Add a Context trait similar to anyhow::Context. (#2676)

* Add a Context trait similar to anyhow::Context.

* Switch two unwrap to context.

candle-core/src/error.rs

@@ -9,8 +9,14 @@ pub struct MatMulUnexpectedStriding {
     pub msg: &'static str,
 }
 
+impl std::fmt::Debug for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{self}")
+    }
+}
+
 /// Main library error type.
-#[derive(thiserror::Error, Debug)]
+#[derive(thiserror::Error)]
 pub enum Error {
     // === DType Errors ===
     #[error("{msg}, expected: {expected:?}, got: {got:?}")]
@@ -199,8 +205,14 @@ pub enum Error {
     UnsupportedSafeTensorDtype(safetensors::Dtype),
 
     /// Arbitrary errors wrapping.
-    #[error(transparent)]
-    Wrapped(Box<dyn std::error::Error + Send + Sync>),
+    #[error("{0}")]
+    Wrapped(Box<dyn std::fmt::Display + Send + Sync>),
+
+    #[error("{context}\n{inner}")]
+    Context {
+        inner: Box<Self>,
+        context: Box<dyn std::fmt::Display + Send + Sync>,
+    },
 
     /// Adding path information to an error.
     #[error("path: {path:?} {inner}")]
@@ -218,16 +230,19 @@ pub enum Error {
     /// User generated error message, typically created via `bail!`.
     #[error("{0}")]
     Msg(String),
+
+    #[error("unwrap none")]
+    UnwrapNone,
 }
 
 pub type Result<T> = std::result::Result<T, Error>;
 
 impl Error {
-    pub fn wrap(err: impl std::error::Error + Send + Sync + 'static) -> Self {
+    pub fn wrap(err: impl std::fmt::Display + Send + Sync + 'static) -> Self {
         Self::Wrapped(Box::new(err)).bt()
     }
 
-    pub fn msg(err: impl std::error::Error) -> Self {
+    pub fn msg(err: impl std::fmt::Display) -> Self {
         Self::Msg(err.to_string()).bt()
     }
 
@@ -253,6 +268,13 @@ impl Error {
             path: p.as_ref().to_path_buf(),
         }
     }
+
+    pub fn context(self, c: impl std::fmt::Display + Send + Sync + 'static) -> Self {
+        Self::Context {
+            inner: Box::new(self),
+            context: Box::new(c),
+        }
+    }
 }
 
 #[macro_export]
@@ -275,3 +297,41 @@ pub fn zip<T, U>(r1: Result<T>, r2: Result<U>) -> Result<(T, U)> {
         (_, Err(e)) => Err(e),
     }
 }
+
+// Taken from anyhow.
+pub trait Context<T> {
+    /// Wrap the error value with additional context.
+    fn context<C>(self, context: C) -> Result<T>
+    where
+        C: std::fmt::Display + Send + Sync + 'static;
+
+    /// Wrap the error value with additional context that is evaluated lazily
+    /// only once an error does occur.
+    fn with_context<C, F>(self, f: F) -> Result<T>
+    where
+        C: std::fmt::Display + Send + Sync + 'static,
+        F: FnOnce() -> C;
+}
+
+impl<T> Context<T> for Option<T> {
+    fn context<C>(self, context: C) -> Result<T>
+    where
+        C: std::fmt::Display + Send + Sync + 'static,
+    {
+        match self {
+            Some(v) => Ok(v),
+            None => Err(Error::UnwrapNone.context(context).bt()),
+        }
+    }
+
+    fn with_context<C, F>(self, f: F) -> Result<T>
+    where
+        C: std::fmt::Display + Send + Sync + 'static,
+        F: FnOnce() -> C,
+    {
+        match self {
+            Some(v) => Ok(v),
+            None => Err(Error::UnwrapNone.context(f()).bt()),
+        }
+    }
+}