macros: Parse :meta properly

This allows us to match attribute bodies in macro invocations, which we
can use later down the line to perform conditional compilation

gcc/rust/expand/rust-macro-expand.cc

@@ -490,11 +490,7 @@ MacroExpander::match_fragment (Parser<MacroInvocLexer> &parser,
 
       // is meta attributes?
     case AST::MacroFragSpec::META:
-      // parser.parse_inner_attribute ?
-      // parser.parse_outer_attribute ?
-      // parser.parse_attribute_body ?
-      // parser.parse_doc_comment ?
-      gcc_unreachable ();
+      parser.parse_attribute_body ();
       break;
 
     case AST::MacroFragSpec::TT:

gcc/rust/parse/rust-parse.h

@@ -143,6 +143,7 @@ public:
   AST::Visibility parse_visibility ();
   std::unique_ptr<AST::IdentifierPattern> parse_identifier_pattern ();
   std::unique_ptr<AST::TokenTree> parse_token_tree ();
+  AST::Attribute parse_attribute_body ();
 
 private:
   void skip_after_semicolon ();
@@ -162,7 +163,6 @@ private:
   AST::Attribute parse_inner_attribute ();
   AST::AttrVec parse_outer_attributes ();
   AST::Attribute parse_outer_attribute ();
-  AST::Attribute parse_attribute_body ();
   std::unique_ptr<AST::AttrInput> parse_attr_input ();
   AST::Attribute parse_doc_comment ();
 

gcc/testsuite/rust/execute/torture/macros27.rs

@@ -0,0 +1,24 @@
+// { dg-additional-options "-frust-cfg=A" }
+
+macro_rules! attr {
+    (#[$attr:meta] $s:stmt) => {
+        #[$attr]
+        $s;
+    };
+}
+
+fn main() -> i32 {
+    let mut a = 0;
+
+    attr! {
+    #[cfg(A)]
+        a = 3
+    };
+
+    attr! {
+    #[cfg(B)]
+        a = 40
+    };
+
+    a - 3
+}