_derive/src/choice.rs

@@ -5,9 +5,9 @@
 mod variant;
 
 use self::variant::ChoiceVariant;
-use crate::{default_lifetime, TypeAttrs};
+use crate::{default_lifetime, ErrorType, TypeAttrs};
 use proc_macro2::TokenStream;
-use quote::quote;
+use quote::{quote, ToTokens};
 use syn::{DeriveInput, GenericParam, Generics, Ident, LifetimeParam};
 
 /// Derive the `Choice` trait for an enum.
@@ -20,6 +20,9 @@ pub(crate) struct DeriveChoice {
 
     /// Variants of this `Choice`.
     variants: Vec<ChoiceVariant>,
+
+    /// Error type for `DecodeValue` implementation.
+    error: ErrorType,
 }
 
 impl DeriveChoice {
@@ -44,6 +47,7 @@ impl DeriveChoice {
             ident: input.ident,
             generics: input.generics.clone(),
             variants,
+            error: type_attrs.error.clone(),
         })
     }
 
@@ -84,6 +88,8 @@ impl DeriveChoice {
             tagged_body.push(variant.to_tagged_tokens());
         }
 
+        let error = self.error.to_token_stream();
+
         quote! {
             impl #impl_generics ::der::Choice<#lifetime> for #ident #ty_generics #where_clause {
                 fn can_decode(tag: ::der::Tag) -> bool {
@@ -92,17 +98,20 @@ impl DeriveChoice {
             }
 
             impl #impl_generics ::der::Decode<#lifetime> for #ident #ty_generics #where_clause {
-                type Error = ::der::Error;
+                type Error = #error;
 
-                fn decode<R: ::der::Reader<#lifetime>>(reader: &mut R) -> ::der::Result<Self> {
+                fn decode<R: ::der::Reader<#lifetime>>(reader: &mut R) -> ::core::result::Result<Self, #error> {
                     use der::Reader as _;
                     match ::der::Tag::peek(reader)? {
                         #(#decode_body)*
-                        actual => Err(der::ErrorKind::TagUnexpected {
-                            expected: None,
-                            actual
-                        }
-                        .into()),
+                        actual => Err(::der::Error::new(
+                            ::der::ErrorKind::TagUnexpected {
+                                expected: None,
+                                actual
+                            },
+                            reader.position()
+                        ).into()
+                        ),
                     }
                 }
             }

der_derive/src/enumerated.rs

@@ -2,11 +2,10 @@
 //! the purposes of decoding/encoding ASN.1 `ENUMERATED` types as mapped to
 //! enum variants.
 
-use crate::attributes::AttrNameValue;
-use crate::{default_lifetime, ATTR_NAME};
+use crate::{default_lifetime, ErrorType, ATTR_NAME};
 use proc_macro2::TokenStream;
-use quote::quote;
-use syn::{DeriveInput, Expr, ExprLit, Ident, Lit, LitInt, Variant};
+use quote::{quote, ToTokens};
+use syn::{DeriveInput, Expr, ExprLit, Ident, Lit, LitInt, LitStr, Path, Variant};
 
 /// Valid options for the `#[repr]` attribute on `Enumerated` types.
 const REPR_TYPES: &[&str] = &["u8", "u16", "u32"];
@@ -24,6 +23,9 @@ pub(crate) struct DeriveEnumerated {
 
     /// Variants of this enum.
     variants: Vec<EnumeratedVariant>,
+
+    /// Error type for `DecodeValue` implementation.
+    error: ErrorType,
 }
 
 impl DeriveEnumerated {
@@ -40,22 +42,30 @@ impl DeriveEnumerated {
         // Reject `asn1` attributes, parse the `repr` attribute
         let mut repr: Option<Ident> = None;
         let mut integer = false;
+        let mut error: Option<ErrorType> = None;
 
         for attr in &input.attrs {
             if attr.path().is_ident(ATTR_NAME) {
-                let kvs = match AttrNameValue::parse_attribute(attr) {
-                    Ok(kvs) => kvs,
-                    Err(e) => abort!(attr, e),
-                };
-                for anv in kvs {
-                    if anv.name.is_ident("type") {
-                        match anv.value.value().as_str() {
+                attr.parse_nested_meta(|meta| {
+                    if meta.path.is_ident("type") {
+                        let value: LitStr = meta.value()?.parse()?;
+                        match value.value().as_str() {
                             "ENUMERATED" => integer = false,
                             "INTEGER" => integer = true,
-                            s => abort!(anv.value, format_args!("`type = \"{s}\"` is unsupported")),
+                            s => abort!(value, format_args!("`type = \"{s}\"` is unsupported")),
                         }
+                    } else if meta.path.is_ident("error") {
+                        let path: Path = meta.value()?.parse()?;
+                        error = Some(ErrorType::Custom(path));
+                    } else {
+                        return Err(syn::Error::new_spanned(
+                            &meta.path,
+                            "invalid `asn1` attribute (valid options are `type` and `error`)",
+                        ));
                     }
-                }
+
+                    Ok(())
+                })?;
             } else if attr.path().is_ident("repr") {
                 if repr.is_some() {
                     abort!(
@@ -97,6 +107,7 @@ impl DeriveEnumerated {
             })?,
             variants,
             integer,
+            error: error.unwrap_or_default(),
         })
     }
 
@@ -115,14 +126,16 @@ impl DeriveEnumerated {
             try_from_body.push(variant.to_try_from_tokens());
         }
 
+        let error = self.error.to_token_stream();
+
         quote! {
             impl<#default_lifetime> ::der::DecodeValue<#default_lifetime> for #ident {
-                type Error = ::der::Error;
+                type Error = #error;
 
                 fn decode_value<R: ::der::Reader<#default_lifetime>>(
                     reader: &mut R,
                     header: ::der::Header
-                ) -> ::der::Result<Self> {
+                ) -> ::core::result::Result<Self, #error> {
                     <#repr as ::der::DecodeValue>::decode_value(reader, header)?.try_into()
                 }
             }
@@ -142,12 +155,12 @@ impl DeriveEnumerated {
             }
 
             impl TryFrom<#repr> for #ident {
-                type Error = ::der::Error;
+                type Error = #error;
 
-                fn try_from(n: #repr) -> ::der::Result<Self> {
+                fn try_from(n: #repr) -> ::core::result::Result<Self, #error> {
                     match n {
                         #(#try_from_body)*
-                        _ => Err(#tag.value_error())
+                        _ => Err(#tag.value_error().into())
                     }
                 }
             }