auto merge of #9336 : alexcrichton/rust/issue-7981, r=catamorphism

Progress on #7981

This doesn't completely close the issue because `struct A;` is still allowed, and it's a much larger change to disallow that. I'm also not entirely sure that we want to disallow that. Regardless, punting that discussion to the issue instead.

src/libsyntax/parse/obsolete.rs

@@ -65,6 +65,7 @@ pub enum ObsoleteSyntax {
     ObsoletePrivVisibility,
     ObsoleteTraitFuncVisibility,
     ObsoleteConstPointer,
+    ObsoleteEmptyImpl,
 }
 
 impl to_bytes::IterBytes for ObsoleteSyntax {
@@ -256,6 +257,10 @@ impl ParserObsoleteMethods for Parser {
                 "instead of `&const Foo` or `@const Foo`, write `&Foo` or \
                  `@Foo`"
             ),
+            ObsoleteEmptyImpl => (
+                "empty implementation",
+                "instead of `impl A;`, write `impl A {}`"
+            ),
         };
 
         self.report(sp, kind, kind_str, desc);

src/libsyntax/parse/parser.rs

@@ -3852,7 +3852,9 @@ impl Parser {
         }
 
         let mut meths = ~[];
-        if !self.eat(&token::SEMI) {
+        if self.eat(&token::SEMI) {
+            self.obsolete(*self.span, ObsoleteEmptyImpl);
+        } else {
             self.expect(&token::LBRACE);
             while !self.eat(&token::RBRACE) {
                 meths.push(self.parse_method());

src/libsyntax/print/pprust.rs

@@ -598,18 +598,12 @@ pub fn print_item(s: @ps, item: &ast::item) {
 
         print_type(s, ty);
 
-        if methods.len() == 0 {
-            word(s.s, ";");
-            end(s); // end the head-ibox
-            end(s); // end the outer cbox
-        } else {
-            space(s.s);
-            bopen(s);
-            for meth in methods.iter() {
-               print_method(s, *meth);
-            }
-            bclose(s, item.span);
+        space(s.s);
+        bopen(s);
+        for meth in methods.iter() {
+           print_method(s, *meth);
         }
+        bclose(s, item.span);
       }
       ast::item_trait(ref generics, ref traits, ref methods) => {
         head(s, visibility_qualified(item.vis, "trait"));