6 files changed, 63 insertions, 43 deletions

diff --git a/crates/ra_hir_expand/src/db.rs b/crates/ra_hir_expand/src/db.rs
index f1918817e..d171d2dfd 100644
--- a/crates/ra_hir_expand/src/db.rs
+++ b/crates/ra_hir_expand/src/db.rs
@@ -2,7 +2,7 @@
 
 use std::sync::Arc;
 
-use mbe::MacroRules;
+use mbe::{ExpandResult, MacroRules};
 use ra_db::{salsa, SourceDatabase};
 use ra_parser::FragmentKind;
 use ra_prof::profile;
@@ -31,12 +31,8 @@ impl TokenExpander {
         match self {
             TokenExpander::MacroRules(it) => it.expand(tt),
             // FIXME switch these to ExpandResult as well
-            TokenExpander::Builtin(it) => it
-                .expand(db, id, tt)
-                .map_or_else(|e| (tt::Subtree::default(), Some(e)), |r| (r, None)),
-            TokenExpander::BuiltinDerive(it) => it
-                .expand(db, id, tt)
-                .map_or_else(|e| (tt::Subtree::default(), Some(e)), |r| (r, None)),
+            TokenExpander::Builtin(it) => it.expand(db, id, tt).into(),
+            TokenExpander::BuiltinDerive(it) => it.expand(db, id, tt).into(),
         }
     }
 
@@ -204,7 +200,7 @@ fn macro_expand_with_arg(
         Some(it) => it,
         None => return (None, Some("Fail to find macro definition".into())),
     };
-    let (tt, err) = macro_rules.0.expand(db, lazy_id, &macro_arg.0);
+    let ExpandResult(tt, err) = macro_rules.0.expand(db, lazy_id, &macro_arg.0);
     // Set a hard limit for the expanded tt
     let count = tt.count();
     if count > 65536 {
diff --git a/crates/ra_mbe/src/lib.rs b/crates/ra_mbe/src/lib.rs
index 3adec4978..6a9037bfc 100644
--- a/crates/ra_mbe/src/lib.rs
+++ b/crates/ra_mbe/src/lib.rs
@@ -30,8 +30,6 @@ pub enum ExpandError {
     InvalidRepeat,
 }
 
-pub type ExpandResult<T> = (T, Option<ExpandError>);
-
 pub use crate::syntax_bridge::{
     ast_to_token_tree, parse_to_token_tree, syntax_node_to_token_tree, token_tree_to_syntax_node,
     TokenMap,
@@ -211,5 +209,35 @@ fn validate(pattern: &tt::Subtree) -> Result<(), ParseError> {
     Ok(())
 }
 
+pub struct ExpandResult<T>(pub T, pub Option<ExpandError>);
+
+impl<T> ExpandResult<T> {
+    pub fn ok(t: T) -> ExpandResult<T> {
+        ExpandResult(t, None)
+    }
+
+    pub fn only_err(err: ExpandError) -> ExpandResult<T>
+    where
+        T: Default,
+    {
+        ExpandResult(Default::default(), Some(err))
+    }
+
+    pub fn map<U>(self, f: impl FnOnce(T) -> U) -> ExpandResult<U> {
+        ExpandResult(f(self.0), self.1)
+    }
+
+    pub fn result(self) -> Result<T, ExpandError> {
+        self.1.map(Err).unwrap_or(Ok(self.0))
+    }
+}
+
+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))
+    }
+}
+
 #[cfg(test)]
 mod tests;
diff --git a/crates/ra_mbe/src/mbe_expander.rs b/crates/ra_mbe/src/mbe_expander.rs
index 204c30e3d..0a4d73dda 100644
--- a/crates/ra_mbe/src/mbe_expander.rs
+++ b/crates/ra_mbe/src/mbe_expander.rs
@@ -18,12 +18,13 @@ fn expand_rules(rules: &[crate::Rule], input: &tt::Subtree) -> ExpandResult<tt::
     let mut match_: Option<(matcher::Match, &crate::Rule)> = None;
     let mut err = Some(ExpandError::NoMatchingRule);
     for rule in rules {
-        let (new_match, bindings_err) = matcher::match_(&rule.lhs, input);
+        let ExpandResult(new_match, bindings_err) = matcher::match_(&rule.lhs, input);
         if bindings_err.is_none() {
             // if we find a rule that applies without errors, we're done
-            let (res, transcribe_err) = transcriber::transcribe(&rule.rhs, &new_match.bindings);
+            let ExpandResult(res, transcribe_err) =
+                transcriber::transcribe(&rule.rhs, &new_match.bindings);
             if transcribe_err.is_none() {
-                return (res, None);
+                return ExpandResult::ok(res);
             }
         }
         // use the rule if we matched more tokens, or had fewer patterns left
@@ -43,10 +44,11 @@ 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 (result, transcribe_err) = transcriber::transcribe(&rule.rhs, &match_.bindings);
-        (result, err.or(transcribe_err))
+        let ExpandResult(result, transcribe_err) =
+            transcriber::transcribe(&rule.rhs, &match_.bindings);
+        ExpandResult(result, err.or(transcribe_err))
     } else {
-        (tt::Subtree::default(), err)
+        ExpandResult(tt::Subtree::default(), err)
     }
 }
 
