Rollup merge of #34750 - GuillaumeGomez:error_doc, r=steveklabnik

Add examples for std::Error module

Fixes #29352.

r? @steveklabnik

Author: GuillaumeGomez

src/libstd/error.rs

@@ -66,10 +66,80 @@ pub trait Error: Debug + Display + Reflect {
     /// The description should not contain newlines or sentence-ending
     /// punctuation, to facilitate embedding in larger user-facing
     /// strings.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    ///
+    /// match "xc".parse::<u32>() {
+    ///     Err(e) => {
+    ///         println!("Error: {}", e.description());
+    ///     }
+    ///     _ => println!("No error"),
+    /// }
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     fn description(&self) -> &str;
 
     /// The lower-level cause of this error, if any.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::error::Error;
+    /// use std::fmt;
+    ///
+    /// #[derive(Debug)]
+    /// struct SuperError {
+    ///     side: SuperErrorSideKick,
+    /// }
+    ///
+    /// impl fmt::Display for SuperError {
+    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    ///         write!(f, "SuperError is here!")
+    ///     }
+    /// }
+    ///
+    /// impl Error for SuperError {
+    ///     fn description(&self) -> &str {
+    ///         "I'm the superhero of errors!"
+    ///     }
+    ///
+    ///     fn cause(&self) -> Option<&Error> {
+    ///         Some(&self.side)
+    ///     }
+    /// }
+    ///
+    /// #[derive(Debug)]
+    /// struct SuperErrorSideKick;
+    ///
+    /// impl fmt::Display for SuperErrorSideKick {
+    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    ///         write!(f, "SuperErrorSideKick is here!")
+    ///     }
+    /// }
+    ///
+    /// impl Error for SuperErrorSideKick {
+    ///     fn description(&self) -> &str {
+    ///         "I'm SuperError side kick!"
+    ///     }
+    /// }
+    ///
+    /// fn get_super_error() -> Result<(), SuperError> {
+    ///     Err(SuperError { side: SuperErrorSideKick })
+    /// }
+    ///
+    /// fn main() {
+    ///     match get_super_error() {
+    ///         Err(e) => {
+    ///             println!("Error: {}", e.description());
+    ///             println!("Caused by: {}", e.cause().unwrap());
+    ///         }
+    ///         _ => println!("No error"),
+    ///     }
+    /// }
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     fn cause(&self) -> Option<&Error> { None }