Announcing err-per-field crate


This crate provides a more fine-grained control over field-level error handling. Project repository: Evian-Zhang/err-per-field

When we are writing a huge complex project, we may gather a lot of information into a single struct, such as:

struct GatheredInformation {
    info1: Info1,
    info2: Info2,
    info3: Info3,

fn generate_info1() -> Result<Info1, Error1> { /* ... */ }
fn generate_info2() -> Option<Info2> { /* ... */ }
fn generate_info3() -> Info3 { /* ... */ }

However, we may encounter Results or Options when generating such subinfos. The core logic is to deal with GatheredInformation if all subinfos are retrieved successfully. If some fields are failed to retrieve, let's say info2, we should inform the user that failure, and may provide other successfully retrieved infos (such as info1 and info3) to user.

To handle this gracefully, dynamically-typed languages such as Javascript can be rather straight forward:

const gathered_information = gather_information();
if (gathered_information.info1 && gathered_information.info2) {
} else {

For Rust, a statically-typed langugage, things become complicated:

struct GatheredInformationWrapper {
    info1: Result<Info1, Error1>,
    info2: Option<Info2>,
    info3: Info3,

let gathered_information_wrapper = gather_information();
if let GatheredInformationWrapper {
    info1: Ok(info1),
    info2: Some(info2),
} = gathered_information_wrappper {
    let gathered_information = GatheredInformation {
} else {

We must:

  1. Define a wrapper struct of GatheredInformation, whose fields are Results or Options according to the generating functions;
  2. Build the real GatheredInformation after validation checks;
  3. Pass the real gathered information to the downstream functions.

This is a boring process and the extra wrapper structs are very distracting.

To handle this problem, this crate provides a rather simple and graceful method.


use err_per_field::{ErrPerField, Wrapper};

struct AnError;

struct Foo {
    pub bar1: u8,
    pub bar2: u16,
    #[err_per_field(maybe_error = "AnError")]
    pub bar3: u32,
    pub bar4: u64,

fn baz1() -> u8 { 0 }
fn baz2() -> Option<u16> { None }
fn baz3() -> Result<u32, AnError> { Err(AnError) }
fn baz4() -> u64 { 0 }

fn generate_foo() -> Wrapper<Foo> {
    let bar1 = baz1();
    let bar2 = baz2();
    let bar3 = baz3();
    let bar4 = baz4();
    Wrapper::<Foo> {

// If we write this in another file, we can simply import `Wrapper` type
// from `err_per_field` crate.
let result: Result<Foo, Wrapper<Foo>> = generate_foo().try_into();
match result {
    Ok(foo) => {
        // `foo` is of type `Foo`, and you can directly use it without any worries
    Err(foo_wrapper) => {
        // `foo_wrapper` has the same fields as `foo`, but in different types,
        // such as `foo_wrapper.bar2` is of type `Option<u16>` and
        // `foo_wrapper.bar3` is of type `Result<u32, AnError>`

By using this crate, we don't need to write wrapper structs by our own, and we can focus on the core logic of our product. All we need to do is:

  1. Derive the core struct Foo with macro ErrPerField and mark fields which may be Results or Options with attributes;
  2. When generating the struct Foo, use Wrapper<Foo> as return type. This wrapper struct has the same field names with Foo, and their types may be Results or Options as we mark when declaration;
  3. When using the generated struct, we can validate its fields by calling try_into, and if it is valid, the result is Ok and the inner type is Foo, and we can directly deal with it as the core logic; if it is invalid, the result is Err and the inner type is Foo's wrapper, and we can check each fields for error handling.


When we use #[derive(ErrPerField)] on a struct Foo, the macro generates a wrapper struct and we can use Wrapper<Foo> to access it. This wrapper struct has the same field names as Foo, and for a field bar, if has type Bar, then's type is:

  • Result<Bar, AnError> if there is a field-level attribute #[err_per_field(maybe_error = "AnError")];
  • Option<Bar> if there is a field-level attribute #[err_per_field(maybe_none)];
  • Bar if otherwise.

Foo will automatically implements TryFrom<Wrapper<Foo>>. If there is a field being Err or None, the conversion fails and the result is Err, the inner value is foo's wrapper itself without any change; otherwise the conversion succeeds and the result is Ok, the inner type is the final foo, with all fields extracted from Results and Options.

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