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< li > < a id = "all-types" href = "all.html" > All Items< / a > < / li > < / ul > < section > < ul class = "block" > < li > < a href = "#derives" > Derive Macros< / a > < / li > < / ul > < / section > < / div > < / nav > < div class = "sidebar-resizer" > < / div >
< main > < div class = "width-limiter" > < nav class = "sub" > < form class = "search-form" > < span > < / span > < div id = "sidebar-button" tabindex = "-1" > < a href = "../thiserror/all.html" title = "show sidebar" > < / a > < / div > < input class = "search-input" name = "search" aria-label = "Run search in the documentation" autocomplete = "off" spellcheck = "false" placeholder = "Click or press ‘ ’ ‘ ’ type = "search" > < div id = "help-button" tabindex = "-1" > < a href = "../help.html" title = "help" > ?< / a > < / div > < div id = "settings-menu" tabindex = "-1" > < a href = "../settings.html" title = "settings" > < img width = "22" height = "22" alt = "Change settings" src = "../static.files/wheel-7b819b6101059cd0.svg" > < / a > < / div > < / form > < / nav > < section id = "main-content" class = "content" > < div class = "main-heading" > < h1 > Crate < a class = "mod" href = "#" > thiserror< / a > < button id = "copy-path" title = "Copy item path to clipboard" > < img src = "../static.files/clipboard-7571035ce49a181d.svg" width = "19" height = "18" alt = "Copy item path" > < / button > < / h1 > < span class = "out-of-band" > < a class = "src" href = "../src/thiserror/lib.rs.html#1-260" > source< / a > · < button id = "toggle-all-docs" title = "collapse all docs" > [< span > − < / span > ]< / button > < / span > < / div > < details class = "toggle top-doc" open > < summary class = "hideme" > < span > Expand description< / span > < / summary > < div class = "docblock" > < p > < a href = "https://github.com/dtolnay/thiserror" > < img src = "https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github" alt = "github" / > < / a > < a href = "https://crates.io/crates/thiserror" > < img src = "https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust" alt = "crates-io" / > < / a > < a href = "https://docs.rs/thiserror" > < img src = "https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" alt = "docs-rs" / > < / a > < / p >
< br >
< p > This library provides a convenient derive macro for the standard library’
< a href = "https://doc.rust-lang.org/std/error/trait.Error.html" > < code > std::error::Error< / code > < / a > trait.< / p >
< br >
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< h2 id = "example" > < a class = "doc-anchor" href = "#example" > §< / a > Example< / h2 >
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< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "kw" > use < / span > thiserror::Error;
< span class = "attr" > #[derive(Error, Debug)]
< / span > < span class = "kw" > pub enum < / span > DataStoreError {
< span class = "attr" > #[error(< span class = "string" > "data store disconnected"< / span > )]
< / span > Disconnect(< span class = "attr" > #[from] < / span > io::Error),
< span class = "attr" > #[error(< span class = "string" > "the data for key `{0}` is not available"< / span > )]
< / span > Redaction(String),
< span class = "attr" > #[error(< span class = "string" > "invalid header (expected {expected:?}, found {found:?})"< / span > )]
< / span > InvalidHeader {
expected: String,
found: String,
},
< span class = "attr" > #[error(< span class = "string" > "unknown data store error"< / span > )]
< / span > Unknown,
}< / code > < / pre > < / div >
< br >
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< h2 id = "details" > < a class = "doc-anchor" href = "#details" > §< / a > Details< / h2 >
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< ul >
< li >
< p > Thiserror deliberately does not appear in your public API. You get the
same thing as if you had written an implementation of < code > std::error::Error< / code >
by hand, and switching from handwritten impls to thiserror or vice versa
is not a breaking change.< / p >
< / li >
< li >
< p > Errors may be enums, structs with named fields, tuple structs, or unit
structs.< / p >
< / li >
< li >
< p > A < code > Display< / code > impl is generated for your error if you provide
< code > #[error(" ..." )]< / code > messages on the struct or each variant of your enum, as
shown above in the example.< / p >
< p > The messages support a shorthand for interpolating fields from the error.< / p >
< ul >
< li > < code > #[error(" {var}" )]< / code > ⟶ < code > write!(" {}" , self.var)< / code > < / li >
< li > < code > #[error(" {0}" )]< / code > ⟶ < code > write!(" {}" , self.0)< / code > < / li >
< li > < code > #[error(" {var:?}" )]< / code > ⟶ < code > write!(" {:?}" , self.var)< / code > < / li >
< li > < code > #[error(" {0:?}" )]< / code > ⟶ < code > write!(" {:?}" , self.0)< / code > < / li >
< / ul >
< p > These shorthands can be used together with any additional format args,
which may be arbitrary expressions. For example:< / p >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "attr" > #[derive(Error, Debug)]
< / span > < span class = "kw" > pub enum < / span > Error {
< span class = "attr" > #[error(< span class = "string" > "invalid rdo_lookahead_frames {0} (expected < {})"< / span > , i32::MAX)]
< / span > InvalidLookahead(u32),
}< / code > < / pre > < / div >
< p > If one of the additional expression arguments needs to refer to a field of