@@ -171,6 +173,6 @@ mod tests {
             ast_to_token_tree(&macro_invocation.token_tree().unwrap()).unwrap();
 
         let expanded = expand_rules(&rules.rules, &invocation_tt);
-        (expanded.0, expanded.1)
+        expanded
     }
 }
diff --git a/crates/ra_mbe/src/mbe_expander/matcher.rs b/crates/ra_mbe/src/mbe_expander/matcher.rs
index f40d55d4e..22fb5d74c 100644
--- a/crates/ra_mbe/src/mbe_expander/matcher.rs
+++ b/crates/ra_mbe/src/mbe_expander/matcher.rs
@@ -79,7 +79,7 @@ pub(super) fn match_(pattern: &tt::Subtree, src: &tt::Subtree) -> ExpandResult<M
         err = Some(err!("leftover tokens"));
     }
 
-    (res, err)
+    ExpandResult(res, err)
 }
 
 fn match_subtree(
@@ -148,7 +148,7 @@ fn match_subtree(
                         continue;
                     }
                 };
-                let (matched, match_err) = match_meta_var(kind.as_str(), src);
+                let ExpandResult(matched, match_err) = match_meta_var(kind.as_str(), src);
                 match matched {
                     Some(fragment) => {
                         res.bindings.inner.insert(name.clone(), Binding::Fragment(fragment));
@@ -308,17 +308,17 @@ impl<'a> TtIter<'a> {
                 token_trees: res.into_iter().cloned().collect(),
             })),
         };
-        (res, err)
+        ExpandResult(res, err)
     }
 
     pub(crate) fn eat_vis(&mut self) -> Option<tt::TokenTree> {
         let mut fork = self.clone();
         match fork.expect_fragment(Visibility) {
-            (tt, None) => {
+            ExpandResult(tt, None) => {
                 *self = fork;
                 tt
             }
-            (_, Some(_)) => None,
+            ExpandResult(_, Some(_)) => None,
         }
     }
 }
@@ -419,12 +419,11 @@ fn match_meta_var(kind: &str, input: &mut TtIter) -> ExpandResult<Option<Fragmen
                 },
                 _ => Err(ExpandError::UnexpectedToken),
             };
-            return to_expand_result(tt_result.map(|it| it.map(Fragment::Tokens)));
+            return tt_result.map(|it| it.map(Fragment::Tokens)).into();
         }
     };
-    let (tt, err) = input.expect_fragment(fragment);
-    let fragment = if kind == "expr" { tt.map(Fragment::Ast) } else { tt.map(Fragment::Tokens) };
-    (fragment, err)
+    let result = input.expect_fragment(fragment);
+    result.map(|tt| if kind == "expr" { tt.map(Fragment::Ast) } else { tt.map(Fragment::Tokens) })
 }
 
