parsing scaffold

crates/ra_hir/src/macros/mbe.rs

@@ -9,8 +9,8 @@ pub(crate) struct MacroRules {
 
 #[derive(Debug)]
 struct Rule {
-    lhs: TokenTree,
-    rhs: TokenTree,
+    lhs: Subtree,
+    rhs: Subtree,
 }
 
 #[derive(Debug)]
@@ -75,6 +75,47 @@ struct Var {
     text: SmolStr,
 }
 
-pub(crate) fn parse(tt: &tt::Subtree) -> MacroRules {
-    MacroRules { rules: Vec::new() }
+pub(crate) fn parse(tt: &tt::Subtree) -> Option<MacroRules> {
+    let mut parser = RulesParser::new(tt);
+    let mut rules = Vec::new();
+    while !parser.is_eof() {
+        rules.push(parse_rule(&mut parser)?)
+    }
+    Some(MacroRules { rules })
+}
+
+fn parse_rule(p: &mut RulesParser) -> Option<Rule> {
+    let lhs = match p.current()? {
+        tt::TokenTree::Subtree(sub) => parse_subtree(sub)?,
+        _ => return None,
+    };
+    let rhs = unimplemented!();
+    Some(Rule { lhs, rhs })
+}
+
+fn parse_subtree(tt: &tt::Subtree) -> Option<Subtree> {
+    unimplemented!()
+}
+
+struct RulesParser<'a> {
+    subtree: &'a tt::Subtree,
+    pos: usize,
+}
+
+impl<'a> RulesParser<'a> {
+    fn new(subtree: &'a tt::Subtree) -> RulesParser<'a> {
+        RulesParser { subtree, pos: 0 }
+    }
+
+    fn is_eof(&self) -> bool {
+        self.pos == self.subtree.token_trees.len()
+    }
+
+    fn current(&self) -> Option<&'a tt::TokenTree> {
+        self.subtree.token_trees.get(self.pos)
+    }
+
+    fn bump(&mut self) {
+        self.pos += 1;
+    }
 }