Auto merge of #50820 - alexcrichton:no-modules, r=petrochenkov

rustc: Disallow modules and macros in expansions

This commit feature gates generating modules and macro definitions in procedural
macro expansions. Custom derive is exempt from this check as it would be a large
retroactive breaking change (#50587). It's hoped that we can hopefully stem the
bleeding to figure out a better solution here before opening up the floodgates.

The restriction here is specifically targeted at surprising hygiene results [1]
that result in non-"copy/paste" behavior. Hygiene and procedural macros is
intended to be avoided as much as possible for Macros 1.2 by saying everything
is "as if you copy/pasted the code", but modules and macros are sort of weird
exceptions to this rule that aren't fully fleshed out.

[1]: https://github.com/rust-lang/rust/issues/50504#issuecomment-387734625

cc #50504

src/test/compile-fail-fulldeps/proc-macro/auxiliary/more-gates.rs

@@ -0,0 +1,56 @@
+// Copyright 2018 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
+
+#![crate_type = "proc-macro"]
+#![feature(proc_macro)]
+
+extern crate proc_macro;
+
+use proc_macro::*;
+
+#[proc_macro_attribute]
+pub fn attr2mod(_: TokenStream, _: TokenStream) -> TokenStream {
+    "mod test {}".parse().unwrap()
+}
+
+#[proc_macro_attribute]
+pub fn attr2mac1(_: TokenStream, _: TokenStream) -> TokenStream {
+    "macro_rules! foo1 { (a) => (a) }".parse().unwrap()
+}
+
+#[proc_macro_attribute]
+pub fn attr2mac2(_: TokenStream, _: TokenStream) -> TokenStream {
+    "macro foo2(a) { a }".parse().unwrap()
+}
+
+#[proc_macro]
+pub fn mac2mod(_: TokenStream) -> TokenStream {
+    "mod test2 {}".parse().unwrap()
+}
+
+#[proc_macro]
+pub fn mac2mac1(_: TokenStream) -> TokenStream {
+    "macro_rules! foo3 { (a) => (a) }".parse().unwrap()
+}
+
+#[proc_macro]
+pub fn mac2mac2(_: TokenStream) -> TokenStream {
+    "macro foo4(a) { a }".parse().unwrap()
+}
+
+#[proc_macro]
+pub fn tricky(_: TokenStream) -> TokenStream {
+    "fn foo() {
+        mod test {}
+        macro_rules! foo { (a) => (a) }
+    }".parse().unwrap()
+}

src/test/compile-fail-fulldeps/proc-macro/more-gates.rs

@@ -0,0 +1,37 @@
+// Copyright 2018 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:more-gates.rs
+
+#![feature(proc_macro)]
+
+extern crate more_gates as foo;
+
+use foo::*;
+
+#[attr2mod]
+//~^ ERROR: cannot expand to modules
+pub fn a() {}
+#[attr2mac1]
+//~^ ERROR: cannot expand to macro definitions
+pub fn a() {}
+#[attr2mac2]
+//~^ ERROR: cannot expand to macro definitions
+pub fn a() {}
+
+mac2mod!(); //~ ERROR: cannot expand to modules
+mac2mac1!(); //~ ERROR: cannot expand to macro definitions
+mac2mac2!(); //~ ERROR: cannot expand to macro definitions
+
+tricky!();
+//~^ ERROR: cannot expand to modules
+//~| ERROR: cannot expand to macro definitions
+
+fn main() {}

src/test/compile-fail-fulldeps/proc-macro/proc-macro-gates.rs

@@ -14,6 +14,7 @@
 // gate-test-proc_macro_mod line
 // gate-test-proc_macro_expr
 // gate-test-proc_macro_mod
+// gate-test-proc_macro_gen
 
 #![feature(proc_macro, stmt_expr_attributes)]
 
@@ -29,10 +30,12 @@ fn _test_inner() {
 }
 
 #[a] //~ ERROR: custom attributes cannot be applied to modules
+//~| ERROR: procedural macros cannot expand to modules
 mod _test2 {}
 
 mod _test2_inner {
     #![a] //~ ERROR: custom attributes cannot be applied to modules
+    //~| ERROR: procedural macros cannot expand to modules
 }
 
 #[a = y] //~ ERROR: must only be followed by a delimiter token

src/test/run-pass-fulldeps/macro-quote-test.rs

@@ -13,7 +13,7 @@
 // aux-build:hello_macro.rs
 // ignore-stage1
 
-#![feature(use_extern_macros, proc_macro_non_items)]
+#![feature(use_extern_macros, proc_macro_non_items, proc_macro_gen)]
 
 extern crate hello_macro;