Crate minicbor_derive

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Procedural macros to derive minicbor’s Encode, Decode, and CborLen traits.

Deriving is supported for structs and enums. The encoding is optimised for forward and backward compatibility and the overall approach is influenced by Google’s Protocol Buffers.

The goal is that ideally a change to a type still allows older software, which is unaware of the changes, to decode values of the changed type (forward compatibility) and newer software, to decode values of types encoded by older software, which do not include the changes made to the type (backward compatibility).

In order to reach this goal, the encoding has the following characteristics:

  1. The encoding does not contain any names, i.e. no field names, type names or variant names. Instead, every field and every constructor needs to be annotated with an (unsigned) index number, e.g. #[n(1)].

  2. Unknown fields are ignored during decoding.1

  3. Optional types default to None if their value is not present during decoding.

  4. Optional enums default to None if an unknown variant is encountered during decoding.

Item 1 ensures that names can be changed freely without compatibility concerns. Item 2 ensures that new fields do not affect older software. Item 3 ensures that newer software can stop producing optional values. Item 4 ensures that enums can get new variants that older software is not aware of. By “fields” we mean the elements of structs and tuple structs as well as enum structs and enum tuples. In addition, it is a compatible change to turn a unit variant into a struct or tuple variant if all fields are optional.

From the above it should be obvious that non-optional fields need to be present forever, so they should only be part of a type after careful consideration.

It should be emphasised that an enum itself can not be changed in a compatible way. An unknown variant causes an error. It is only when they are declared as an optional field type that unknown variants of an enum are mapped to None. In other words, only structs can be used as top-level types in a forward and backward compatible way, enums can not.

Example

use minicbor::{Encode, Decode};

#[derive(Encode, Decode)]
struct Point {
    #[n(0)] x: f64,
    #[n(1)] y: f64
}

#[derive(Encode, Decode)]
struct ConvexHull {
    #[n(0)] left: Point,
    #[n(1)] right: Point,
    #[n(2)] points: Vec<Point>,
    #[n(3)] state: Option<State>
}

#[derive(Encode, Decode)]
enum State {
    #[n(0)] Start,
    #[n(1)] Search { #[n(0)] info: u64 }
}

In this example the following changes would be compatible in both directions:

  • Renaming every identifier.

  • Adding optional fields to Point, ConvexHull, State::Start or State::Search.

  • Adding more variants to State iff State is only decoded as part of ConvexHull. Direct decoding of State would produce an UnknownVariant error for those new variants.

Supported attributes

#[n(...)] and #[b(...)] (or #[cbor(n(...))] and #[cbor(b(...))])

Each field and variant needs to be annotated with an index number, which is used instead of the name. For the encoding it makes no difference which one to choose. For decoding, b indicates that the value borrows from the decoding input, whereas n produces non-borrowed values (but see section Implicit borrowing below). This means that if a type is annotated with #[b(...)], all its lifetimes will be constrained to the input lifetime ('bytes). Further, if the type is a Cow<'_, str>, Cow<'_, minicbor::bytes::ByteSlice> or Cow<'_, [u8]> the generated code will decode the str, ByteSlice or [u8] and construct a Cow::Borrowed variant, contrary to the regular Cow impls of Decode and DecodeBytes which produce owned values.

#[cbor(array)]

Uses a CBOR array to encode the annotated struct, enum or enum variant. When used with an enum it applies to all its variants but can be overriden per variant. See section CBOR encoding for details.

If neither #[cbor(array)] nor #[cbor(map)] are specified, #[cbor(array)] is used by default.

#[cbor(map)]

Use a CBOR map to encode the annotated struct, enum or enum variant. When used with an enum it applies to all its variants but can be overriden per variant. See section CBOR encoding for details.

If neither #[cbor(array)] nor #[cbor(map)] are specified, #[cbor(array)] is used by default.

#[cbor(index_only)]

Enumerations which do not contain fields may have this attribute attached to them. This changes the encoding to encode only the variant index (cf. section CBOR encoding for details).

#[cbor(transparent)]

This attribute can be attached to structs with exactly one field (aka newtypes). If present, the generated Encode and Decode impls will just forward the respective encode and decode calls to the inner type, i.e. the resulting CBOR representation will be identical to the one of the inner type.

