Attempt to implement ranking of rules when none matches perfectly (wip)

crates/ra_ide/src/completion/complete_scope.rs

@@ -811,7 +811,44 @@ mod tests {
                 }
                 "
             ),
-            @"[]"
+            @r###"
+        [
+            CompletionItem {
+                label: "m!",
+                source_range: [145; 145),
+                delete: [145; 145),
+                insert: "m!($0)",
+                kind: Macro,
+                detail: "macro_rules! m",
+            },
+            CompletionItem {
+                label: "quux(…)",
+                source_range: [145; 145),
+                delete: [145; 145),
+                insert: "quux(${1:x})$0",
+                kind: Function,
+                lookup: "quux",
+                detail: "fn quux(x: i32)",
+                trigger_call_info: true,
+            },
+            CompletionItem {
+                label: "x",
+                source_range: [145; 145),
+                delete: [145; 145),
+                insert: "x",
+                kind: Binding,
+                detail: "i32",
+            },
+            CompletionItem {
+                label: "y",
+                source_range: [145; 145),
+                delete: [145; 145),
+                insert: "y",
+                kind: Binding,
+                detail: "i32",
+            },
+        ]
+        "###
         );
     }
 

crates/ra_mbe/src/mbe_expander.rs

@@ -14,24 +14,27 @@ pub(crate) fn expand(
     rules: &crate::MacroRules,
     input: &tt::Subtree,
 ) -> ExpandResult<tt::Subtree> {
-    let (mut result, mut err) = (tt::Subtree::default(), Some(ExpandError::NoMatchingRule));
+    let (mut result, mut left_over, mut err) = (tt::Subtree::default(), usize::max_value(), Some(ExpandError::NoMatchingRule));
     for rule in &rules.rules {
-        let (res, e) = expand_rule(rule, input);
+        let ((res, left), e) = expand_rule(rule, input);
         if e.is_none() {
             // if we find a rule that applies without errors, we're done
             return (res, None);
         }
-        // TODO decide which result is better
+        // use the rule if we matched more tokens
-        result = res;
+        if left < left_over {
-        err = e;
+            result = res;
+            err = e;
+            left_over = left;
+        }
     }
     (result, err)
 }
 
-fn expand_rule(rule: &crate::Rule, input: &tt::Subtree) -> ExpandResult<tt::Subtree> {
+fn expand_rule(rule: &crate::Rule, input: &tt::Subtree) -> ExpandResult<(tt::Subtree, usize)> {
-    let (bindings, bindings_err) = dbg!(matcher::match_(&rule.lhs, input));
+    let ((bindings, left_over), bindings_err) = dbg!(matcher::match_(&rule.lhs, input));
     let (res, transcribe_err) = dbg!(transcriber::transcribe(&rule.rhs, &bindings));
-    (res, bindings_err.or(transcribe_err))
+    ((res, left_over), bindings_err.or(transcribe_err))
 }
 
 /// The actual algorithm for expansion is not too hard, but is pretty tricky.

crates/ra_mbe/src/mbe_expander/matcher.rs

@@ -65,7 +65,7 @@ macro_rules! bail {
     };
 }
 
-pub(super) fn match_(pattern: &tt::Subtree, src: &tt::Subtree) -> ExpandResult<Bindings> {
+pub(super) fn match_(pattern: &tt::Subtree, src: &tt::Subtree) -> ExpandResult<(Bindings, usize)> {
     assert!(pattern.delimiter == None);
 
     let mut res = Bindings::default();
@@ -77,7 +77,7 @@ pub(super) fn match_(pattern: &tt::Subtree, src: &tt::Subtree) -> ExpandResult<B
         err = Some(err!("leftover tokens"));
     }
 
-    (res, err)
+    ((res, src.len()), err)
 }
 
 fn match_subtree(