Make working with Errors a bit easier

examples/actix-web-app/src/main.rs

@@ -96,7 +96,7 @@ async fn not_found_handler(req: HttpRequest) -> Result<impl Responder> {
         };
         Ok(HttpResponse::NotFound()
             .insert_header(ContentType::html())
-            .body(tmpl.render().map_err(<Box<dyn std::error::Error>>::from)?))
+            .body(tmpl.render().map_err(|err| err.into_io_error())?))
     } else {
         Ok(HttpResponse::MethodNotAllowed().finish())
     }
@@ -155,9 +155,7 @@ async fn index_handler(
         name: query.name,
     };
     Ok(Html::new(
-        template
-            .render()
-            .map_err(<Box<dyn std::error::Error>>::from)?,
+        template.render().map_err(|err| err.into_io_error())?,
     ))
 }
 
@@ -192,8 +190,6 @@ async fn greeting_handler(
         name: query.name,
     };
     Ok(Html::new(
-        template
-            .render()
-            .map_err(<Box<dyn std::error::Error>>::from)?,
+        template.render().map_err(|err| err.into_io_error())?,
     ))
 }

rinja/src/error.rs

@@ -1,62 +1,94 @@
 use std::convert::Infallible;
-use std::fmt::{self, Display};
+use std::error::Error as StdError;
+use std::{fmt, io};
 
 /// The [`Result`](std::result::Result) type with [`Error`] as default error type
 pub type Result<I, E = Error> = std::result::Result<I, E>;
 
-/// rinja error type
+/// rinja's error type
 ///
-/// # Feature Interaction
-///
-/// If the feature `serde_json` is enabled an
-/// additional error variant `Json` is added.
-///
-/// # Why not `failure`/`error-chain`?
-///
-/// Error from `error-chain` are not `Sync` which
-/// can lead to problems e.g. when this is used
-/// by a crate which use `failure`. Implementing
-/// `Fail` on the other hand prevents the implementation
-/// of `std::error::Error` until specialization lands
-/// on stable. While errors impl. `Fail` can be
-/// converted to a type impl. `std::error::Error`
-/// using a adapter the benefits `failure` would
-/// bring to this crate are small, which is why
-/// `std::error::Error` was used.
+/// Used as error value for e.g. [`Template::render()`][crate::Template::render()]
+/// and custom filters.
 #[non_exhaustive]
 #[derive(Debug)]
 pub enum Error {
-    /// formatting error
+    /// Generic, unspecified formatting error
     Fmt,
-    /// an error raised by using `?` in a template
-    Custom(Box<dyn std::error::Error + Send + Sync>),
-    /// json conversion error
+    /// An error raised by using `?` in a template
+    Custom(Box<dyn StdError + Send + Sync>),
+    /// JSON conversion error
     #[cfg(feature = "serde_json")]
     Json(serde_json::Error),
 }
 
-impl std::error::Error for Error {
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
-        match *self {
-            Error::Fmt => None,
-            Error::Custom(ref err) => Some(err.as_ref()),
+impl Error {
+    /// Capture an [`StdError`]
+    #[inline]
+    pub fn custom(err: impl Into<Box<dyn StdError + Send + Sync>>) -> Self {
+        Self::Custom(err.into())
+    }
+
+    /// Convert this [`Error`] into a
+    /// <code>[Box]&lt;dyn [StdError] + [Send] + [Sync]&gt;</code>
+    pub fn into_box(self) -> Box<dyn StdError + Send + Sync> {
+        match self {
+            Error::Fmt => fmt::Error.into(),
+            Error::Custom(err) => err,
             #[cfg(feature = "serde_json")]
-            Error::Json(ref err) => Some(err),
+            Error::Json(err) => err.into(),
+        }
+    }
+
+    /// Convert this [`Error`] into an [`io::Error`]
+    ///
+    /// Not this error itself, but the contained [`source`][StdError::source] is returned.
+    pub fn into_io_error(self) -> io::Error {
+        io::Error::other(match self {
+            Error::Custom(err) => match err.downcast() {
+                Ok(err) => return *err,
+                Err(err) => err,
+            },
+            err => err.into_box(),
+        })
+    }
+}
+
+impl StdError for Error {
+    fn source(&self) -> Option<&(dyn StdError + 'static)> {
+        match self {
+            Error::Fmt => Some(&fmt::Error),
+            Error::Custom(err) => Some(err.as_ref()),
+            #[cfg(feature = "serde_json")]
+            Error::Json(err) => Some(err),
         }
     }
 }
 
-impl Display for Error {
-    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
-            Error::Fmt => write!(formatter, "formatting error"),
-            Error::Custom(err) => write!(formatter, "{err}"),
+            Error::Fmt => fmt::Error.fmt(f),
+            Error::Custom(err) => err.fmt(f),
             #[cfg(feature = "serde_json")]
-            Error::Json(err) => write!(formatter, "json conversion error: {err}"),
+            Error::Json(err) => err.fmt(f),
         }
     }
 }
 
+impl From<Error> for fmt::Error {
+    #[inline]
+    fn from(_: Error) -> Self {
+        Self
+    }
+}
+
+impl From<Error> for io::Error {
+    #[inline]
+    fn from(err: Error) -> Self {
+        err.into_io_error()
+    }
+}
+
 impl From<fmt::Error> for Error {
     #[inline]
     fn from(_: fmt::Error) -> Self {
@@ -64,10 +96,103 @@ impl From<fmt::Error> for Error {
     }
 }
 