 fn collect_vars(buf: &mut Vec<SmolStr>, pattern: &tt::Subtree) -> Result<(), ExpandError> {
@@ -438,7 +437,3 @@ fn collect_vars(buf: &mut Vec<SmolStr>, pattern: &tt::Subtree) -> Result<(), Exp
     }
     Ok(())
 }
-
-fn to_expand_result<T: Default>(result: Result<T, ExpandError>) -> ExpandResult<T> {
-    result.map_or_else(|e| (Default::default(), Some(e)), |it| (it, None))
-}
diff --git a/crates/ra_mbe/src/mbe_expander/transcriber.rs b/crates/ra_mbe/src/mbe_expander/transcriber.rs
index c53c2d35e..4b173edd3 100644
--- a/crates/ra_mbe/src/mbe_expander/transcriber.rs
+++ b/crates/ra_mbe/src/mbe_expander/transcriber.rs
@@ -87,23 +87,23 @@ fn expand_subtree(ctx: &mut ExpandCtx, template: &tt::Subtree) -> ExpandResult<t
         match op {
             Op::TokenTree(tt @ tt::TokenTree::Leaf(..)) => buf.push(tt.clone()),
             Op::TokenTree(tt::TokenTree::Subtree(tt)) => {
-                let (tt, e) = expand_subtree(ctx, tt);
+                let ExpandResult(tt, e) = expand_subtree(ctx, tt);
                 err = err.or(e);
                 buf.push(tt.into());
             }
             Op::Var { name, kind: _ } => {
-                let (fragment, e) = expand_var(ctx, name);
+                let ExpandResult(fragment, e) = expand_var(ctx, name);
                 err = err.or(e);
                 push_fragment(&mut buf, fragment);
             }
             Op::Repeat { subtree, kind, separator } => {
-                let (fragment, e) = expand_repeat(ctx, subtree, kind, separator);
+                let ExpandResult(fragment, e) = expand_repeat(ctx, subtree, kind, separator);
                 err = err.or(e);
                 push_fragment(&mut buf, fragment)
             }
         }
     }
-    (tt::Subtree { delimiter: template.delimiter, token_trees: buf }, err)
+    ExpandResult(tt::Subtree { delimiter: template.delimiter, token_trees: buf }, err)
 }
 
 fn expand_var(ctx: &mut ExpandCtx, v: &SmolStr) -> ExpandResult<Fragment> {
@@ -112,7 +112,7 @@ fn expand_var(ctx: &mut ExpandCtx, v: &SmolStr) -> ExpandResult<Fragment> {
         let tt =
             tt::Leaf::from(tt::Ident { text: "$crate".into(), id: tt::TokenId::unspecified() })
                 .into();
-        (Fragment::Tokens(tt), None)
+        ExpandResult::ok(Fragment::Tokens(tt))
     } else if !ctx.bindings.contains(v) {
         // Note that it is possible to have a `$var` inside a macro which is not bound.
         // For example:
@@ -141,11 +141,11 @@ fn expand_var(ctx: &mut ExpandCtx, v: &SmolStr) -> ExpandResult<Fragment> {
             ],
         }
         .into();
-        (Fragment::Tokens(tt), None)
+        ExpandResult::ok(Fragment::Tokens(tt))
     } else {
         ctx.bindings.get(&v, &mut ctx.nesting).map_or_else(
-            |e| (Fragment::Tokens(tt::TokenTree::empty()), Some(e)),
-            |b| (b.clone(), None),
+            |e| ExpandResult(Fragment::Tokens(tt::TokenTree::empty()), Some(e)),
+            |b| ExpandResult::ok(b.clone()),
         )
     }
 }
@@ -165,7 +165,7 @@ fn expand_repeat(
     let mut counter = 0;
 
     loop {
-        let (mut t, e) = expand_subtree(ctx, template);
+        let ExpandResult(mut t, e) = expand_subtree(ctx, template);
         let nesting_state = ctx.nesting.last_mut().unwrap();
         if nesting_state.at_end || !nesting_state.hit {
             break;
@@ -225,9 +225,9 @@ fn expand_repeat(
     let tt = tt::Subtree { delimiter: None, token_trees: buf }.into();
 
     if RepeatKind::OneOrMore == kind && counter == 0 {
-        return (Fragment::Tokens(tt), Some(ExpandError::UnexpectedToken));
+        return ExpandResult(Fragment::Tokens(tt), Some(ExpandError::UnexpectedToken));
     }
-    (Fragment::Tokens(tt), None)
+    ExpandResult::ok(Fragment::Tokens(tt))
 }
 
 fn push_fragment(buf: &mut Vec<tt::TokenTree>, fragment: Fragment) {
diff --git a/crates/ra_mbe/src/tests.rs b/crates/ra_mbe/src/tests.rs
index faf88c1b1..44f381938 100644
--- a/crates/ra_mbe/src/tests.rs
+++ b/crates/ra_mbe/src/tests.rs
@@ -1430,8 +1430,7 @@ impl MacroFixture {
         let (invocation_tt, _) =
             ast_to_token_tree(&macro_invocation.token_tree().unwrap()).unwrap();
 
-        let (tt, err) = self.rules.expand(&invocation_tt);
-        err.map(Err).unwrap_or(Ok(tt))
+        self.rules.expand(&invocation_tt).result()
     }
 
     fn assert_expand_err(&self, invocation: &str, err: &ExpandError) {