Add computed contexts

This was requested on the issue tracker and the answer was "do it
yourself" so here I go doing it myself!

Author: Lexicality

src/parse/error.rs

@@ -62,7 +62,9 @@ impl std::fmt::Display for MBusError {
 		let mut level = 0;
 		for context in ctx.context() {
 			match context {
-				MBusContext::Label(_) | MBusContext::Assertion(_) => {
+				MBusContext::Label(_)
+				| MBusContext::ComputedLabel(_)
+				| MBusContext::Assertion(_) => {
 					if first {
 						first = false;
 					} else {
@@ -138,6 +140,8 @@ impl ErrorConvert<MBusError> for ContextError<MBusContext> {
 pub enum MBusContext {
 	/// Description of what is currently being parsed
 	Label(&'static str),
+	/// Computed description of what is currently being parsed
+	ComputedLabel(String),
 	/// Grammar item that was expected
 	Expected(StrContextValue),
 	/// Failed assertion
@@ -148,6 +152,7 @@ impl std::fmt::Display for MBusContext {
 	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
 		match self {
 			Self::Label(name) => write!(f, "invalid {name}"),
+			Self::ComputedLabel(name) => write!(f, "invalid {name}"),
 			Self::Expected(value) => write!(f, "expected {value}"),
 			Self::Assertion(text) => write!(f, "assertion failed: {text}"),
 		}

src/parse/types/number.rs

@@ -8,7 +8,7 @@ use winnow::prelude::*;
 use winnow::token::take;
 use winnow::Bytes;
 
-use crate::parse::error::{MBResult, MBusError};
+use crate::parse::error::{MBResult, MBusContext, MBusError};
 
 use super::BitsInput;
 
@@ -62,7 +62,12 @@ pub fn parse_bcd<'a>(bytes: usize) -> impl Parser<&'a Bytes, i64, MBusError> {
 
 	(peek(take(bytes)).void(), binary::bits::bits(parser))
 		.map(|(_, n)| n)
-		.context(StrContext::Label("signed BCD number"))
+		.context_with(move || {
+			[MBusContext::ComputedLabel(format!(
+				"signed {bytes} byte BCD number"
+			))]
+			.into_iter()
+		})
 }
 
 #[cfg(test)]
@@ -161,7 +166,7 @@ mod test_parse_bcd {
 
 		let ctx = result.inner().context().next().unwrap().to_string();
 
-		assert_eq!(ctx, "invalid signed BCD number");
+		assert_eq!(ctx, "invalid signed 3 byte BCD number");
 	}
 
 	#[test]
@@ -172,7 +177,7 @@ mod test_parse_bcd {
 
 		let ctx = result.inner().context().next().unwrap().to_string();
 
-		assert_eq!(ctx, "invalid signed BCD number");
+		assert_eq!(ctx, "invalid signed 2 byte BCD number");
 	}
 
 	#[test]