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#![deny(missing_docs)]
#![cfg_attr(not(any(feature = "std", test)), no_std)]
#![cfg_attr(feature = "unstable-backtraces-impl-std", feature(backtrace))]
//! # SNAFU
//!
//! SNAFU is a library to easily assign underlying errors into
//! domain-specific errors while adding context. For detailed
//! information, please see the [user's guide](guide).
//!
//! ## Quick example
//!
//! This example mimics a (very poor) authentication process that
//! opens a file, writes to a file, and checks the user's ID. While
//! two of our operations involve an [`io::Error`](std::io::Error),
//! these are different conceptual errors to us.
//!
//! SNAFU creates a *context selector* type for each variant in the
//! error enum. These context selectors are used with the
//! [`context`](ResultExt::context) method to provide ergonomic error
//! handling.
//!
//! ```rust
//! use snafu::{ensure, Backtrace, ErrorCompat, ResultExt, Snafu};
//! use std::{
//! fs,
//! path::{Path, PathBuf},
//! };
//!
//! #[derive(Debug, Snafu)]
//! enum Error {
//! #[snafu(display("Could not open config from {}: {}", filename.display(), source))]
//! OpenConfig {
//! filename: PathBuf,
//! source: std::io::Error,
//! },
//! #[snafu(display("Could not save config to {}: {}", filename.display(), source))]
//! SaveConfig {
//! filename: PathBuf,
//! source: std::io::Error,
//! },
//! #[snafu(display("The user id {} is invalid", user_id))]
//! UserIdInvalid { user_id: i32, backtrace: Backtrace },
//! }
//!
//! type Result<T, E = Error> = std::result::Result<T, E>;
//!
//! fn log_in_user<P>(config_root: P, user_id: i32) -> Result<bool>
//! where
//! P: AsRef<Path>,
//! {
//! let config_root = config_root.as_ref();
//! let filename = &config_root.join("config.toml");
//!
//! let config = fs::read(filename).context(OpenConfig { filename })?;
//! // Perform updates to config
//! fs::write(filename, config).context(SaveConfig { filename })?;
//!
//! ensure!(user_id == 42, UserIdInvalid { user_id });
//!
//! Ok(true)
//! }
//!
//! # const CONFIG_DIRECTORY: &str = "/does/not/exist";
//! # const USER_ID: i32 = 0;
//! # #[cfg(not(feature = "backtraces-impl-backtrace-crate"))]
//! fn log_in() {
//! match log_in_user(CONFIG_DIRECTORY, USER_ID) {
//! Ok(true) => println!("Logged in!"),
//! Ok(false) => println!("Not logged in!"),
//! Err(e) => {
//! eprintln!("An error occurred: {}", e);
//! if let Some(backtrace) = ErrorCompat::backtrace(&e) {
//! println!("{}", backtrace);
//! }
//! }
//! }
//! }
//! ```
#[cfg(all(
not(feature = "backtraces"),
not(feature = "backtraces-impl-backtrace-crate"),
not(feature = "unstable-backtraces-impl-std"),
))]
mod backtrace_inert;
#[cfg(all(
not(feature = "backtraces"),
not(feature = "backtraces-impl-backtrace-crate"),
not(feature = "unstable-backtraces-impl-std"),
))]
pub use crate::backtrace_inert::*;
#[cfg(all(
feature = "backtraces",
not(feature = "backtraces-impl-backtrace-crate"),
not(feature = "unstable-backtraces-impl-std"),
))]
mod backtrace_shim;
#[cfg(all(
feature = "backtraces",
not(feature = "backtraces-impl-backtrace-crate"),
not(feature = "unstable-backtraces-impl-std"),
))]
pub use crate::backtrace_shim::*;
#[cfg(feature = "backtraces-impl-backtrace-crate")]
pub use backtrace::Backtrace;
#[cfg(feature = "unstable-backtraces-impl-std")]
