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//! # SNAFU //! //! ## Design philosophy //! //! SNAFU believes that it should be easy to bin one underlying error //! type (such as [`io::Error`](std::io::Error)) into multiple //! domain-specific errors while also optionally adding contextual //! information. //! //! SNAFU is designed to be used in libraries, not just end-user applications. //! //! ## 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 *context selectors* mirroring each error //! variant. These are used with the [`context`](ResultExt::context) //! method to provide ergonomic error handling. //! //! ```rust //! use snafu::{Snafu, ResultExt, Backtrace, ErrorCompat}; //! 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 })?; //! //! if user_id != 42 { //! UserIdInvalid { user_id }.fail()?; //! } //! //! Ok(true) //! } //! //! # const CONFIG_DIRECTORY: &str = "/does/not/exist"; //! # const USER_ID: i32 = 0; //! 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); //! } //! } //! } //! } //! ``` //! //! ## The `Snafu` macro //! //! This procedural macro implements the [`Error`](std::error::Error) //! trait and produces the corresponding context selectors. //! //! ### Detailed example //! //! ```rust //! use snafu::Snafu; //! use std::path::PathBuf; //! //! #[derive(Debug, Snafu)] //! enum Error { //! #[snafu_display("Could not open config at {}: {}", "filename.display()", "source")] //! OpenConfig { filename: PathBuf, source: std::io::Error }, //! #[snafu_display("Could not open config: {}", "source")] //! SaveConfig { source: std::io::Error }, //! #[snafu_display("The user id {} is invalid", "user_id")] //! UserIdInvalid { user_id: i32 }, //! } //! ``` //! //! #### Generated code //! //! This will generate three additional types called *context //! selectors*: //! //! ```rust,ignore //! struct OpenConfig<P> { filename: P } //! struct SaveConfig<P> { filename: P } //! struct UserIdInvalid<I> { user_id: I } //! ``` //! //! Notably: //! //! 1. One struct is created for each enum variant. //! 1. The name of the struct is the same as the enum variant's name. //! 1. The `source` and `backtrace` fields have been removed; the //! library will automatically handle this for you. //! 1. Each remaining field's type has been replaced with a generic //! type. //! //! If the original variant had a `source` field, its context selector //! will have an implementation of [`From`](std::convert::From) for a //! `snafu::Context`: //! //! ```rust,ignore //! impl<P> From<Context<Error, OpenConfig<P>>> for Error //! where //! P: Into<PathBuf>, //! ``` //! //! Otherwise, the context selector will have an inherent method //! `fail`: //! //! ```rust,ignore //! impl<I> UserIdInvalid<I> //! where //! I: Into<i32>, //! { //! fn fail<T>(self) -> Result<T, Error> { /* ... */ } //! } //! ``` //! //! If the original variant had a `backtrace` field, the backtrace //! will be automatically constructed when either `From` or `fail` are //! called. //! //! ### Attributes //! //! #### Controlling `Display` //! //! For backwards compatibility purposes, there are a number of ways //! you can specify how the `Display` trait will be implemented for //! each variant: //! //! - `#[snafu::display("a format string with arguments: {}", info)]` //! //! No special escaping is needed; this looks just like the arguments to a call to `println!`. //! //! - `#[snafu_display("a format string with arguments: {}", "info")]` //! //! Every argument is quoted as a string literal separately. //! //! - `#[snafu_display = r#"("a format string with arguments: {}", info)"#]` //! //! The entire //! //! Each choice has the same capabilities. All of the fields of the //! variant will be available and you can call methods on them, such //! as `filename.display()`. //! //! ## Version compatibility //! //! SNAFU is tested and compatible back to Rust 1.18, released on //! 2017-06-08. Compatibility is controlled by Cargo feature flags. //! //! ### Default //! //! - Targets the current stable version of Rust at the time of //! release of the crate. Check the Cargo.toml for the exact //! version. //! //! ### No features - supports Rust 1.18 //! //! - Implements `Error` and `Display`. //! - Creates context selectors. //! //! ### `rust_1_30` - supports Rust 1.30 //! //! - Adds an implementation for `Error::source` //! - Adds support for re-exporting the `Snafu` macro directly from //! the `snafu` crate. //! //! ### `unstable_display_attribute` - supports Rust Nightly //! //! - Adds support for the `snafu::display` attribute. //! //! ## Other feature flags //! //! ### `backtraces` //! //! When enabled, you can use the [`Backtrace`](Backtrace) type in //! your enum variant. If you never use backtraces, you can omit this //! feature to speed up compilation a small amount. #[cfg(feature = "backtraces")] extern crate backtrace; #[cfg(feature = "rust_1_30")] extern crate snafu_derive; #[cfg(feature = "rust_1_30")] pub use snafu_derive::Snafu; /// A combination of an underlying error and additional information /// about the error. It is not expected for users of this crate to /// interact with this type. pub struct Context<E, C> { /// The underlying error pub error: E, /// Information that provides a context for the underlying error pub context: C, } /// Additions to [`Result`](std::result::Result). pub trait ResultExt<T, E>: Sized { /// Extend a `Result` with additional context-sensitive