#[cbor(decode_with = "<path>")]

When applied to a field of type T, the function denoted by <path> will be used to decode T. The function needs to be equivalent to the following type:

use minicbor::decode::{Decoder, Error};

fn decode<'b, Ctx, T: 'b>(d: &mut Decoder<'b>, ctx: &mut Ctx) -> Result<T, Error> {
    todo!()
}

Please note that if the decode function is generic in its context parameter that the derive macro uses the type variable name Ctx.

#[cbor(encode_with = "<path>")]

When applied to a field of type T, the function denoted by <path> will be used to encode T. The function needs to be equivalent to the following type:

use minicbor::encode::{Encoder, Error, Write};

fn encode<Ctx, T, W: Write>(v: &T, e: &mut Encoder<W>, ctx: &mut Ctx) -> Result<(), Error<W::Error>> {
    todo!()
}

Please note that if the encode function is generic in its context parameter that the derive macro uses the type variable name Ctx.

#[cbor(with = "<path>")]

Combines #[cbor(decode_with = "...")] and #[cbor(encode_with = "...")]. Here, <path> denotes a module that contains functions named encode and decode that satisfy the respective type signatures mentioned in encode_with and decode_with. If CborLen is also derived, the module is assumed to contain a function named cbor_len with a signature matching the one described in #[cbor(cbor_len = "...")] below.

#[cbor(nil = "<path>")]

Only valid in conjuction with #[cbor(decode_with = "...")]. If present, <path> denotes a function to create a nil-like value of type T. See minicbor::Decode::nil for details. The function needs to be equivalent to the following type:

fn nil<T>() -> Option<T> {
    todo!()
}

#[cbor(has_nil)]

Only valid in conjuction with #[cbor(with = "...")]. If present, the attribute signals that the module denoted by with also contains functions nil and is_nil to create nil values and to check if a value is a nil value.

#[cbor(is_nil = "<path>")]

Only valid in conjuction with #[cbor(encode_with = "...")]. If present, <path> denotes a function to check if a value of type T is a nil-like value. See minicbor::Encode::is_nil for details. The function needs to be equivalent to the following type:

fn is_nil<T>(v: &T) -> bool {
    todo!()
}

#[cbor(cbor_len = "<path>")]

Only applicable when deriving CborLen. When applied to a field of type T, the function denoted by <path> will be used to calculate the CBOR length in bytes. The function needs to be equivalent to the following type:

fn cbor_len<Ctx, T>(val: &T, ctx: &mut Ctx) -> usize {
    todo!()
}

Please note that if the cbor_len function is generic in its context parameter that the derive macro uses the type variable name Ctx.

#[cbor(decode_bound = "...")]

When applied to a generic field, this attribute overrides any implicit type parameter bounds generated by minicbor-derive for the derived Decode impl.

#[cbor(encode_bound = "...")]

When applied to a generic field, this attribute overrides any implicit type parameter bounds generated by minicbor-derive for the derived Encode impl.

#[cbor(bound)]

Combines #[cbor(encode_bound = "...")] and #[cbor(decode_bound = "...")], i.e. the bound applies to the derived Encode and Decode impl.

#[cbor(context_bound = "...")]

When deriving Encode or Decode for a type which has parts that constrain the generic context type parameter, this attribute can be used to add the required trait bounds to the context type parameter. The attribute can either be repeated or the bounds can be listed as ‘+’-separated value, e.g. “A + B + C”.

Example

A combined context. 
use minicbor::{Encode, Decode};
use minicbor::decode::{self, Decoder};

// Some decodable type that uses a custom context.
struct A(u8);

// `A`'s context type.
struct AC { a: u8 }

impl AsMut<AC> for AC {
    fn as_mut(&mut self) -> &mut AC { self }
}

impl<'b, C: AsMut<AC>> Decode<'b, C> for A {
    fn decode(d: &mut Decoder<'b>, ctx: &mut C) -> Result<Self, decode::Error> {
        Ok(A(ctx.as_mut().a))
    }
}

// Another decodable type that uses a different context.
struct B(u8);

// `B`'s context type.
struct BC { b: u8 }

impl AsMut<BC> for BC {
    fn as_mut(&mut self) -> &mut BC { self }
}

impl<'b, C: AsMut<BC>> Decode<'b, C> for B {
    fn decode(d: &mut Decoder<'b>, ctx: &mut C) -> Result<Self, decode::Error> {
        Ok(B(ctx.as_mut().b))
    }
}

// Finally, a type that combines `A` and `B` and therefore also needs to provide
// a context that can be used by both of them.
#[derive(Decode)]
#[cbor(context_bound = "AsMut<AC> + AsMut<BC>")]
struct C {
    #[n(0)] a: A,
    #[n(1)] b: B
}