pub use std::backtrace::Backtrace;
#[cfg(feature = "futures-01")]
pub mod futures01;
#[cfg(feature = "futures")]
pub mod futures;
pub use snafu_derive::Snafu;
#[cfg(feature = "guide")]
macro_rules! generate_guide {
(pub mod $name:ident; $($rest:tt)*) => {
generate_guide!(@gen ".", pub mod $name { } $($rest)*);
};
(pub mod $name:ident { $($children:tt)* } $($rest:tt)*) => {
generate_guide!(@gen ".", pub mod $name { $($children)* } $($rest)*);
};
(@gen $prefix:expr, ) => {};
(@gen $prefix:expr, pub mod $name:ident; $($rest:tt)*) => {
generate_guide!(@gen $prefix, pub mod $name { } $($rest)*);
};
(@gen $prefix:expr, @code pub mod $name:ident; $($rest:tt)*) => {
pub mod $name;
generate_guide!(@gen $prefix, $($rest)*);
};
(@gen $prefix:expr, pub mod $name:ident { $($children:tt)* } $($rest:tt)*) => {
doc_comment::doc_comment! {
include_str!(concat!($prefix, "/", stringify!($name), ".md")),
pub mod $name {
generate_guide!(@gen concat!($prefix, "/", stringify!($name)), $($children)*);
}
}
generate_guide!(@gen $prefix, $($rest)*);
};
}
#[cfg(feature = "guide")]
generate_guide! {
pub mod guide {
pub mod attributes;
pub mod comparison {
pub mod failure;
}
pub mod compatibility;
pub mod feature_flags;
pub mod generics;
pub mod opaque;
pub mod philosophy;
pub mod structs;
pub mod the_macro;
pub mod upgrading;
@code pub mod examples;
}
}
doc_comment::doctest!("../README.md", readme_tests);
#[cfg(any(feature = "std", test))]
#[doc(hidden)]
pub use std::error::Error;
#[cfg(not(any(feature = "std", test)))]
mod no_std_error;
#[cfg(not(any(feature = "std", test)))]
#[doc(hidden)]
pub use no_std_error::Error;
/// Ensure a condition is true. If it is not, return from the function
/// with an error.
///
/// ```rust
/// use snafu::{ensure, Snafu};
///
/// #[derive(Debug, Snafu)]
/// enum Error {
/// InvalidUser { user_id: i32 },
/// }
///
/// fn example(user_id: i32) -> Result<(), Error> {
/// ensure!(user_id > 0, InvalidUser { user_id });
/// // After this point, we know that `user_id` is positive.
/// let user_id = user_id as u32;
/// Ok(())
/// }
/// ```
#[macro_export]
macro_rules! ensure {
($predicate:expr, $context_selector:expr $(,)*) => {
if !$predicate {
return $context_selector
.fail()
.map_err(::core::convert::Into::into);
}
};
}
/// Additions to [`Result`](std::result::Result).
pub trait ResultExt<T, E>: Sized {
/// Extend a [`Result`]'s error with additional context-sensitive information.
///
/// [`Result`]: std::result::Result
///
/// ```rust
/// use snafu::{ResultExt, Snafu};
///
/// #[derive(Debug, Snafu)]
/// enum Error {
/// Authenticating {
/// user_name: String,
/// user_id: i32,
/// source: ApiError,
/// },
/// }
///
/// fn example() -> Result<(), Error> {
/// another_function().context(Authenticating {
/// user_name: "admin",
/// user_id: 42,
/// })?;
/// Ok(())
/// }
///
/// # type ApiError = Box<dyn std::error::Error>;
/// fn another_function() -> Result<i32, ApiError> {
/// /* ... */
/// # Ok(42)
/// }
/// ```
///
/// Note that the context selector will call
/// [`Into::into`](std::convert::Into::into) on each field, so the types
/// are not required to exactly match.
fn context<C, E2>(self, context: C) -> Result<T, E2>
where
C: IntoError<E2, Source = E>,
E2: Error + ErrorCompat;
/// Extend a [`Result`][]'s error with lazily-generated context-sensitive information.
