config: Make error handling a little bit nicer

* Implement the std::error::Error trait and add a Result type
 * Propagate generic io::Error and serde_yaml::Error using From
 * Move the Result and Error types to the bottom of the module

src/config.rs

@@ -18,16 +18,7 @@
  */
 
 use serde::Deserialize;
-use std::{fs, io};
-
-#[derive(Debug)]
-#[allow(dead_code)]
-pub enum Error {
-    FileNotFoundError(String),
-    PermissionDeniedError(),
-    InvalidFormatError(serde_yaml::Error),
-    IoError(io::Error),
-}
+use std::{fs, io, fmt};
 
 #[derive(Deserialize, Clone, Debug)]
 pub struct FeedConfig {
@@ -46,19 +37,49 @@ pub struct Config {
 }
 
 impl Config {
-    pub fn load(config_file: &str) -> Result<Self, Error> {
+    pub fn load(config_file: &str) -> Result<Self> {
 
         let serialized_config = fs::read_to_string(config_file).map_err(|e| {
             match e.kind() {
                 io::ErrorKind::NotFound => Error::FileNotFoundError(config_file.into()),
                 io::ErrorKind::PermissionDenied => Error::PermissionDeniedError(),
-                _ => Error::IoError(e),
+                _ => e.into(),
             }
         })?;
 
-        let config: Config = serde_yaml::from_str(&serialized_config)
-            .map_err(|e| Error::InvalidFormatError(e))?;
+        let config: Config = serde_yaml::from_str(&serialized_config)?;
 
         Ok(config)
     }
 }
+
+#[derive(Debug)]
+#[allow(dead_code)] // we use Debug, so inner fields are never read
+pub enum Error {
+    FileNotFoundError(String),
+    PermissionDeniedError(),
+    InvalidFormatError(serde_yaml::Error),
+    IoError(io::Error),
+}
+
+impl From<serde_yaml::Error> for Error {
+    fn from(e: serde_yaml::Error) -> Self {
+        Self::InvalidFormatError(e)
+    }
+}
+
+impl From<io::Error> for Error {
+    fn from(e: io::Error) -> Self {
+        Self::IoError(e)
+    }
+}
+
+impl std::error::Error for Error {}
+
+impl fmt::Display for Error {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> std::result::Result<(), fmt::Error> {
+        write!(fmt, "{self:?}")
+    }
+}
+
+pub type Result<T> = std::result::Result<T, Error>;