Handle None-delimited groups when parsing macro_rules! macro

When a `macro_rules!` macro expands to another `macro_rules!` macro, we
may see `None`-delimited groups in odd places when another crate
deserializes the 'inner' macro. This commit 'unwraps' an outer
`None`-delimited group to avoid breaking existing code.

See #73569 (comment)
for more details.

The proper fix is to handle `None`-delimited groups systematically
throughout the parser, but that will require significant work. In the
meantime, this hack lets us fix important hygiene bugs in macros

src/librustc_expand/mbe/quoted.rs

@@ -90,72 +90,88 @@ pub(super) fn parse(
 /// # Parameters
 ///
 /// - `tree`: the tree we wish to convert.
-/// - `trees`: an iterator over trees. We may need to read more tokens from it in order to finish
+/// - `outer_trees`: an iterator over trees. We may need to read more tokens from it in order to finish
 ///   converting `tree`
 /// - `expect_matchers`: same as for `parse` (see above).
 /// - `sess`: the parsing session. Any errors will be emitted to this session.
 /// - `features`, `attrs`: language feature flags and attributes so that we know whether to use
 ///   unstable features or not.
 fn parse_tree(
     tree: tokenstream::TokenTree,
-    trees: &mut impl Iterator<Item = tokenstream::TokenTree>,
+    outer_trees: &mut impl Iterator<Item = tokenstream::TokenTree>,
     expect_matchers: bool,
     sess: &ParseSess,
     node_id: NodeId,
 ) -> TokenTree {
     // Depending on what `tree` is, we could be parsing different parts of a macro
     match tree {
         // `tree` is a `$` token. Look at the next token in `trees`
-        tokenstream::TokenTree::Token(Token { kind: token::Dollar, span }) => match trees.next() {
-            // `tree` is followed by a delimited set of token trees. This indicates the beginning
-            // of a repetition sequence in the macro (e.g. `$(pat)*`).
-            Some(tokenstream::TokenTree::Delimited(span, delim, tts)) => {
-                // Must have `(` not `{` or `[`
-                if delim != token::Paren {
-                    let tok = pprust::token_kind_to_string(&token::OpenDelim(delim));
-                    let msg = format!("expected `(`, found `{}`", tok);
-                    sess.span_diagnostic.span_err(span.entire(), &msg);
-                }
-                // Parse the contents of the sequence itself
-                let sequence = parse(tts, expect_matchers, sess, node_id);
-                // Get the Kleene operator and optional separator
-                let (separator, kleene) = parse_sep_and_kleene_op(trees, span.entire(), sess);
-                // Count the number of captured "names" (i.e., named metavars)
-                let name_captures = macro_parser::count_names(&sequence);
-                TokenTree::Sequence(
-                    span,
-                    Lrc::new(SequenceRepetition {
-                        tts: sequence,
-                        separator,
-                        kleene,
-                        num_captures: name_captures,
-                    }),
-                )
+        tokenstream::TokenTree::Token(Token { kind: token::Dollar, span }) => {
+            // FIXME: Handle `None`-delimited groups in a more systematic way
+            // during parsing.
+            let mut next = outer_trees.next();
+            let mut trees: Box<dyn Iterator<Item = tokenstream::TokenTree>>;
+            if let Some(tokenstream::TokenTree::Delimited(_, token::NoDelim, tts)) = next {
+                trees = Box::new(tts.into_trees());
+                next = trees.next();
+            } else {
+                trees = Box::new(outer_trees);
             }
 