-impl From<Error> for fmt::Error {
+/// This conversion inspects the argument and chooses the best fitting [`Error`] variant
+impl From<Box<dyn StdError + Send + Sync>> for Error {
     #[inline]
-    fn from(_: Error) -> Self {
-        Self
+    fn from(err: Box<dyn StdError + Send + Sync>) -> Self {
+        error_from_stderror(err, MAX_ERROR_UNWRAP_COUNT)
+    }
+}
+
+/// This conversion inspects the argument and chooses the best fitting [`Error`] variant
+impl From<io::Error> for Error {
+    #[inline]
+    fn from(err: io::Error) -> Self {
+        from_from_io_error(err, MAX_ERROR_UNWRAP_COUNT)
+    }
+}
+
+const MAX_ERROR_UNWRAP_COUNT: usize = 5;
+
+fn error_from_stderror(err: Box<dyn StdError + Send + Sync>, unwraps: usize) -> Error {
+    let Some(unwraps) = unwraps.checked_sub(1) else {
+        return Error::Custom(err);
+    };
+    match ErrorKind::inspect(err.as_ref()) {
+        ErrorKind::Fmt => Error::Fmt,
+        ErrorKind::Custom => Error::Custom(err),
+        #[cfg(feature = "serde_json")]
+        ErrorKind::Json => match err.downcast() {
+            Ok(err) => Error::Json(*err),
+            Err(_) => Error::Fmt, // unreachable
+        },
+        ErrorKind::Io => match err.downcast() {
+            Ok(err) => from_from_io_error(*err, unwraps),
+            Err(_) => Error::Fmt, // unreachable
+        },
+        ErrorKind::Rinja => match err.downcast() {
+            Ok(err) => *err,
+            Err(_) => Error::Fmt, // unreachable
+        },
+    }
+}
+
+fn from_from_io_error(err: io::Error, unwraps: usize) -> Error {
+    let Some(inner) = err.get_ref() else {
+        return Error::custom(err);
+    };
+    let Some(unwraps) = unwraps.checked_sub(1) else {
+        return match err.into_inner() {
+            Some(err) => Error::Custom(err),
+            None => Error::Fmt, // unreachable
+        };
+    };
+    match ErrorKind::inspect(inner) {
+        ErrorKind::Fmt => Error::Fmt,
+        ErrorKind::Rinja => match err.downcast() {
+            Ok(err) => err,
+            Err(_) => Error::Fmt, // unreachable
+        },
+        #[cfg(feature = "serde_json")]
+        ErrorKind::Json => match err.downcast() {
+            Ok(err) => Error::Json(err),
+            Err(_) => Error::Fmt, // unreachable
+        },
+        ErrorKind::Custom => match err.into_inner() {
+            Some(err) => Error::Custom(err),
+            None => Error::Fmt, // unreachable
+        },
+        ErrorKind::Io => match err.downcast() {
+            Ok(inner) => from_from_io_error(inner, unwraps),
+            Err(_) => Error::Fmt, // unreachable
+        },
+    }
+}
+
+enum ErrorKind {
+    Fmt,
+    Custom,
+    #[cfg(feature = "serde_json")]
+    Json,
+    Io,
+    Rinja,
+}
+
+impl ErrorKind {
+    fn inspect(err: &(dyn StdError + 'static)) -> ErrorKind {
+        if err.is::<fmt::Error>() {
+            ErrorKind::Fmt
+        } else if err.is::<io::Error>() {
+            ErrorKind::Io
+        } else if err.is::<Error>() {
+            ErrorKind::Rinja
+        } else {
+            #[cfg(feature = "serde_json")]
+            if err.is::<serde_json::Error>() {
+                return ErrorKind::Json;
+            }
+            ErrorKind::Custom
+        }
     }
 }
 
@@ -87,10 +212,7 @@ impl From<Infallible> for Error {
 }
 
 #[cfg(test)]
-mod tests {
-    use super::Error;
-
-    #[allow(dead_code)]
+const _: () = {
     trait AssertSendSyncStatic: Send + Sync + 'static {}
     impl AssertSendSyncStatic for Error {}
-}
+};

rinja/src/helpers.rs

@@ -263,3 +263,11 @@ impl<L: FastWritable, R: FastWritable> FastWritable for Concat<L, R> {
         self.1.write_into(dest)
     }
 }
+
+#[inline]
+pub fn map_try<T, E>(result: Result<T, E>) -> Result<T, crate::Error>
+where
+    E: Into<Box<dyn std::error::Error + Send + Sync>>,
+{
+    result.map_err(crate::Error::custom)
+}

rinja_derive/src/generator.rs

@@ -1493,9 +1493,9 @@ impl<'a, 'h> Generator<'a, 'h> {
         buf: &mut Buffer,
         expr: &WithSpan<'_, Expr<'_>>,
     ) -> Result<DisplayWrap, CompileError> {
-        buf.write("rinja::helpers::core::result::Result::map_err(");
+        buf.write("rinja::helpers::map_try(");
         self.visit_expr(ctx, buf, expr)?;
-        buf.write(", |err| rinja::shared::Error::Custom(rinja::helpers::core::convert::Into::into(err)))?");
+        buf.write(")?");
         Ok(DisplayWrap::Unwrapped)
     }