Merge #1105

1105: [WIP] Implement ra_mbe meta variables support  r=matklad a=edwin0cheng

This PR implements the following meta variable support in `ra_mba` crate (issue  #720):

- [x] `path`
- [ ] `expr`
- [ ] `ty`
- [ ]  `pat`
- [ ] `stmt`
- [ ]  `block`
- [ ]  `meta`
- [ ] `item`

*Implementation Details*

In the macro expanding lhs phase, if we see a meta variable type, we try to create a `tt:TokenTree` from the remaining input. And then we use a special set of `ra_parser` to parse it to `SyntaxNode`. 


Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>

crates/ra_parser/src/grammar.rs

@@ -49,6 +49,10 @@ pub(crate) fn root(p: &mut Parser) {
     m.complete(p, SOURCE_FILE);
 }
 
+pub(crate) fn path(p: &mut Parser) {
+    paths::type_path(p);
+}
+
 pub(crate) fn reparser(
     node: SyntaxKind,
     first_child: Option<SyntaxKind>,

crates/ra_parser/src/lib.rs

@@ -61,6 +61,14 @@ pub fn parse(token_source: &dyn TokenSource, tree_sink: &mut dyn TreeSink) {
     event::process(tree_sink, events);
 }
 
+/// Parse given tokens into the given sink as a path
+pub fn parse_path(token_source: &dyn TokenSource, tree_sink: &mut dyn TreeSink) {
+    let mut p = parser::Parser::new(token_source);
+    grammar::path(&mut p);
+    let events = p.finish();
+    event::process(tree_sink, events);
+}
+
 /// A parsing function for a specific braced-block.
 pub struct Reparser(fn(&mut parser::Parser));