-            // `tree` is followed by an `ident`. This could be `$meta_var` or the `$crate` special
-            // metavariable that names the crate of the invocation.
-            Some(tokenstream::TokenTree::Token(token)) if token.is_ident() => {
-                let (ident, is_raw) = token.ident().unwrap();
-                let span = ident.span.with_lo(span.lo());
-                if ident.name == kw::Crate && !is_raw {
-                    TokenTree::token(token::Ident(kw::DollarCrate, is_raw), span)
-                } else {
-                    TokenTree::MetaVar(span, ident)
+            match next {
+                // `tree` is followed by a delimited set of token trees. This indicates the beginning
+                // of a repetition sequence in the macro (e.g. `$(pat)*`).
+                Some(tokenstream::TokenTree::Delimited(span, delim, tts)) => {
+                    // Must have `(` not `{` or `[`
+                    if delim != token::Paren {
+                        let tok = pprust::token_kind_to_string(&token::OpenDelim(delim));
+                        let msg = format!("expected `(`, found `{}`", tok);
+                        sess.span_diagnostic.span_err(span.entire(), &msg);
+                    }
+                    // Parse the contents of the sequence itself
+                    let sequence = parse(tts, expect_matchers, sess, node_id);
+                    // Get the Kleene operator and optional separator
+                    let (separator, kleene) =
+                        parse_sep_and_kleene_op(&mut trees, span.entire(), sess);
+                    // Count the number of captured "names" (i.e., named metavars)
+                    let name_captures = macro_parser::count_names(&sequence);
+                    TokenTree::Sequence(
+                        span,
+                        Lrc::new(SequenceRepetition {
+                            tts: sequence,
+                            separator,
+                            kleene,
+                            num_captures: name_captures,
+                        }),
+                    )
                 }
-            }
 
-            // `tree` is followed by a random token. This is an error.
-            Some(tokenstream::TokenTree::Token(token)) => {
-                let msg =
-                    format!("expected identifier, found `{}`", pprust::token_to_string(&token),);
-                sess.span_diagnostic.span_err(token.span, &msg);
-                TokenTree::MetaVar(token.span, Ident::invalid())
-            }
+                // `tree` is followed by an `ident`. This could be `$meta_var` or the `$crate` special
+                // metavariable that names the crate of the invocation.
+                Some(tokenstream::TokenTree::Token(token)) if token.is_ident() => {
+                    let (ident, is_raw) = token.ident().unwrap();
+                    let span = ident.span.with_lo(span.lo());
+                    if ident.name == kw::Crate && !is_raw {
+                        TokenTree::token(token::Ident(kw::DollarCrate, is_raw), span)
+                    } else {
+                        TokenTree::MetaVar(span, ident)
+                    }
+                }
 
-            // There are no more tokens. Just return the `$` we already have.
-            None => TokenTree::token(token::Dollar, span),
-        },
+                // `tree` is followed by a random token. This is an error.
+                Some(tokenstream::TokenTree::Token(token)) => {
+                    let msg = format!(
+                        "expected identifier, found `{}`",
+                        pprust::token_to_string(&token),
+                    );
+                    sess.span_diagnostic.span_err(token.span, &msg);
+                    TokenTree::MetaVar(token.span, Ident::invalid())
+                }
+
+                // There are no more tokens. Just return the `$` we already have.
+                None => TokenTree::token(token::Dollar, span),
+            }
+        }
 
         // `tree` is an arbitrary token. Keep it.
         tokenstream::TokenTree::Token(token) => TokenTree::Token(token),

src/test/ui/proc-macro/auxiliary/meta-delim.rs

@@ -0,0 +1,12 @@
+macro_rules! produce_it {
+    ($dollar_one:tt $foo:ident $my_name:ident) => {
+        #[macro_export]
+        macro_rules! meta_delim {
+            ($dollar_one ($dollar_one $my_name:ident)*) => {
+                stringify!($dollar_one ($dollar_one $my_name)*)
+            }
+        }
+    }
+}
+
+produce_it!($my_name name);

src/test/ui/proc-macro/meta-delim.rs

@@ -0,0 +1,12 @@
+// aux-build:meta-delim.rs
+// edition:2018
+// run-pass
+
+// Tests that we can properly deserialize a macro with strange delimiters
+// See https://github.com/rust-lang/rust/pull/73569#issuecomment-650860457
+
+extern crate meta_delim;
+
+fn main() {
+    assert_eq!("a bunch of idents", meta_delim::meta_delim!(a bunch of idents));
+}