misc: option_parser: streamline #[source] and Error

This streamlines the code base to follow best practices for
error handling in Rust: Each error struct implements
std::error::Error (most due via thiserror::Error derive macro)
and sets its source accordingly.

This allows future work that nicely prints the error chains,
for example.

So far, the convention is that each error prints its
sub error as part of its Display::fmt() impl.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

option_parser/Cargo.toml

@@ -3,3 +3,6 @@ authors = ["The Cloud Hypervisor Authors"]
 edition = "2021"
 name = "option_parser"
 version = "0.1.0"
+
+[dependencies]
+thiserror = "2.0.6"

option_parser/src/lib.rs

@@ -4,10 +4,11 @@
 //
 
 use std::collections::HashMap;
-use std::fmt;
 use std::num::ParseIntError;
 use std::str::FromStr;
 
+use thiserror::Error;
+
 #[derive(Default)]
 pub struct OptionParser {
     options: HashMap<String, OptionParserValue>,
@@ -18,26 +19,17 @@ struct OptionParserValue {
     requires_value: bool,
 }
 
-#[derive(Debug)]
+#[derive(Error, Debug)]
 pub enum OptionParserError {
+    #[error("unknown option: {0}")]
     UnknownOption(String),
+    #[error("unknown option: {0}")]
     InvalidSyntax(String),
-    Conversion(String, String),
+    #[error("unable to convert {1} for {0}")]
+    Conversion(String /* field */, String /* value */),
+    #[error("invalid value: {0}")]
     InvalidValue(String),
 }
-
-impl fmt::Display for OptionParserError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        match self {
-            OptionParserError::UnknownOption(s) => write!(f, "unknown option: {s}"),
-            OptionParserError::InvalidSyntax(s) => write!(f, "invalid syntax:{s}"),
-            OptionParserError::Conversion(field, value) => {
-                write!(f, "unable to convert {value} for {field}")
-            }
-            OptionParserError::InvalidValue(s) => write!(f, "invalid value: {s}"),
-        }
-    }
-}
 type OptionParserResult<T> = std::result::Result<T, OptionParserError>;
 
 fn split_commas(s: &str) -> OptionParserResult<Vec<String>> {