Using postcard with serde

1

Hi, I am trying to use serde with postcard to serialize and deserialze structs. But I am getting an error type annotations needed for heapless::vec::Vec<u8, _> when calling postcard::to_vec.

use postcard;

struct Test {
  field1: i32,
  field2: i32
}

fn serialize() {
  let data = Test { field1: 10, field2: 10 };
  let output = postcard::to_vec(&data).unwrap; // error: type annotations needed for heapless::vec::Vec<u8, _>
  println!("serialized {:?}", output);
  let out = postcard::from_bytes(output.deref());
  println!("serialized {:?}", output);
}

According to postcard documentation use heapless::Vec should do it, but postcard does not expose it and installing healpless as a separate dependency gives mismatched error when I use it.

I tried postcard with use-std feature it does not work

cargo check details:

error[E0284]: type annotations needed for `heapless::vec::Vec<u8, _>`
   --> src/bin_codec/encode.rs:15:9
    |
15  |     let output= postcard::to_vec(transit_packet).unwrap();
    |         ^^^^^^  -------------------------------- type must be known at this point
    |
note: required by a const generic parameter in `to_vec`
   --> /Users/machine/.cargo/registry/src/index.crates.io-6f17d22bba15001f/postcard-1.1.1/src/ser/mod.rs:154:18
    |
154 | pub fn to_vec<T, const B: usize>(value: &T) -> Result<Vec<u8, B>>
    |                  ^^^^^^^^^^^^^^ required by this const generic parameter in `to_vec`
help: consider giving `output` an explicit type, where the value of const parameter `B` is specified
    |
15  |     let output: heapless::vec::Vec<u8, B>= postcard::to_vec(transit_packet).unwrap();
    |               +++++++++++++++++++++++++++

error[E0284]: type annotations needed for `heapless::vec::Vec<u8, _>`
  --> src/bin_codec/encode.rs:15:9
   |
15 |     let output= postcard::to_vec(transit_packet).unwrap();
   |         ^^^^^^
16 |     println!("serialized {:?}", output);
17 |     let out = postcard::from_bytes(output.deref()).unwrap();
   |                                           ----- type must be known at this point
   |
   = note: required for `heapless::vec::Vec<u8, _>` to implement `Deref`
help: consider giving `output` an explicit type, where the value of const parameter `B` is specified
   |
15 |     let output: heapless::vec::Vec<u8, B>= postcard::to_vec(transit_packet).unwrap();
   |               +++++++++++++++++++++++++++

error[E0284]: type annotations needed for `heapless::vec::Vec<u8, _>`
   --> src/bin_codec/encode.rs:15:9
    |
15  |     let output= postcard::to_vec(transit_packet).unwrap();
    |         ^^^^^^
16  |     println!("serialized {:?}", output);
    |                          ---- type must be known at this point
    |
    = note: required for `heapless::vec::Vec<u8, _>` to implement `Debug`
note: required by a bound in `core::fmt::rt::Argument::<'a>::new_debug`
   --> /Users/machine/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/fmt/rt.rs:117:29
    |
117 |     pub fn new_debug<'b, T: Debug>(x: &'b T) -> Argument<'b> {
    |                             ^^^^^ required by this bound in `Argument::<'a>::new_debug`
help: consider giving `output` an explicit type, where the value of const parameter `B` is specified
    |
15  |     let output: heapless::vec::Vec<u8, B>= postcard::to_vec(transit_packet).unwrap();
    |               +++++++++++++++++++++++++++
2

Please post the full text of the error as output by the cargo check command, including all notes and source code snippets. This will help us understand the problem.

3

updated the post

4

Try doing this:

+ use heapless::Vec;

- let output = postcard::to_vec(&data).unwrap;
+ let output: Vec<u8, 32> = postcard::to_vec(&data).unwrap;
5

Yes, that's exactly what I did to fix it. Thanks.

I am getting a new error at
let out = postcard::from_bytes(output.deref()).unwrap();
Error:

cannot satisfy `_: Deserialize<'_>`
the following types implement trait `Deserialize<'de>`:```

I have derived the `Serialize` and `Deserialize` traits on `Test`.

Any idea about it?
6

You also need to hint the compiler on what type you are trying to deserialize into:

let out: Test = postcard::from_bytes(output.deref()).unwrap();
7

Thanks for answering a trivial question, I'm new to rust

1 Like
8

No worries. Welcome!