// The combined context type.
struct CC(AC, BC);

impl AsMut<AC> for CC {
    fn as_mut(&mut self) -> &mut AC {
        &mut self.0
    }
}

impl AsMut<BC> for CC {
    fn as_mut(&mut self) -> &mut BC {
        &mut self.1
    }
}

Implicit borrowing

Apart from the explicit borrowing with #[b(...)], the following types implicitly borrow from the decoding input, which means their lifetimes are constrained by the input lifetime:

  • &'_ str
  • &'_ minicbor::bytes::ByteSlice
  • Option<&'_ str>
  • Option<&'_ minicbor::bytes::ByteSlice>

What about &[u8]?

&[u8] is a special case of &[T]. The lack of trait impl specialisation in Rust makes it difficult to provide optimised support for byte slices. The generic [T] impl of Encode produces an array of Ts. To specifically encode to and decode from CBOR bytes, the types ByteSlice, ByteArray and ByteVec are provided by minicbor. In addition, the attributes encode_with, decode_with and with can be used with &[u8] when deriving, e.g.

use minicbor::{Encode, Decode};

#[derive(Encode, Decode)]
struct Foo<'a> {
    #[cbor(n(0), with = "minicbor::bytes")]
    field0: &'a [u8],

    #[n(1)]
    #[cbor(encode_with = "minicbor::bytes::encode")]
    #[cbor(decode_with = "minicbor::bytes::decode")]
    field1: &'a [u8],

    #[cbor(n(2), with = "minicbor::bytes")]
    field2: Option<&'a [u8]>,

    #[cbor(n(3), with = "minicbor::bytes")]
    field3: Vec<u8>,

    #[cbor(n(4), with = "minicbor::bytes")]
    field4: [u8; 16]
}

CBOR encoding

The CBOR values produced by a derived Encode implementation are of the following formats.

Structs

Array encoding

By default or if a struct has the #[cbor(array)] attribute, it will be represented as a CBOR array. Its index numbers are represened by the position of the field value in this array. Any gaps between index numbers are filled with CBOR NULL values and Options which are None likewise end up as NULLs in this array.

<<struct-as-array encoding>> =
    `array(n)`
        item_0
        item_1
        ...
        item_n

Map encoding

If a struct has the #[cbor(map)] attribute attached, then it will be represented as a CBOR map with keys corresponding to the numeric index value:

<<struct-as-map encoding>> =
    `map(n)`
        `0` item_0
        `1` item_1
        ...
         n  item_n

Optional fields whose value is None are not encoded.

Enums

Unless the #[cbor(index_only)] attribute is used for enums without any fields, each enum variant is encoded as a two-element array. The first element is the variant index and the second the actual variant value. Otherwise, if enums do not have fields and the index_only attribute is present, only the variant index is encoded:

<<enum encoding>> =
    | `array(2)` n <<struct-as-array encoding>> ; if #[cbor(array)]
    | `array(2)` n <<struct-as-map encoding>>   ; if #[cbor(map)]
    | n                                         ; if #[cbor(index_only)]

Which encoding to use?

The map encoding needs to represent the indexes explicitly in the encoding which costs at least one extra byte per field value, whereas the array encoding does not need to encode the indexes. On the other hand, absent values, i.e. Nones and gaps between indexes are not encoded with maps but need to be encoded explicitly with arrays as NULLs which need one byte each. Which encoding to choose depends therefore on the nature of the type that should be encoded:

  • Dense types are types which contain only few Options or their Options are assumed to be Somes usually. They are best encoded as arrays.

  • Sparse types are types with many Options and their Options are usually Nones. They are best encoded as maps.

When selecting the encoding, future changes to the type should be considered as they may turn a dense type into a sparse one over time. This also applies to #[cbor(index_only)] which should be used only with enums which are not expected to ever have fields in their variants.

  1. CBOR items are ignored using Decoder::skip. This method requires feature “alloc” to work for all possible CBOR items. Without “alloc”, indefinite maps or arrays inside of regular maps or arrays can not be skipped over. If such a combination occurs and Decoder::skip was compiled without feature “alloc”, a decoding error is returned. ↩

Derive Macros

CborLen
Derive the minicbor::CborLen trait for a struct or enum.
Decode
Derive the minicbor::Decode trait for a struct or enum.
Encode
Derive the minicbor::Encode trait for a struct or enum.