Implement Display for macro expansion errors

crates/mbe/src/lib.rs

@@ -12,6 +12,8 @@ mod subtree_source;
 #[cfg(test)]
 mod tests;
 
+use std::fmt;
+
 pub use tt::{Delimiter, Punct};
 
 use crate::{
@@ -42,6 +44,20 @@ impl From<tt::ExpansionError> for ExpandError {
     }
 }
 
+impl fmt::Display for ExpandError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ExpandError::NoMatchingRule => f.write_str("no rule matches input tokens"),
+            ExpandError::UnexpectedToken => f.write_str("unexpected token in input"),
+            ExpandError::BindingError(e) => f.write_str(e),
+            ExpandError::ConversionError => f.write_str("could not convert tokens"),
+            ExpandError::InvalidRepeat => f.write_str("invalid repeat expression"),
+            ExpandError::ProcMacroError(e) => write!(f, "{}", e),
+            ExpandError::Other(e) => f.write_str(e),
+        }
+    }
+}
+
 pub use crate::syntax_bridge::{
     ast_to_token_tree, parse_to_token_tree, syntax_node_to_token_tree, token_tree_to_syntax_node,
     TokenMap,

crates/tt/src/lib.rs

@@ -240,6 +240,17 @@ pub enum ExpansionError {
     ExpansionError(String),
 }
 
+impl fmt::Display for ExpansionError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ExpansionError::IOError(e) => write!(f, "I/O error: {}", e),
+            ExpansionError::JsonError(e) => write!(f, "JSON decoding error: {}", e),
+            ExpansionError::Unknown(e) => write!(f, "{}", e),
+            ExpansionError::ExpansionError(e) => write!(f, "proc macro returned error: {}", e),
+        }
+    }
+}
+
 pub trait TokenExpander: Debug + Send + Sync + RefUnwindSafe {
     fn expand(&self, subtree: &Subtree, attrs: Option<&Subtree>)
         -> Result<Subtree, ExpansionError>;