This library provides a convenient derive macro for the standard library's
 = "1.0"
Compiler support: requires rustc 1.31+
Thiserror deliberately does not appear in your public API. You get the same thing as if you had written an implementation of
std::error::Errorby 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.
Displayimpl 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.
These shorthands can be used together with any additional format args, which may be arbitrary expressions. For example:
If one of the additional expression arguments needs to refer to a field of the struct or enum, then refer to named fields as
.varand tuple fields as
Fromimpl is generated for each variant containing a
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
Fromimpl if there is a field for it.
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.
#[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
std::error::Erroror dereferences to
dyn std::error::Errorwill work as a source.
The Error trait's
provide()method is implemented to provide whichever field has a type named
Backtrace, if any, as a
If a field is both a source (named
source, or has
#[from]attribute) and is marked
#[backtrace], then the Error trait's
provide()method is forwarded to the source's
provideso that both layers of the error share the same backtrace.
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.
Another use case is hiding implementation details of an error representation behind an opaque error type, so that the representation is able to evolve without breaking the crate's public API.
// PublicError is public, but opaque and easy to keep compatible. ; // Private and free to change across minor version of the crate.
See also the
anyhowlibrary for a convenient single error type to use in application code.
Comparison to anyhow
Use thiserror if you care about designing your own dedicated error type(s) so that the caller receives exactly the information that you choose in the event of failure. This most often applies to library-like code. Use Anyhow if you don't care what error type your functions return, you just want it to be easy. This is common in application-like code.