perf(error): Allow custmizing formatting

For now, there isn't much a custom implementation can do.

Going from `Rich` to `Null` drops about 6 KiB from the binary

This is a part of #1365 and #1384

src/builder/mod.rs

@@ -33,6 +33,7 @@ pub use possible_value::PossibleValue;
 pub use range::ValueRange;
 pub use resettable::IntoResettable;
 pub use resettable::Resettable;
+pub use styled_str::StyledStr;
 pub use value_hint::ValueHint;
 pub use value_parser::_AutoValueParser;
 pub use value_parser::via_prelude;
@@ -55,7 +56,6 @@ pub use value_parser::_AnonymousValueParser;
 
 #[allow(unused_imports)]
 pub(crate) use self::str::Inner as StrInner;
-pub(crate) use self::styled_str::StyledStr;
 pub(crate) use action::CountType;
 pub(crate) use arg::render_arg_val;
 pub(crate) use arg_settings::{ArgFlags, ArgSettings};

src/builder/styled_str.rs

@@ -1,10 +1,12 @@
+/// Terminal-styling container
 #[derive(Clone, Default, Debug, PartialEq, Eq)]
-pub(crate) struct StyledStr {
+pub struct StyledStr {
     pieces: Vec<(Option<Style>, String)>,
 }
 
 impl StyledStr {
-    pub(crate) const fn new() -> Self {
+    /// Create an empty buffer
+    pub const fn new() -> Self {
         Self { pieces: Vec::new() }
     }
 

src/error/context.rs

@@ -34,6 +34,35 @@ pub enum ContextKind {
     Custom,
 }
 
+impl ContextKind {
+    /// End-user description of the error case, where relevant
+    pub fn as_str(self) -> Option<&'static str> {
+        match self {
+            Self::InvalidSubcommand => Some("Invalid Subcommand"),
+            Self::InvalidArg => Some("Invalid Argument"),
+            Self::PriorArg => Some("Prior Argument"),
+            Self::ValidValue => Some("Value Value"),
+            Self::InvalidValue => Some("Invalid Value"),
+            Self::ActualNumValues => Some("Actual Number of Values"),
+            Self::ExpectedNumValues => Some("Expected Number of Values"),
+            Self::MinValues => Some("Minimum Number of Values"),
+            Self::SuggestedCommand => Some("Suggested Command"),
+            Self::SuggestedSubcommand => Some("Suggested Subcommand"),
+            Self::SuggestedArg => Some("Suggested Argument"),
+            Self::SuggestedValue => Some("Suggested Value"),
+            Self::TrailingArg => Some("Trailing Argument"),
+            Self::Usage => None,
+            Self::Custom => None,
+        }
+    }
+}
+
+impl std::fmt::Display for ContextKind {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.as_str().unwrap_or_default().fmt(f)
+    }
+}
+
 /// A piece of error information
 #[derive(Clone, Debug, PartialEq, Eq)]
 #[non_exhaustive]
@@ -49,3 +78,15 @@ pub enum ContextValue {
     /// A single value
     Number(isize),
 }
+
+impl std::fmt::Display for ContextValue {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::None => "".fmt(f),
+            Self::Bool(v) => v.fmt(f),
+            Self::String(v) => v.fmt(f),
+            Self::Strings(v) => v.join(", ").fmt(f),
+            Self::Number(v) => v.fmt(f),
+        }
+    }
+}