///
/// [`Result`]: std::result::Result
///
/// ```rust
/// use snafu::{ResultExt, Snafu};
///
/// #[derive(Debug, Snafu)]
/// enum Error {
/// Authenticating {
/// user_name: String,
/// user_id: i32,
/// source: ApiError,
/// },
/// }
///
/// fn example() -> Result<(), Error> {
/// another_function().with_context(|| Authenticating {
/// user_name: "admin".to_string(),
/// user_id: 42,
/// })?;
/// Ok(())
/// }
///
/// # type ApiError = std::io::Error;
/// fn another_function() -> Result<i32, ApiError> {
/// /* ... */
/// # Ok(42)
/// }
/// ```
///
/// Note that this *may not* be needed in many cases because the context
/// selector will call [`Into::into`](std::convert::Into::into) on each
/// field.
fn with_context<F, C, E2>(self, context: F) -> Result<T, E2>
where
F: FnOnce() -> C,
C: IntoError<E2, Source = E>,
E2: Error + ErrorCompat;
#[doc(hidden)]
#[deprecated(since = "0.4.0", note = "use ResultExt::context instead")]
fn eager_context<C, E2>(self, context: C) -> Result<T, E2>
where
C: IntoError<E2, Source = E>,
E2: Error + ErrorCompat,
{
self.context(context)
}
#[doc(hidden)]
#[deprecated(since = "0.4.0", note = "use ResultExt::with_context instead")]
fn with_eager_context<F, C, E2>(self, context: F) -> Result<T, E2>
where
F: FnOnce() -> C,
C: IntoError<E2, Source = E>,
E2: Error + ErrorCompat,
{
self.with_context(context)
}
}
impl<T, E> ResultExt<T, E> for Result<T, E> {
fn context<C, E2>(self, context: C) -> Result<T, E2>
where
C: IntoError<E2, Source = E>,
E2: Error + ErrorCompat,
{
self.map_err(|error| context.into_error(error))
}
fn with_context<F, C, E2>(self, context: F) -> Result<T, E2>
where
F: FnOnce() -> C,
C: IntoError<E2, Source = E>,
E2: Error + ErrorCompat,
{
self.map_err(|error| {
let context = context();
context.into_error(error)
})
}
}
/// A temporary error type used when converting an [`Option`][] into a
/// [`Result`][]
///
/// [`Option`]: std::option::Option
/// [`Result`]: std::result::Result
pub struct NoneError;
/// Additions to [`Option`](std::option::Option).
pub trait OptionExt<T>: Sized {
/// Convert an [`Option`][] into a [`Result`][] with additional
/// context-sensitive information.
///
/// [Option]: std::option::Option
/// [Result]: std::option::Result
///
/// ```rust
/// use snafu::{OptionExt, Snafu};
///
/// #[derive(Debug, Snafu)]
/// enum Error {
/// UserLookup { user_id: i32 },
/// }
///
/// fn example(user_id: i32) -> Result<(), Error> {
/// let name = username(user_id).context(UserLookup { user_id })?;
/// println!("Username was {}", name);
/// Ok(())
/// }
///
/// fn username(user_id: i32) -> Option<String> {
/// /* ... */
/// # None
/// }
/// ```
///
/// Note that the context selector will call
/// [`Into::into`](std::convert::Into::into) on each field, so the types
/// are not required to exactly match.
fn context<C, E>(self, context: C) -> Result<T, E>
where
C: IntoError<E, Source = NoneError>,
E: Error + ErrorCompat;
/// Convert an [`Option`][] into a [`Result`][] with
/// lazily-generated context-sensitive information.
