The RustcEncodable and RustcDecoadable derives serve no purpose on their own and are deprecated, undocumented, and widely unused. The derives produce code for rustc-serialize, an external crate which has not had a release since April 2017 and is officially deprecated in favor of serde.
The rustc-serialize crate will, in its current state, fail to compile at some point in the future, as it contains code with a future compatibility warning:
> warning: impl method assumes more implied bounds than the corresponding trait method
> --> /home/jhpratt/.cargo/registry/src/index.crates.io-6f17d22bba15001f/rustc-serialize-0.3.24/src/serialize.rs:1155:41
> |
> 1155 | fn decode<D: Decoder>(d: &mut D) -> Result<Cow<'static, T>, D::Error> {
> | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace this type to make the impl signature compatible: `Result<Cow<'a, T>, <D as serialize::Decoder>::Error>`
> |
> = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
> = note: for more information, see issue #105572 <https://github.com/rust-lang/rust/issues/105572>
> = note: `#[allow(implied_bounds_entailment)]` on by default
>
Solution sketch
The RustcEncodable and RustcDecodable derives are to be soft-destabilized, as mentioned in rust-lang/rust#92594. This turns any use of them into a deny-by-default lint, which is stronger than the warn-by-default deprecation warning that is currently present.
Additionally, the RustcEncodable and RustcDecodable traits are to be removed from the standard library's preludes in all future editions (i.e. ≥2024). rustfix will change existing uses to use the path to the previous prelude.
Alternatives
A full destabilization is not proposed due to breakage of existing code. If/when rustc-serialize fails to compile on stable as mentioned above, a full destabilization would make more sense.
While this proposes both soft-destabilizing and removing the derives from the prelude, there is no reason one could not be accepted without the other if it is so desired.
Proposal
Problem statement
The
RustcEncodable
andRustcDecoadable
derives serve no purpose on their own and are deprecated, undocumented, and widely unused. The derives produce code forrustc-serialize
, an external crate which has not had a release since April 2017 and is officially deprecated in favor ofserde
.The
rustc-serialize
crate will, in its current state, fail to compile at some point in the future, as it contains code with a future compatibility warning:Solution sketch
The
RustcEncodable
andRustcDecodable
derives are to be soft-destabilized, as mentioned in rust-lang/rust#92594. This turns any use of them into a deny-by-default lint, which is stronger than the warn-by-default deprecation warning that is currently present.Additionally, the
RustcEncodable
andRustcDecodable
traits are to be removed from the standard library's preludes in all future editions (i.e. ≥2024).rustfix
will change existing uses to use the path to the previous prelude.Alternatives
A full destabilization is not proposed due to breakage of existing code. If/when
rustc-serialize
fails to compile on stable as mentioned above, a full destabilization would make more sense.While this proposes both soft-destabilizing and removing the derives from the prelude, there is no reason one could not be accepted without the other if it is so desired.
Links and related work
rustc-serialize
is archived.Encodable
andDecodable
derives were previously removed outright..RustcEncodable
andRustcDecodable
were deprecated in March 2021. This was uncontroversial enough that FCP was functionally skipped.RustcEncodable
andRustcDecodable
was previously proposed, but closed due to inactivity.