Add ContextCompat trait for porting from anyhow (#15)

Cargo.toml

@@ -20,9 +20,10 @@ rustversion = "1.0"
 thiserror = "1.0"
 trybuild = { version = "1.0.19", features = ["diff"] }
 backtrace = "0.3.46"
+anyhow = "1.0.28"
 
 [dependencies]
-indenter = "0.1.3"
+indenter = "0.2.0"
 
 [package.metadata.docs.rs]
 targets = ["x86_64-unknown-linux-gnu"]

README.md

@@ -1,4 +1,4 @@
-Eyre
+eyre
 ====
 
 [![Build Status][actions-badge]][actions-url]
@@ -158,27 +158,71 @@ This crate does its best to be usable as a drop in replacement of `anyhow` and
 vice-versa by `re-exporting` all of the renamed APIs with the names used in
 `anyhow`.
 
-It is not 100% compatible because there are some cases where `eyre` encounters
-type inference errors but it should mostly work as a drop in replacement.
-Specifically, the following works in anyhow:
+There are two main incompatibilities that you might encounter when porting a
+codebase from `anyhow` to `eyre`:
+
+- type inference errors when using `eyre!`
+- `.context` not being implemented for `Option`
+
+#### Type Inference Errors
+
+The type inference issue is caused by the generic parameter, which isn't
+present in `anyhow::Error`. Specifically, the following works in anyhow:
 
 ```rust
+use anyhow::anyhow;
+
 // Works
-let val = get_optional_val.ok_or_else(|| anyhow!("failed to get value")).unwrap();
+let val = get_optional_val().ok_or_else(|| anyhow!("failed to get value")).unwrap_err();
 ```
 
 Where as with `eyre!` this will fail due to being unable to infer the type for
 the Context parameter. The solution to this problem, should you encounter it,
 is to give the compiler a hint for what type it should be resolving to, either
 via your return type or a type annotation.
 
-```rust
+```rust,compile_fail
+use eyre::eyre;
+
 // Broken
-let val = get_optional_val.ok_or_else(|| eyre!("failed to get value")).unwrap();
+let val = get_optional_val().ok_or_else(|| eyre!("failed to get value")).unwrap();
 
