4 files changed, 62 insertions, 28 deletions

diff --git a/crates/mbe/src/lib.rs b/crates/mbe/src/lib.rs
index f854ca09a..2d0763c47 100644
--- a/crates/mbe/src/lib.rs
+++ b/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::{
@@ -33,6 +35,7 @@ pub enum ExpandError {
     ConversionError,
     InvalidRepeat,
     ProcMacroError(tt::ExpansionError),
+    Other(String),
 }
 
 impl From<tt::ExpansionError> for ExpandError {
@@ -41,6 +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) => e.fmt(f),
+            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,
@@ -246,33 +263,42 @@ fn validate(pattern: &tt::Subtree) -> Result<(), ParseError> {
     Ok(())
 }
 
-#[derive(Debug)]
-pub struct ExpandResult<T>(pub T, pub Option<ExpandError>);
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct ExpandResult<T> {
+    pub value: T,
+    pub err: Option<ExpandError>,
+}
 
 impl<T> ExpandResult<T> {
-    pub fn ok(t: T) -> ExpandResult<T> {
-        ExpandResult(t, None)
+    pub fn ok(value: T) -> Self {
+        Self { value, err: None }
+    }
+
+    pub fn only_err(err: ExpandError) -> Self
+    where
+        T: Default,
+    {
+        Self { value: Default::default(), err: Some(err) }
     }
 
-    pub fn only_err(err: ExpandError) -> ExpandResult<T>
+    pub fn str_err(err: String) -> Self
     where
         T: Default,
     {
-        ExpandResult(Default::default(), Some(err))
+        Self::only_err(ExpandError::Other(err))
     }
 
     pub fn map<U>(self, f: impl FnOnce(T) -> U) -> ExpandResult<U> {
-        ExpandResult(f(self.0), self.1)
+        ExpandResult { value: f(self.value), err: self.err }
     }
 
     pub fn result(self) -> Result<T, ExpandError> {
-        self.1.map(Err).unwrap_or(Ok(self.0))
+        self.err.map(Err).unwrap_or(Ok(self.value))
     }
 }
 
 impl<T: Default> From<Result<T, ExpandError>> for ExpandResult<T> {
-    fn from(result: Result<T, ExpandError>) -> ExpandResult<T> {
-        result
-            .map_or_else(|e| ExpandResult(Default::default(), Some(e)), |it| ExpandResult(it, None))
+    fn from(result: Result<T, ExpandError>) -> Self {
+        result.map_or_else(|e| Self::only_err(e), |it| Self::ok(it))
     }
 }
diff --git a/crates/mbe/src/mbe_expander.rs b/crates/mbe/src/mbe_expander.rs
index 1ad8b9f8a..97bce0536 100644
--- a/crates/mbe/src/mbe_expander.rs
+++ b/crates/mbe/src/mbe_expander.rs
@@ -28,10 +28,10 @@ fn expand_rules(rules: &[crate::Rule], input: &tt::Subtree) -> ExpandResult<tt::
             // If we find a rule that applies without errors, we're done.
             // Unconditionally returning the transcription here makes the
             // `test_repeat_bad_var` test fail.
-            let ExpandResult(res, transcribe_err) =
+            let ExpandResult { value, err: transcribe_err } =
                 transcriber::transcribe(&rule.rhs, &new_match.bindings);
             if transcribe_err.is_none() {
-                return ExpandResult::ok(res);
+                return ExpandResult::ok(value);
             }
         }
         // Use the rule if we matched more tokens, or had fewer errors
@@ -47,11 +47,11 @@ fn expand_rules(rules: &[crate::Rule], input: &tt::Subtree) -> ExpandResult<tt::
     }
     if let Some((match_, rule)) = match_ {
         // if we got here, there was no match without errors
-        let ExpandResult(result, transcribe_err) =
+        let ExpandResult { value, err: transcribe_err } =
             transcriber::transcribe(&rule.rhs, &match_.bindings);
-        ExpandResult(result, match_.err.or(transcribe_err))
+        ExpandResult { value, err: match_.err.or(transcribe_err) }
     } else {
-        ExpandResult(tt::Subtree::default(), Some(ExpandError::NoMatchingRule))
+        ExpandResult::only_err(ExpandError::NoMatchingRule)
     }
 }
 