///
/// [`Option`]: std::option::Option
/// [`Result`]: std::result::Result
///
/// ```
/// use snafu::{OptionExt, Snafu};
///
/// #[derive(Debug, Snafu)]
/// enum Error {
/// UserLookup {
/// user_id: i32,
/// previous_ids: Vec<i32>,
/// },
/// }
///
/// fn example(user_id: i32) -> Result<(), Error> {
/// let name = username(user_id).with_context(|| UserLookup {
/// user_id,
/// previous_ids: Vec::new(),
/// })?;
/// println!("Username was {}", name);
/// Ok(())
/// }
///
/// fn username(user_id: i32) -> Option<String> {
/// /* ... */
/// # None
/// }
/// ```
///
/// Note that this *may not* be needed in many cases because the context
/// selector will call [`Into::into`](std::convert::Into::into) on each
/// field.
fn with_context<F, C, E>(self, context: F) -> Result<T, E>
where
F: FnOnce() -> C,
C: IntoError<E, Source = NoneError>,
E: Error + ErrorCompat;
#[doc(hidden)]
#[deprecated(since = "0.4.0", note = "use OptionExt::context instead")]
fn eager_context<C, E>(self, context: C) -> Result<T, E>
where
C: IntoError<E, Source = NoneError>,
E: Error + ErrorCompat,
{
self.context(context).map_err(Into::into)
}
#[doc(hidden)]
#[deprecated(since = "0.4.0", note = "use OptionExt::with_context instead")]
fn with_eager_context<F, C, E>(self, context: F) -> Result<T, E>
where
F: FnOnce() -> C,
C: IntoError<E, Source = NoneError>,
E: Error + ErrorCompat,
{
self.with_context(context).map_err(Into::into)
}
}
impl<T> OptionExt<T> for Option<T> {
fn context<C, E>(self, context: C) -> Result<T, E>
where
C: IntoError<E, Source = NoneError>,
E: Error + ErrorCompat,
{
self.ok_or_else(|| context.into_error(NoneError))
}
fn with_context<F, C, E>(self, context: F) -> Result<T, E>
where
F: FnOnce() -> C,
C: IntoError<E, Source = NoneError>,
E: Error + ErrorCompat,
{
self.ok_or_else(|| context().into_error(NoneError))
}
}
/// Backports changes to the [`Error`](std::error::Error) trait to
/// versions of Rust lacking them.
///
/// It is recommended to always call these methods explicitly so that
/// it is easy to replace usages of this trait when you start
/// supporting a newer version of Rust.
///
/// ```
/// # use snafu::{Snafu, ErrorCompat};
/// # #[derive(Debug, Snafu)] enum Example {};
/// # fn example(error: Example) {
/// ErrorCompat::backtrace(&error); // Recommended
/// error.backtrace(); // Discouraged
/// # }
/// ```
pub trait ErrorCompat {
/// Returns a [`Backtrace`](Backtrace) that may be printed.
fn backtrace(&self) -> Option<&Backtrace> {
None
}
}
impl<'a, E> ErrorCompat for &'a E
where
E: ErrorCompat,
{
fn backtrace(&self) -> Option<&Backtrace> {
(**self).backtrace()
}
}
#[cfg(any(feature = "std", test))]
impl<E> ErrorCompat for Box<E>
where
E: ErrorCompat,
{
fn backtrace(&self) -> Option<&Backtrace> {
(**self).backtrace()
}
}
/// Converts the receiver into an [`Error`][] trait object, suitable
/// for use in [`Error::source`][].
///
/// It is expected that most users of SNAFU will not directly interact
/// with this trait.