 // Works
-let val: Report = get_optional_val.ok_or_else(|| eyre!("failed to get value")).unwrap();
+let val: Report = get_optional_val().ok_or_else(|| eyre!("failed to get value")).unwrap();
+```
+
+#### `Context` and `Option`
+
+As part of renaming `Context` to `WrapErr` we also intentionally do not
+implement `WrapErr` for `Option`. This decision was made because `wrap_err`
+implies that you're creating a new error that saves the old error as its
+`source`. With `Option` there is no source error to wrap, so `wrap_err` ends up
+being somewhat meaningless.
+
+Instead `eyre` intends for users to use the combinator functions provided by
+`std` for converting `Option`s to `Result`s. So where you would write this with
+anyhow:
+
+```rust
+use anyhow::Context;
+
+let opt: Option<()> = None;
+let result = opt.context("new error message");
+```
+
+With `eyre` we want users to write:
+
+```rust
+use eyre::{eyre, Result};
+
+let opt: Option<()> = None;
+let result: Result<()> = opt.ok_or_else(|| eyre!("new error message"));
 ```
+
+However, to help with porting we do provide a `ContextCompat` trait which
+implements `context` for options which you can import to make existing
+`.context` calls compile.
+
 [Report]: https://docs.rs/eyre/*/eyre/struct.Report.html
 [`eyre::EyreContext`]: https://docs.rs/eyre/*/eyre/trait.EyreContext.html
 [`eyre::WrapErr`]: https://docs.rs/eyre/*/eyre/trait.WrapErr.html

src/chain.rs

@@ -24,6 +24,26 @@ pub(crate) enum ChainState<'a> {
 }
 
 impl<'a> Chain<'a> {
+    /// Construct an iterator over a chain of errors via the `source` method
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use std::error::Error;
+    /// use std::fmt::{self, Write};
+    /// use eyre::Chain;
+    /// use indenter::indented;
+    ///
+    /// fn report(error: &(dyn Error + 'static), f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///     let mut errors = Chain::new(error).enumerate();
+    ///     for (i, error) in errors {
+    ///         writeln!(f)?;
+    ///         write!(indented(f).ind(i), "{}", error)?;
+    ///     }
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
     pub fn new(head: &'a (dyn StdError + 'static)) -> Self {
         Chain {
             state: ChainState::Linked { next: Some(head) },

src/context.rs

@@ -1,5 +1,5 @@
 use crate::error::ContextError;
-use crate::{EyreContext, Report, StdError, WrapErr};
+use crate::{ContextCompat, EyreContext, Report, StdError, WrapErr};
 use core::fmt::{self, Debug, Display, Write};
 
 #[cfg(backtrace)]
@@ -80,12 +80,47 @@ where
     }
 }
 
+impl<T, C> ContextCompat<T, C> for Option<T>
+where
+    C: EyreContext,
+{
+    fn wrap_err<D>(self, msg: D) -> Result<T, Report<C>>
+    where
+        D: Display + Send + Sync + 'static,
+    {
+        self.context(msg)
+    }
+
+    fn wrap_err_with<D, F>(self, msg: F) -> Result<T, Report<C>>
+    where
+        D: Display + Send + Sync + 'static,
+        F: FnOnce() -> D,
+    {
+        self.with_context(msg)
+    }
+
+    fn context<D>(self, msg: D) -> Result<T, Report<C>>
+    where
+        D: Display + Send + Sync + 'static,
+    {
+        self.ok_or_else(|| Report::from_display(msg))
+    }
+
+    fn with_context<D, F>(self, msg: F) -> Result<T, Report<C>>
+    where
+        D: Display + Send + Sync + 'static,
+        F: FnOnce() -> D,
+    {
+        self.ok_or_else(|| Report::from_display(msg()))
+    }
+}
+
 impl<D, E> Debug for ContextError<D, E>
 where
     D: Display,
     E: Debug,
 {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("Error")
             .field("msg", &Quoted(&self.msg))
             .field("source", &self.error)
@@ -97,7 +132,7 @@ impl<D, E> Display for ContextError<D, E>
 where
     D: Display,
 {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         Display::fmt(&self.msg, f)
     }
 }
@@ -133,7 +168,7 @@ impl<D> Debug for Quoted<D>
 where
     D: Display,
 {
-    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         formatter.write_char('"')?;
         Quoted(&mut *formatter).write_fmt(format_args!("{}", self.0))?;
         formatter.write_char('"')?;
@@ -153,4 +188,5 @@ pub(crate) mod private {
     pub trait Sealed<C: EyreContext> {}
 
     impl<T, E, C: EyreContext> Sealed<C> for Result<T, E> where E: ext::StdError<C> {}
+    impl<T, C: EyreContext> Sealed<C> for Option<T> {}
 }

src/error.rs

@@ -116,6 +116,28 @@ where
         unsafe { Report::construct(error, vtable, context) }
     }
 
+    pub(crate) fn from_display<M>(message: M) -> Self
+    where
+        M: Display + Send + Sync + 'static,
+    {
+        use crate::wrapper::{DisplayError, NoneError};
+        let error: DisplayError<M> = DisplayError(message);
+        let vtable = &ErrorVTable {
+            object_drop: object_drop::<DisplayError<M>, C>,
+            object_ref: object_ref::<DisplayError<M>, C>,
+            #[cfg(feature = "std")]
+            object_mut: object_mut::<DisplayError<M>, C>,
+            object_boxed: object_boxed::<DisplayError<M>, C>,
+            object_downcast: object_downcast::<M, C>,
+            object_drop_rest: object_drop_front::<M, C>,
+        };
+
+        // Safety: DisplayError is repr(transparent) so it is okay for the
+        // vtable to allow casting the DisplayError<M> to M.
+        let context = Some(C::default(&NoneError));
+        unsafe { Report::construct(error, vtable, context) }
+    }
+
     #[cfg(feature = "std")]
     pub(crate) fn from_msg<D, E>(msg: D, error: E) -> Self
     where
@@ -280,7 +302,7 @@ where
     /// }
     /// ```
     #[cfg(feature = "std")]
-    pub fn chain(&self) -> Chain {
+    pub fn chain(&self) -> Chain<'_> {
         self.inner.chain()
     }
 
@@ -448,13 +470,13 @@ impl<C: EyreContext> DerefMut for Report<C> {
 }
 
 impl<C: EyreContext> Display for Report<C> {
-    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.inner.display(formatter)
     }
 }
 
 impl<C: EyreContext> Debug for Report<C> {
-    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.inner.debug(formatter)
     }
 }
@@ -674,8 +696,8 @@ where
 // ContextError<ManuallyDrop<D>, E> and ContextError<D, ManuallyDrop<E>>.
 #[repr(C)]
 pub(crate) struct ContextError<D, E> {
-    pub msg: D,
-    pub error: E,
+    pub(crate) msg: D,
+    pub(crate) error: E,
 }
 
 impl<E, C> ErrorImpl<E, C>
@@ -707,7 +729,7 @@ where
         unsafe { &mut *(self.vtable.object_mut)(self) }
     }
 
-    pub(crate) fn chain(&self) -> Chain {
+    pub(crate) fn chain(&self) -> Chain<'_> {
         Chain::new(self.error())
     }
 }
@@ -727,7 +749,7 @@ where
     C: EyreContext,
     E: Debug,
 {
-    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.erase().debug(formatter)
     }
 }
@@ -737,7 +759,7 @@ where
     C: EyreContext,
     E: Display,
 {
-    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         Display::fmt(&self.erase().error(), formatter)
     }
 }

src/fmt.rs

@@ -6,14 +6,14 @@ impl<C> ErrorImpl<(), C>
 where
     C: EyreContext,
 {
-    pub(crate) fn display(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    pub(crate) fn display(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.context
             .as_ref()
             .map(|context| context.display(self.error(), f))
             .unwrap_or_else(|| std::fmt::Display::fmt(self.error(), f))
     }
 
-    pub(crate) fn debug(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    pub(crate) fn debug(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.context
             .as_ref()
             .map(|context| context.debug(self.error(), f))

src/kind.rs

@@ -1,3 +1,4 @@
+#![allow(missing_debug_implementations, missing_docs)]
 // Tagged dispatch mechanism for resolving the behavior of `eyre!($expr)`.
 //
 // When eyre! is given a single expr argument to turn into eyre::Report, we