information. /// /// ```rust /// use snafu::{Snafu, ResultExt}; /// /// #[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 [`From`](std::convert::From) implementation /// generated by the macro will call /// [`Into::into`](std::convert::Into::into) on each field, so the /// types are not required to exactly match. fn context<C>(self, context: C) -> Result<T, Context<E, C>>; /// Extend a `Result` with lazily-generated context-sensitive information. /// /// ```rust /// use snafu::{Snafu, ResultExt}; /// /// #[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 /// [`From`](std::convert::From) implementation generated by the /// macro will call [`Into::into`](std::convert::Into::into) on /// each field. fn with_context<F, C>(self, context: F) -> Result<T, Context<E, C>> where F: FnOnce() -> C; /// Extend a `Result` with additional context-sensitive /// information and immediately convert it to another `Result`. /// /// This is most useful when using `Result`'s combinators and when /// the final `Result` type is already constrained. /// /// ```rust /// use snafu::{Snafu, ResultExt}; /// /// #[derive(Debug, Snafu)] /// enum Error { /// Authenticating { user_name: String, user_id: i32, source: ApiError }, /// } /// /// fn example() -> Result<i32, Error> { /// another_function() /// .map(|v| v + 10) /// .eager_context(Authenticating { user_name: "admin", user_id: 42 }) /// } /// /// # type ApiError = std::io::Error; /// fn another_function() -> Result<i32, ApiError> { /// /* ... */ /// # Ok(42) /// } /// ``` fn eager_context<C, E2>(self, context: C) -> Result<T, E2> where E2: From<Context<E, C>>, { self.context(context).map_err(Into::into) } /// Extend a `Result` with lazily-generated context-sensitive /// information and immediately convert it to another `Result`. /// /// This is most useful when using `Result`'s combinators and when /// the final `Result` type is already constrained. /// /// ```rust /// use snafu::{Snafu, ResultExt}; /// /// #[derive(Debug, Snafu)] /// enum Error { /// Authenticating { user_name: String, user_id: i32, source: ApiError }, /// } /// /// fn example() -> Result<i32, Error> { /// another_function() /// .map(|v| v + 10) /// .with_eager_context(|| Authenticating { /// user_name: "admin".to_string(), /// user_id: 42, /// }) /// } /// /// # 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 /// [`From`](std::convert::From) implementation generated by the /// macro will call [`Into::into`](std::convert::Into::into) on /// each field. fn with_eager_context<F, C, E2>(self, context: F) -> Result<T, E2> where F: FnOnce() -> C, E2: From<Context<E, C>>, { self.with_context(context).map_err(Into::into) } } impl<T, E> ResultExt<T, E> for std::result::Result<T, E> { fn context<C>(self, context: C) -> Result<T, Context<E, C>> { self.map_err(|error| Context { error, context }) } fn with_context<F, C>(self, context: F) -> Result<T, Context<E, C>> where F: FnOnce() -> C, { self.map_err(|error| { let context = context(); Context { error, context } }) } } /// 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. #[cfg(feature = "backtraces")] fn backtrace(&self) -> Option<&Backtrace> { None } } #[cfg(feature = "backtraces")] pub use backtrace_shim::*; #[cfg(feature = "backtraces")] mod backtrace_shim { use backtrace; use std::{fmt, path}; /// A backtrace starting from the beginning of the thread. #[derive(Debug)] pub struct Backtrace(backtrace::Backtrace); impl Backtrace { /// Creates the backtrace. // Inlining in an attempt to remove this function from the backtrace #[inline(always)] pub fn new() -> Self { Backtrace(backtrace::Backtrace::new()) } } impl Default for Backtrace { // Inlining in an attempt to remove this function from the backtrace #[inline(always)] fn default() -> Self { Self::new() } } impl fmt::Display for Backtrace { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { let frames = self.0.frames(); let width = (frames.len() as f32).log10().floor() as usize + 1; for (index, frame) in frames.iter().enumerate() { let mut symbols = frame.symbols().iter().map(SymbolDisplay); if let Some(symbol) = symbols.next() { writeln!(f, "{index:width$} {name}", index = index, width = width, name = symbol.name())?; if let Some(location) = symbol.location() { writeln!(f, "{index:width$} {location}", index = "", width = width, location = location)?; } for symbol in symbols { writeln!(f, "{index:width$} {name}", index = "", width = width, name = symbol.name())?; if let Some(location) = symbol.location() { writeln!(f, "{index:width$} {location}", index = "", width = width, location = location)?; } } } } Ok(()) } } struct SymbolDisplay<'a>(&'a backtrace::BacktraceSymbol); impl<'a> SymbolDisplay<'a> { fn name(&self) -> SymbolNameDisplay<'a> { SymbolNameDisplay(self.0) } fn location(&self) -> Option<SymbolLocationDisplay<'a>> { self.0.filename().map(|f| SymbolLocationDisplay(self.0, f)) } } struct SymbolNameDisplay<'a>(&'a backtrace::BacktraceSymbol); impl<'a> fmt::Display for SymbolNameDisplay<'a> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { match self.0.name() { Some(n) => write!(f, "{}", n)?, None => write!(f, "<unknown>")?, } Ok(()) } } struct SymbolLocationDisplay<'a>(&'a backtrace::BacktraceSymbol, &'a path::Path); impl<'a> fmt::Display for SymbolLocationDisplay<'a> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "{}", self.1.display())?; if let Some(l) = self.0.lineno() { write!(f, ":{}", l)?; } Ok(()) } } }