use of unstable library feature 'rustc_private'

[DavidM-D]

If you try and compile the following code in a rust project without serde as a dependency:

use candid::Deserialize;

#[derive(Deserialize)]
pub struct A();

You will get the following error

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
 --> rust_canisters/dfn_http/src/lib.rs:3:10
  |
3 | #[derive(Deserialize)]
  |          ^^^^^^^^^^^
  |
  = note: see issue #27812 <https://github.com/rust-lang/rust/issues/27812> for more information
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.

This is the case for any struct, but it is resolved as soon as you add serde as a dependency.

This is true for 0.25 and master at the time of writing.

[chenyan-dfinity]

There is no supported way of doing this without adding serde as a dependency, see serde-rs/serde#1465. I will update the doc to reflect this.