Introduce #[rustc_dummy] attribute and use it in tests

Unlike other built-in attributes, this attribute accepts any input

Author: petrochenkov

src/libsyntax/feature_gate.rs

@@ -1336,6 +1336,11 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
                                                 "internal implementation detail",
                                                 cfg_fn!(rustc_attrs))),
 
+    (sym::rustc_dummy, Normal, template!(Word /* doesn't matter*/), Gated(Stability::Unstable,
+                                         sym::rustc_attrs,
+                                         "used by the test suite",
+                                         cfg_fn!(rustc_attrs))),
+
     // FIXME: #14408 whitelist docs since rustdoc looks at them
     (
         sym::doc,
@@ -1962,12 +1967,10 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         }
 
         match attr_info {
-            Some(&(name, _, template, _)) => self.check_builtin_attribute(
-                attr,
-                name,
-                template
-            ),
-            None => if let Some(TokenTree::Token(token)) = attr.tokens.trees().next() {
+            // `rustc_dummy` doesn't have any restrictions specific to built-in attributes.
+            Some(&(name, _, template, _)) if name != sym::rustc_dummy =>
+                self.check_builtin_attribute(attr, name, template),
+            _ => if let Some(TokenTree::Token(token)) = attr.tokens.trees().next() {
                 if token == token::Eq {
                     // All key-value attributes are restricted to meta-item syntax.
                     attr.parse_meta(self.context.parse_sess).map_err(|mut err| err.emit()).ok();

src/libsyntax_pos/symbol.rs

@@ -527,6 +527,7 @@ symbols! {
         rustc_diagnostic_macros,
         rustc_dirty,
         rustc_doc_only_macro,
+        rustc_dummy,
         rustc_dump_env_program_clauses,
         rustc_dump_program_clauses,
         rustc_dump_user_substs,

src/test/pretty/attr-fn-inner.rs

@@ -1,15 +1,16 @@
-// pp-exact
 // Testing that both the inner item and next outer item are
 // preserved, and that the first outer item parsed in main is not
 // accidentally carried over to each inner function
 
-#![feature(custom_attribute)]
+// pp-exact
+
+#![feature(rustc_attrs)]
 
 fn main() {
-    #![inner_attr]
-    #[outer_attr]
+    #![rustc_dummy]
+    #[rustc_dummy]
     fn f() { }
 
-    #[outer_attr]
+    #[rustc_dummy]
     fn g() { }
 }

src/test/pretty/attr-literals.rs

@@ -1,13 +1,14 @@
-// pp-exact
 // Tests literals in attributes.
 
-#![feature(custom_attribute)]
+// pp-exact
+
+#![feature(rustc_attrs)]
 
 fn main() {
-    #![hello("hi", 1, 2, 1.012, pi = 3.14, bye, name("John"))]
-    #[align = 8]
+    #![rustc_dummy("hi", 1, 2, 1.012, pi = 3.14, bye, name("John"))]
+    #[rustc_dummy = 8]
     fn f() { }
 
-    #[vector(1, 2, 3)]
+    #[rustc_dummy(1, 2, 3)]
     fn g() { }
 }