#enum #error #struct #types #thiserror

thiserror_core2

derive(Error) (no_std-enabled using core2)

1 stable release

2.0.0 Feb 3, 2022
1.0.30 Jan 29, 2022
1.0.23 Feb 13, 2021

#991 in Rust patterns

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423 downloads per month
Used in flatbuffers

MIT/Apache

23KB
222 lines

thiserror_core2

thiserror but uses core2 for no_std support.


lib.rs:

githubcrates-iodocs-rs


This library provides a convenient derive macro for the standard library's core2::error::Error trait.


Example

# use core2::io;
use thiserror_core2::Error;

#[derive(Error, Debug)]
pub enum DataStoreError {
    #[error("data store disconnected")]
    Disconnect(#[from] io::Error),
    #[error("the data for key `{0}` is not available")]
    Redaction(String),
    #[error("invalid header (expected {expected:?}, found {found:?})")]
    InvalidHeader {
        expected: String,
        found: String,
    },
    #[error("unknown data store error")]
    Unknown,
}

Details

  • Thiserror deliberately does not appear in your public API. You get the same thing as if you had written an implementation of core2::error::Error by hand, and switching from handwritten impls to thiserror or vice versa is not a breaking change.

  • Errors may be enums, structs with named fields, tuple structs, or unit structs.

  • A Display impl is generated for your error if you provide #[error("...")] messages on the struct or each variant of your enum, as shown above in the example.

    The messages support a shorthand for interpolating fields from the error.

    • #[error("{var}")] ⟶ write!("{}", self.var)
    • #[error("{0}")] ⟶ write!("{}", self.0)
    • #[error("{var:?}")] ⟶ write!("{:?}", self.var)
    • #[error("{0:?}")] ⟶ write!("{:?}", self.0)

    These shorthands can be used together with any additional format args, which may be arbitrary expressions. For example:

    # use core::i32;
    # use thiserror_core2::Error;
    #
    #[derive(Error, Debug)]
    pub enum Error {
        #[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)]
        InvalidLookahead(u32),
    }
    

    If one of the additional expression arguments needs to refer to a field of the struct or enum, then refer to named fields as .var and tuple fields as .0.

    # use thiserror_core2::Error;
    #
    # fn first_char(s: &String) -> char {
    #     s.chars().next().unwrap()
    # }
    #
    # #[derive(Debug)]
    # struct Limits {
    #     lo: usize,
    #     hi: usize,
    # }
    #
    #[derive(Error, Debug)]
    pub enum Error {
        #[error("first letter must be lowercase but was {:?}", first_char(.0))]
        WrongCase(String),
        #[error("invalid index {idx}, expected at least {} and at most {}", .limits.lo, .limits.hi)]
        OutOfBounds { idx: usize, limits: Limits },
    }
    
  • A From impl is generated for each variant containing a #[from] attribute.

    Note that the variant must not contain any other fields beyond the source error and possibly a backtrace. A backtrace is captured from within the From impl if there is a field for it.

    # const IGNORE: &str = stringify! {
    #[derive(Error, Debug)]
    pub enum MyError {
        Io {
            #[from]
            source: io::Error,
            backtrace: Backtrace,
        },
    }
    # };
    
  • The Error trait's source() method is implemented to return whichever field has a #[source] attribute or is named source, if any. This is for identifying the underlying lower level error that caused your error.

    The #[from] attribute always implies that the same field is #[source], so you don't ever need to specify both attributes.

    Any error type that implements core2::error::Error or dereferences to dyn core2::error::Error will work as a source.

    # use core::fmt::{self, Display};
    # use thiserror_core2::Error;
    # use core2::io::Error;
    #
    #[derive(Error, Debug)]
    pub struct MyError {
        msg: String,
        #[source]  // optional if field name is `source`
        source: Error,
    }
    #
    # impl Display for MyError {
    #     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
    #         unimplemented!()
    #     }
    # }
    
  • The Error trait's backtrace() method is implemented to return whichever field has a type named Backtrace, if any.

    # const IGNORE: &str = stringify! {
    use core2::backtrace::Backtrace;
    
    #[derive(Error, Debug)]
    pub struct MyError {
        msg: String,
        backtrace: Backtrace,  // automatically detected
    }
    # };
    
  • If a field is both a source (named source, or has #[source] or #[from] attribute) and is marked #[backtrace], then the Error trait's backtrace() method is forwarded to the source's backtrace.

    # const IGNORE: &str = stringify! {
    #[derive(Error, Debug)]
    pub enum MyError {
        Io {
            #[backtrace]
            source: io::Error,
        },
    }
    # };
    
  • Errors may use error(transparent) to forward the source and Display methods straight through to an underlying error without adding an additional message. This would be appropriate for enums that need an "anything else" variant.

    # use thiserror_core2::Error;
    # use core2::io::Error;
    #
    #[derive(Error, Debug)]
    pub enum MyError {
        # /*
        ...
        # */
    
        #[error(transparent)]
        Other(#[from] Error),  // source and Display delegate to Error
    }
    

Dependencies

~0.5–1MB
~21K SLoC