Combine all tokensource to one and refactoring

crates/ra_mbe/src/subtree_parser.rs

@@ -0,0 +1,59 @@
+use crate::subtree_source::SubtreeTokenSource;
+
+use ra_parser::{TokenSource, TreeSink};
+use ra_syntax::{SyntaxKind};
+
+struct OffsetTokenSink {
+    token_pos: usize,
+}
+
+impl TreeSink for OffsetTokenSink {
+    fn token(&mut self, _kind: SyntaxKind, n_tokens: u8) {
+        self.token_pos += n_tokens as usize;
+    }
+    fn start_node(&mut self, _kind: SyntaxKind) {}
+    fn finish_node(&mut self) {}
+    fn error(&mut self, _error: ra_parser::ParseError) {}
+}
+
+pub(crate) struct Parser<'a> {
+    subtree: &'a tt::Subtree,
+    pos: &'a mut usize,
+}
+
+impl<'a> Parser<'a> {
+    pub fn new(pos: &'a mut usize, subtree: &'a tt::Subtree) -> Parser<'a> {
+        Parser { pos, subtree }
+    }
+
+    pub fn parse_path(self) -> Option<tt::TokenTree> {
+        self.parse(ra_parser::parse_path)
+    }
+
+    fn parse<F>(self, f: F) -> Option<tt::TokenTree>
+    where
+        F: FnOnce(&dyn TokenSource, &mut dyn TreeSink),
+    {
+        let mut src = SubtreeTokenSource::new(self.subtree);
+        src.advance(*self.pos, true);
+        let mut sink = OffsetTokenSink { token_pos: 0 };
+
+        f(&src, &mut sink);
+
+        self.finish(sink.token_pos, &mut src)
+    }
+
+    fn finish(self, parsed_token: usize, src: &mut SubtreeTokenSource) -> Option<tt::TokenTree> {
+        let res = src.bump_n(parsed_token, self.pos);
+        let res: Vec<_> = res.into_iter().cloned().collect();
+
+        match res.len() {
+            0 => None,
+            1 => Some(res[0].clone()),
+            _ => Some(tt::TokenTree::Subtree(tt::Subtree {
+                delimiter: tt::Delimiter::None,
+                token_trees: res,
+            })),
+        }
+    }
+}