Rust：为什么我不能在这个库中使用 anyhow::Context？

Question

I'm trying to simplify some code which uses the psa_crypto library .

More in details I'm using their first encryption example from their docs from their docs:

use psa_crypto::types::algorithm::{Aead, AeadWithDefaultLengthTag};
use psa_crypto::types::key::{Attributes, Type, Lifetime, Policy, UsageFlags};
use psa_crypto::operations::{key_management, aead};
let alg = Aead::AeadWithDefaultLengthTag(AeadWithDefaultLengthTag::Ccm);
let mut usage_flags: UsageFlags = Default::default();
usage_flags.set_encrypt();
let attributes = Attributes {
key_type: Type::Aes,
     bits: 0,
     lifetime: Lifetime::Volatile,
     policy: Policy {
         usage_flags,
         permitted_algorithms: alg.into(),
     },
};
psa_crypto::init().unwrap();
let my_key = key_management::import(attributes, None, &KEY_DATA).unwrap();
let output_buffer_size = attributes.aead_encrypt_output_size(alg.into(), INPUT_DATA.len()).unwrap();
let mut output_buffer = vec![0; output_buffer_size];
let length = aead::encrypt(my_key, alg, &NONCE, &ADDITIONAL_DATA, &INPUT_DATA, &mut output_buffer).unwrap();
output_buffer.resize(length, 0);

I put this code inside a function using anyhow::Result I would like to replace the unwrap s with some more idiomatic code. Unfortunately if I try to change the first unwrap with `.context("error") I get an error:

let my_key = key_management::import(attributes, None, &KEY_DATA).context("error")?;
                                                                 ^^^^^^^ method cannot be called on `Result<Id, psa_crypto::types::status::Error>` due to unsatisfied trait bounds


    |
504 | pub enum Result<T, E> {
    | --------------------- doesn't satisfy `_: anyhow::Context<Id, psa_crypto::types::status::Error>`
    |
   ::: /home/fedboz01/.cargo/registry/src/github.com-1ecc6299db9ec823/psa-crypto-0.9.1/src/types/status.rs:50:1
    |
50  | pub enum Error {
    | -------------- doesn't satisfy `_: anyhow::context::ext::StdError`
    |
    = note: the following trait bounds were not satisfied:
            `psa_crypto::types::status::Error: anyhow::context::ext::StdError`
            which is required by `Result<Id, psa_crypto::types::status::Error>: anyhow::Context<Id, psa_crypto::types::status::Error>`

What does this error mean? How can I change this code to make it work?

Answer 1

anyhow::Error requires that the error it wraps implement std::error::Error . psa_crypto::types::status::Error does not implement that trait, so in turn, it does not implement the Context extension trait.

This part of the error message hints at this, though it's a bit obscured, since it prints anyhow's internal alias for std::error::Error :

50  | pub enum Error {
    | -------------- doesn't satisfy `_: anyhow::context::ext::StdError`
    |
    = note: the following trait bounds were not satisfied:
            `psa_crypto::types::status::Error: anyhow::context::ext::StdError`
            which is required by `Result<Id, psa_crypto::types::status::Error>: anyhow::Context<Id, psa_crypto::types::status::Error>`

This seems like an oversight in the psa_crypto library. It may be worth filing a bug asking them to implement that trait. As a workaround, you have a few options:

• Since psa_crypto::types::status::Error implements std::fmt::Debug , you can use this to get a String error message: psa_crypto::init().map_err(|e| format!("{:?}", e).context("While initializing")
• A prettier solution would be to implement a wrapper type that does implement Error (and its requirement, Display ).