Forbid pub mymacro!();

It's not clear what this means, because a macro in item position can expand to
zero or more items.  For now we disallow it, which is technically a

    [breaking-change]

but is landing without an RFC.  The `pub` keyword previously had no effect,
which seems quite unintended.

Fixes #18317.
Fixes #14660.

src/test/compile-fail/pub-item-macro.rs

@@ -0,0 +1,28 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Issue #14660
+
+macro_rules! priv_x { () => {
+    static x: u32 = 0;
+}}
+
+macro_rules! pub_x { () => {
+    pub priv_x!(); //~ ERROR can't qualify macro invocation with `pub`
+    //~^ HELP try adjusting the macro to put `pub` inside the invocation
+}}
+
+mod foo {
+    pub_x!();
+}
+
+fn main() {
+    let y: u32 = foo::x;
+}

src/test/compile-fail/pub-method-macro.rs

@@ -0,0 +1,34 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Issue #18317
+
+mod bleh {
+    macro_rules! defn {
+        ($n:ident) => (
+            fn $n (&self) -> i32 {
+                println!("{}", stringify!($n));
+                1
+            }
+        )
+    }
+
+    #[derive(Copy)]
+    pub struct S;
+
+    impl S {
+        pub defn!(f); //~ ERROR can't qualify macro invocation with `pub`
+        //~^ HELP try adjusting the macro to put `pub` inside the invocation
+    }
+}
+
+fn main() {
+    bleh::S.f();
+}

src/test/run-pass/pub-item-inside-macro.rs

@@ -0,0 +1,25 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Issue #14660
+
+mod bleh {
+    macro_rules! foo {
+        () => {
+            pub fn bar() { }
+        }
+    }
+
+    foo!();
+}
+
+fn main() {
+    bleh::bar();
+}