the struct or enum, then refer to named fields as < code > .var< / code > and tuple fields
as < code > .0< / code > .< / p >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "attr" > #[derive(Error, Debug)]
< / span > < span class = "kw" > pub enum < / span > Error {
< span class = "attr" > #[error(< span class = "string" > "first letter must be lowercase but was {:?}"< / span > , first_char(.< span class = "number" > 0< / span > ))]
< / span > WrongCase(String),
< span class = "attr" > #[error(< span class = "string" > "invalid index {idx}, expected at least {} and at most {}"< / span > , .limits.lo, .limits.hi)]
< / span > OutOfBounds { idx: usize, limits: Limits },
}< / code > < / pre > < / div >
< / li >
< li >
< p > A < code > From< / code > impl is generated for each variant containing a < code > #[from]< / code >
attribute.< / p >
< p > 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
< code > From< / code > impl if there is a field for it.< / p >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "attr" > #[derive(Error, Debug)]
< / span > < span class = "kw" > pub enum < / span > MyError {
Io {
< span class = "attr" > #[from]
< / span > source: io::Error,
backtrace: Backtrace,
},
}< / code > < / pre > < / div >
< / li >
< li >
< p > The Error trait’ < code > source()< / code > method is implemented to return whichever
field has a < code > #[source]< / code > attribute or is named < code > source< / code > , if any. This is
for identifying the underlying lower level error that caused your error.< / p >
< p > The < code > #[from]< / code > attribute always implies that the same field is < code > #[source]< / code > ,
so you don’ < / p >
< p > Any error type that implements < code > std::error::Error< / code > or dereferences to < code > dyn std::error::Error< / code > will work as a source.< / p >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "attr" > #[derive(Error, Debug)]
< / span > < span class = "kw" > pub struct < / span > MyError {
msg: String,
< span class = "attr" > #[source] < / span > < span class = "comment" > // optional if field name is `source`
< / span > source: anyhow::Error,
}< / code > < / pre > < / div >
< / li >
< li >
< p > The Error trait’ < code > provide()< / code > method is implemented to provide whichever
field has a type named < code > Backtrace< / code > , if any, as a
< code > std::backtrace::Backtrace< / code > .< / p >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "kw" > use < / span > std::backtrace::Backtrace;
< span class = "attr" > #[derive(Error, Debug)]
< / span > < span class = "kw" > pub struct < / span > MyError {
msg: String,
backtrace: Backtrace, < span class = "comment" > // automatically detected
< / span > }< / code > < / pre > < / div >
< / li >
< li >
< p > If a field is both a source (named < code > source< / code > , or has < code > #[source]< / code > or
< code > #[from]< / code > attribute) < em > and< / em > is marked < code > #[backtrace]< / code > , then the Error
trait’ < code > provide()< / code > method is forwarded to the source’ < code > provide< / code > so that
both layers of the error share the same backtrace.< / p >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "attr" > #[derive(Error, Debug)]
< / span > < span class = "kw" > pub enum < / span > MyError {
Io {
< span class = "attr" > #[backtrace]
< / span > source: io::Error,
},
}< / code > < / pre > < / div >
< / li >
< li >
< p > Errors may use < code > error(transparent)< / code > 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.< / p >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "attr" > #[derive(Error, Debug)]
< / span > < span class = "kw" > pub enum < / span > MyError {
...
< span class = "attr" > #[error(transparent)]
< / span > Other(< span class = "attr" > #[from] < / span > anyhow::Error), < span class = "comment" > // source and Display delegate to anyhow::Error
< / span > }< / code > < / pre > < / div >
< p > 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’ < / p >
< div class = "example-wrap" > < pre class = "rust rust-example-rendered" > < code > < span class = "comment" > // PublicError is public, but opaque and easy to keep compatible.
< / span > < span class = "attr" > #[derive(Error, Debug)]
#[error(transparent)]
< / span > < span class = "kw" > pub struct < / span > PublicError(< span class = "attr" > #[from] < / span > ErrorRepr);
< span class = "kw" > impl < / span > PublicError {
< span class = "comment" > // Accessors for anything we do want to expose publicly.
< / span > }
< span class = "comment" > // Private and free to change across minor version of the crate.
< / span > < span class = "attr" > #[derive(Error, Debug)]
< / span > < span class = "kw" > enum < / span > ErrorRepr {
...
}< / code > < / pre > < / div >
< / li >
< li >
< p > See also the < a href = "https://github.com/dtolnay/anyhow" > < code > anyhow< / code > < / a > library for a convenient single error type to use
in application code.< / p >
< / li >
< / ul >
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< / div > < / details > < h2 id = "derives" class = "section-header" > Derive Macros< a href = "#derives" class = "anchor" > §< / a > < / h2 > < ul class = "item-table" > < li > < div class = "item-name" > < a class = "derive" href = "derive.Error.html" title = "derive thiserror::Error" > Error< / a > < / div > < / li > < / ul > < / section > < / div > < / main > < / body > < / html >
