rustc: Fix proc_macro expansions on trait methods

This commit fixes procedural macro attributes being attached to trait methods,
ensuring that they get resolved and expanded as other procedural macro
attributes. The bug here was that `current_module` on the resolver was
accidentally set to be a trait when it's otherwise only ever expecting a
`mod`/block module. The actual fix here came from @jseyfried, I'm just helping
to land it in the compiler!

Closes #42493

Author: alexcrichton

src/librustc_resolve/macros.rs

@@ -402,7 +402,8 @@ impl<'a> Resolver<'a> {
         let ast::Path { ref segments, span } = *path;
         let path: Vec<_> = segments.iter().map(|seg| respan(seg.span, seg.identifier)).collect();
         let invocation = self.invocations[&scope];
-        self.current_module = invocation.module.get();
+        let module = invocation.module.get();
+        self.current_module = if module.is_trait() { module.parent.unwrap() } else { module };
 
         if path.len() > 1 {
             if !self.use_extern_macros && self.gated_errors.insert(span) {

src/test/run-pass-fulldeps/proc-macro/attr-on-trait.rs

@@ -0,0 +1,28 @@
+// Copyright 2017 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.
+
+// aux-build:attr-on-trait.rs
+
+#![feature(proc_macro)]
+
+extern crate attr_on_trait;
+
+trait Foo {
+    #[attr_on_trait::foo]
+    fn foo() {}
+}
+
+impl Foo for i32 {
+    fn foo(&self) {}
+}
+
+fn main() {
+    3i32.foo();
+}

src/test/run-pass-fulldeps/proc-macro/auxiliary/attr-on-trait.rs

@@ -0,0 +1,25 @@
+// Copyright 2017 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.
+
+// no-prefer-dynamic
+
+#![feature(proc_macro)]
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn foo(attr: TokenStream, item: TokenStream) -> TokenStream {
+    drop(attr);
+    assert_eq!(item.to_string(), "fn foo() { }");
+    "fn foo(&self);".parse().unwrap()
+}