///
/// [`Error`]: std::error::Error
/// [`Error::source`]: std::error::Error::source
//
// Given an error enum with multiple types of underlying causes:
//
// ```rust
// enum Error {
// BoxTraitObjectSendSync(Box<dyn error::Error + Send + Sync + 'static>),
// BoxTraitObject(Box<dyn error::Error + 'static>),
// Boxed(Box<io::Error>),
// Unboxed(io::Error),
// }
// ```
//
// This trait provides the answer to what consistent expression can go
// in each match arm:
//
// ```rust
// impl error::Error for Error {
// fn source(&self) -> Option<&(dyn error::Error + 'static)> {
// use Error::*;
//
// let v = match *self {
// BoxTraitObjectSendSync(ref e) => ...,
// BoxTraitObject(ref e) => ...,
// Boxed(ref e) => ...,
// Unboxed(ref e) => ...,
// };
//
// Some(v)
// }
// }
//
// Existing methods like returning `e`, `&**e`, `Borrow::borrow(e)`,
// `Deref::deref(e)`, and `AsRef::as_ref(e)` do not work for various
// reasons.
pub trait AsErrorSource {
/// For maximum effectiveness, this needs to be called as a method
/// to benefit from Rust's automatic dereferencing of method
/// receivers.
fn as_error_source(&self) -> &(dyn Error + 'static);
}
impl AsErrorSource for dyn Error + 'static {
fn as_error_source(&self) -> &(dyn Error + 'static) {
self
}
}
impl AsErrorSource for dyn Error + Send + 'static {
fn as_error_source(&self) -> &(dyn Error + 'static) {
self
}
}
impl AsErrorSource for dyn Error + Sync + 'static {
fn as_error_source(&self) -> &(dyn Error + 'static) {
self
}
}
impl AsErrorSource for dyn Error + Send + Sync + 'static {
fn as_error_source(&self) -> &(dyn Error + 'static) {
self
}
}
impl<T: Error + 'static> AsErrorSource for T {
fn as_error_source(&self) -> &(dyn Error + 'static) {
self
}
}
/// Combines an underlying error with additional information
/// about the error.
///
/// It is expected that most users of SNAFU will not directly interact
/// with this trait.
pub trait IntoError<E>
where
E: Error + ErrorCompat,
{
/// The underlying error
type Source;
/// Combine the information to produce the error
fn into_error(self, source: Self::Source) -> E;
}
/// Construct a backtrace, allowing it to be optional.
pub trait GenerateBacktrace {
/// Generate a new backtrace instance
fn generate() -> Self;
/// Retrieve the optional backtrace
fn as_backtrace(&self) -> Option<&Backtrace>;
}
/// Only create a backtrace when an environment variable is set.
///
/// This looks first for the value of `RUST_LIB_BACKTRACE` then
/// `RUST_BACKTRACE`. If the value is set to `1`, backtraces will be
/// enabled.
///
/// This value will be tested only once per program execution;
/// changing the environment variable after it has been checked will
/// have no effect.
#[cfg(any(feature = "std", test))]
impl GenerateBacktrace for Option<Backtrace> {
fn generate() -> Self {
use std::env;
use std::sync::{
atomic::{AtomicBool, Ordering},
Once,
};
static START: Once = Once::new();
static ENABLED: AtomicBool = AtomicBool::new(false);
START.call_once(|| {
// TODO: What values count as "true"?
let enabled = env::var_os("RUST_LIB_BACKTRACE")
.or_else(|| env::var_os("RUST_BACKTRACE"))
.map_or(false, |v| v == "1");
ENABLED.store(enabled, Ordering::SeqCst);
});
if ENABLED.load(Ordering::SeqCst) {
Some(Backtrace::generate())
} else {
None
}
}
fn as_backtrace(&self) -> Option<&Backtrace> {
self.as_ref()
}
}
#[cfg(feature = "backtraces-impl-backtrace-crate")]
impl GenerateBacktrace for Backtrace {
fn generate() -> Self {
Backtrace::new()
}
fn as_backtrace(&self) -> Option<&Backtrace> {
Some(self)
}
}
#[cfg(feature = "unstable-backtraces-impl-std")]
impl GenerateBacktrace for Backtrace {
fn generate() -> Self {
Backtrace::force_capture()
}
fn as_backtrace(&self) -> Option<&Backtrace> {
Some(self)
}
}