feat(error): New error context API

This is meant to replace `Error::info` as part of clap-rs#2628.

src/error/context.rs

@@ -0,0 +1,17 @@
+/// Semantics for a piece of error information
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[non_exhaustive]
+pub enum ContextKind {
+    /// An opaque message to the user
+    Custom,
+}
+
+/// A piece of error information
+#[derive(Clone, Debug, PartialEq, Eq)]
+#[non_exhaustive]
+pub enum ContextValue {
+    /// [`ContextKind`] is self-sufficient, no additional information needed
+    None,
+    /// A single value
+    Value(String),
+}

src/error/mod.rs

@@ -19,8 +19,11 @@ use crate::{
     App, AppSettings,
 };
 
+mod context;
 mod kind;
 
+pub use context::ContextKind;
+pub use context::ContextValue;
 pub use kind::ErrorKind;
 
 /// Short hand for [`Result`] type
@@ -45,9 +48,8 @@ pub struct Error {
 
 #[derive(Debug)]
 struct ErrorInner {
-    /// The type of error
     kind: ErrorKind,
-    /// Formatted error message, enhancing the cause message with extra information
+    context: Vec<(ContextKind, ContextValue)>,
     message: Message,
     source: Option<Box<dyn error::Error + Send + Sync>>,
     wait_on_exit: bool,
@@ -82,6 +84,11 @@ impl Error {
         self.inner.kind
     }
 
+    /// Additional information to further qualify the error
+    pub fn context(&self) -> impl Iterator<Item = (ContextKind, &ContextValue)> {
+        self.inner.context.iter().map(|(k, v)| (*k, v))
+    }
+
     /// Should the message be written to `stdout` or not?
     #[inline]
     pub fn use_stderr(&self) -> bool {
@@ -147,6 +154,7 @@ impl Error {
         Self {
             inner: Box::new(ErrorInner {
                 kind,
+                context: Vec::new(),
                 message: message.into(),
                 source: None,
                 wait_on_exit,