@@ -143,7 +143,10 @@ mod tests {
     }
 
     fn assert_err(macro_body: &str, invocation: &str, err: ExpandError) {
-        assert_eq!(expand_first(&create_rules(&format_macro(macro_body)), invocation).1, Some(err));
+        assert_eq!(
+            expand_first(&create_rules(&format_macro(macro_body)), invocation).err,
+            Some(err)
+        );
     }
 
     fn format_macro(macro_body: &str) -> String {
diff --git a/crates/mbe/src/mbe_expander/matcher.rs b/crates/mbe/src/mbe_expander/matcher.rs
index 39a8eefbd..3f8445897 100644
--- a/crates/mbe/src/mbe_expander/matcher.rs
+++ b/crates/mbe/src/mbe_expander/matcher.rs
@@ -158,7 +158,8 @@ fn match_subtree(
                         continue;
                     }
                 };
-                let ExpandResult(matched, match_err) = match_meta_var(kind.as_str(), src);
+                let ExpandResult { value: matched, err: match_err } =
+                    match_meta_var(kind.as_str(), src);
                 match matched {
                     Some(fragment) => {
                         res.bindings.inner.insert(name.clone(), Binding::Fragment(fragment));
@@ -342,17 +343,17 @@ impl<'a> TtIter<'a> {
                 token_trees: res.into_iter().cloned().collect(),
             })),
         };
-        ExpandResult(res, err)
+        ExpandResult { value: res, err }
     }
 
     pub(crate) fn eat_vis(&mut self) -> Option<tt::TokenTree> {
         let mut fork = self.clone();
         match fork.expect_fragment(Visibility) {
-            ExpandResult(tt, None) => {
+            ExpandResult { value: tt, err: None } => {
                 *self = fork;
                 tt
             }
-            ExpandResult(_, Some(_)) => None,
+            ExpandResult { value: _, err: Some(_) } => None,
         }
     }
 }
diff --git a/crates/mbe/src/mbe_expander/transcriber.rs b/crates/mbe/src/mbe_expander/transcriber.rs
index c9525c5bf..616119ba9 100644
--- a/crates/mbe/src/mbe_expander/transcriber.rs
+++ b/crates/mbe/src/mbe_expander/transcriber.rs
@@ -93,17 +93,18 @@ fn expand_subtree(
         match op {
             Op::TokenTree(tt @ tt::TokenTree::Leaf(..)) => arena.push(tt.clone()),
             Op::TokenTree(tt::TokenTree::Subtree(tt)) => {
-                let ExpandResult(tt, e) = expand_subtree(ctx, tt, arena);
+                let ExpandResult { value: tt, err: e } = expand_subtree(ctx, tt, arena);
                 err = err.or(e);
                 arena.push(tt.into());
             }
             Op::Var { name, kind: _ } => {
-                let ExpandResult(fragment, e) = expand_var(ctx, name);
+                let ExpandResult { value: fragment, err: e } = expand_var(ctx, name);
                 err = err.or(e);
                 push_fragment(arena, fragment);
             }
             Op::Repeat { subtree, kind, separator } => {
-                let ExpandResult(fragment, e) = expand_repeat(ctx, subtree, kind, separator, arena);
+                let ExpandResult { value: fragment, err: e } =
+                    expand_repeat(ctx, subtree, kind, separator, arena);
                 err = err.or(e);
                 push_fragment(arena, fragment)
             }
@@ -111,7 +112,7 @@ fn expand_subtree(
     }
     // drain the elements added in this instance of expand_subtree
     let tts = arena.drain(start_elements..arena.len()).collect();
-    ExpandResult(tt::Subtree { delimiter: template.delimiter, token_trees: tts }, err)
+    ExpandResult { value: tt::Subtree { delimiter: template.delimiter, token_trees: tts }, err }
 }
 
 fn expand_var(ctx: &mut ExpandCtx, v: &SmolStr) -> ExpandResult<Fragment> {
@@ -152,7 +153,7 @@ fn expand_var(ctx: &mut ExpandCtx, v: &SmolStr) -> ExpandResult<Fragment> {
         ExpandResult::ok(Fragment::Tokens(tt))
     } else {
         ctx.bindings.get(&v, &mut ctx.nesting).map_or_else(
-            |e| ExpandResult(Fragment::Tokens(tt::TokenTree::empty()), Some(e)),
+            |e| ExpandResult { value: Fragment::Tokens(tt::TokenTree::empty()), err: Some(e) },
             |b| ExpandResult::ok(b.clone()),
         )
     }
@@ -174,7 +175,7 @@ fn expand_repeat(
     let mut counter = 0;
 
     loop {
-        let ExpandResult(mut t, e) = expand_subtree(ctx, template, arena);
+        let ExpandResult { value: mut t, err: e } = expand_subtree(ctx, template, arena);
         let nesting_state = ctx.nesting.last_mut().unwrap();
         if nesting_state.at_end || !nesting_state.hit {
             break;
@@ -234,7 +235,10 @@ fn expand_repeat(
     let tt = tt::Subtree { delimiter: None, token_trees: buf }.into();
 
     if RepeatKind::OneOrMore == kind && counter == 0 {
-        return ExpandResult(Fragment::Tokens(tt), Some(ExpandError::UnexpectedToken));
+        return ExpandResult {
+            value: Fragment::Tokens(tt),
+            err: Some(ExpandError::UnexpectedToken),
+        };
     }
     ExpandResult::ok(Fragment::Tokens(tt))
 }