Merge #3425

3425: Fix a bug for single dollar sign macro r=matklad a=edwin0cheng

This PR fixed a bug to allow the following valid `macro_rules!` :

```rust
macro_rules! m {
    ($) => ($)
}
```

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

crates/ra_hir_expand/src/db.rs

@@ -86,10 +86,13 @@ pub(crate) fn macro_def(
                 log::warn!("fail on macro_def to token tree: {:#?}", arg);
                 None
             })?;
-            let rules = MacroRules::parse(&tt).ok().or_else(|| {
-                log::warn!("fail on macro_def parse: {:#?}", tt);
-                None
-            })?;
+            let rules = match MacroRules::parse(&tt) {
+                Ok(it) => it,
+                Err(err) => {
+                    log::warn!("fail on macro_def parse: error: {:#?} {:#?}", err, tt);
+                    return None;
+                }
+            };
             Some(Arc::new((TokenExpander::MacroRules(rules), tmap)))
         }
         MacroDefKind::BuiltIn(expander) => {
@@ -150,7 +153,9 @@ pub(crate) fn parse_macro(
             // Note:
             // The final goal we would like to make all parse_macro success,
             // such that the following log will not call anyway.
-            log::warn!("fail on macro_parse: (reason: {})", err,);
+            let loc: MacroCallLoc = db.lookup_intern_macro(macro_call_id);
+            let node = loc.kind.node(db);
+            log::warn!("fail on macro_parse: (reason: {} macro_call: {:#})", err, node.value);
         })
         .ok()?;
 

crates/ra_mbe/src/parser.rs

@@ -90,7 +90,11 @@ fn next_op<'a>(
 ) -> Result<Op<'a>, ExpandError> {
     let res = match first {
         tt::TokenTree::Leaf(tt::Leaf::Punct(tt::Punct { char: '$', .. })) => {
-            let second = src.next().ok_or_else(|| err!("bad var 1"))?;
+            // Note that the '$' itself is a valid token inside macro_rules.
+            let second = match src.next() {
+                None => return Ok(Op::TokenTree(first)),
+                Some(it) => it,
+            };
             match second {
                 tt::TokenTree::Subtree(subtree) => {
                     let (separator, kind) = parse_repeat(src)?;

crates/ra_mbe/src/tests.rs

@@ -23,6 +23,7 @@ mod rule_parsing {
         check("($($i:ident)*) => ($_)");
         check("($($true:ident)*) => ($true)");
         check("($($false:ident)*) => ($false)");
+        check("($) => ($)");
     }
 
     #[test]