implicit dependency of "std" for "serde" feature in published 0.4.2

[kentfredric]

I've just been testing a few things as part of my linux vendoring efforts, and I stumbled upon this:

cargo +system build --no-default-features --features 'serde'

    Updating crates.io index
   Compiling proc-macro2 v1.0.8
   Compiling unicode-xid v0.2.0
   Compiling syn v1.0.14
   Compiling serde v1.0.104
   Compiling quote v1.0.2
   Compiling serde_derive v1.0.104
   Compiling hex v0.4.2 (/home/kent/.cpanm/work/1582182020.22342/hex-0.4.2)
error[E0433]: failed to resolve: use of undeclared type or module `std`
  --> src/serde.rs:19:5
   |
19 | use std::marker::PhantomData;
   |     ^^^ use of undeclared type or module `std`

error[E0432]: unresolved import `std`
  --> src/serde.rs:18:5
   |
18 | use std::fmt;
   |     ^^^ help: a similar path exists: `serde::std`

error[E0412]: cannot find type `String` in this scope
  --> src/serde.rs:31:37
   |
31 |     let s = data.encode_hex_upper::<String>();
   |                                     ^^^^^^ not found in this scope
   |
help: possible candidates are found in other modules, you can import them into scope
   |
15 | use alloc::prelude::v1::String;
   |
15 | use alloc::string::String;
   |

error[E0412]: cannot find type `String` in this scope
  --> src/serde.rs:45:31
   |
45 |     let s = data.encode_hex::<String>();
   |                               ^^^^^^ not found in this scope
   |
help: possible candidates are found in other modules, you can import them into scope
   |
15 | use alloc::prelude::v1::String;
   |
15 | use alloc::string::String;
   |

error[E0412]: cannot find type `PhantomData` in this scope
  --> src/serde.rs:59:29
   |
59 |     struct HexStrVisitor<T>(PhantomData<T>);
   |                             ^^^^^^^^^^^ not found in this scope
   |
help: possible candidates are found in other modules, you can import them into scope
   |
15 | use core::marker::PhantomData;
   |
15 | use serde::export::PhantomData;
   |

error[E0425]: cannot find value `PhantomData` in this scope
  --> src/serde.rs:87:48
   |
87 |     deserializer.deserialize_str(HexStrVisitor(PhantomData))
   |                                                ^^^^^^^^^^^ not found in this scope
   |
help: possible candidates are found in other modules, you can import them into scope
   |
15 | use core::marker::PhantomData;
   |
15 | use serde::export::PhantomData;
   |

error: aborting due to 6 previous errors

Some errors have detailed explanations: E0412, E0425, E0432, E0433.
For more information about an error, try `rustc --explain E0412`.
error: could not compile `hex`.
warning: build failed, waiting for other jobs to finish...
error: build failed

It may not be important shrug. Though I suspect this can be fixed.