1 files changed, 15 insertions, 3 deletions

diff --git a/crates/ra_mbe/src/lib.rs b/crates/ra_mbe/src/lib.rs
index 907402f5f..2c75b7b4f 100644
--- a/crates/ra_mbe/src/lib.rs
+++ b/crates/ra_mbe/src/lib.rs
@@ -24,6 +24,18 @@ use ra_syntax::SmolStr;
 
 pub use tt::{Delimiter, Punct};
 
+#[derive(Debug, PartialEq, Eq)]
+pub enum ParseError {
+    Expected(String),
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum ExpandError {
+    NoMatchingRule,
+    UnexpectedToken,
+    BindingError(String),
+}
+
 pub use crate::syntax_bridge::{ast_to_token_tree, token_tree_to_ast_item_list};
 
 /// This struct contains AST for a single `macro_rules` definition. What might
@@ -36,11 +48,11 @@ pub struct MacroRules {
 }
 
 impl MacroRules {
-    pub fn parse(tt: &tt::Subtree) -> Option<MacroRules> {
+    pub fn parse(tt: &tt::Subtree) -> Result<MacroRules, ParseError> {
         mbe_parser::parse(tt)
     }
-    pub fn expand(&self, tt: &tt::Subtree) -> Option<tt::Subtree> {
-        mbe_expander::exapnd(self, tt)
+    pub fn expand(&self, tt: &tt::Subtree) -> Result<tt::Subtree, ExpandError> {
+        mbe_expander::expand(self, tt)
     }
 }