Auto merge of #66078 - petrochenkov:gateout, r=Centril

expand: Feature gate out-of-line modules in proc macro input

Extracted from #64273.

We are currently gating attributes applied directly to `mod` items because there are unresolved questions about out-of-line modules and their behavior is very likely to change.

However, you can sneak an out-of-line module into an attribute macro input using modules nested into other items like
```rust
#[my_attr]
fn m() {
    #[path = "zzz.rs"]
    mod n; // what tokens does the `my_attr` macro see?
}
```
This PR prevents that and emits a feature gate error for this case as well.

r? @Centril
It would be great to land this before beta.

src/libsyntax_expand/expand.rs

@@ -17,9 +17,10 @@ use syntax::parse::token;
 use syntax::parse::parser::Parser;
 use syntax::print::pprust;
 use syntax::ptr::P;
+use syntax::sess::ParseSess;
 use syntax::symbol::{sym, Symbol};
 use syntax::tokenstream::{TokenStream, TokenTree};
-use syntax::visit::Visitor;
+use syntax::visit::{self, Visitor};
 use syntax::util::map_in_place::MapInPlace;
 
 use errors::{Applicability, FatalError};
@@ -615,6 +616,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
             }
             InvocationKind::Attr { attr, mut item, .. } => match ext {
                 SyntaxExtensionKind::Attr(expander) => {
+                    self.gate_proc_macro_input(&item);
                     self.gate_proc_macro_attr_item(span, &item);
                     let item_tok = TokenTree::token(token::Interpolated(Lrc::new(match item {
                         Annotatable::Item(item) => token::NtItem(item),
@@ -664,6 +666,9 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                     if !item.derive_allowed() {
                         return fragment_kind.dummy(span);
                     }
+                    if let SyntaxExtensionKind::Derive(..) = ext {
+                        self.gate_proc_macro_input(&item);
+                    }
                     let meta = ast::MetaItem { kind: ast::MetaItemKind::Word, span, path };
                     let items = expander.expand(self.cx, span, &meta, item);
                     fragment_kind.expect_from_annotatables(items)
@@ -692,21 +697,16 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
     }
 
     fn gate_proc_macro_attr_item(&self, span: Span, item: &Annotatable) {
-        let (kind, gate) = match *item {
-            Annotatable::Item(ref item) => {
-                match item.kind {
-                    ItemKind::Mod(_) if self.cx.ecfg.proc_macro_hygiene() => return,
-                    ItemKind::Mod(_) => ("modules", sym::proc_macro_hygiene),
-                    _ => return,
-                }
+        let kind = match item {
+            Annotatable::Item(item) => match &item.kind {
+                ItemKind::Mod(m) if m.inline => "modules",
+                _ => return,
             }
-            Annotatable::TraitItem(_) => return,
-            Annotatable::ImplItem(_) => return,
-            Annotatable::ForeignItem(_) => return,
-            Annotatable::Stmt(_) |
-            Annotatable::Expr(_) if self.cx.ecfg.proc_macro_hygiene() => return,
-            Annotatable::Stmt(_) => ("statements", sym::proc_macro_hygiene),
-            Annotatable::Expr(_) => ("expressions", sym::proc_macro_hygiene),
+            Annotatable::TraitItem(_)
+            | Annotatable::ImplItem(_)
+            | Annotatable::ForeignItem(_) => return,
+            Annotatable::Stmt(_) => "statements",
+            Annotatable::Expr(_) => "expressions",
             Annotatable::Arm(..)
             | Annotatable::Field(..)
             | Annotatable::FieldPat(..)
@@ -716,15 +716,49 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
             | Annotatable::Variant(..)
             => panic!("unexpected annotatable"),
         };
+        if self.cx.ecfg.proc_macro_hygiene() {
+            return
+        }
         emit_feature_err(
             self.cx.parse_sess,
-            gate,
+            sym::proc_macro_hygiene,
             span,
             GateIssue::Language,
             &format!("custom attributes cannot be applied to {}", kind),
         );
     }
 
+    fn gate_proc_macro_input(&self, annotatable: &Annotatable) {
+        struct GateProcMacroInput<'a> {
+            parse_sess: &'a ParseSess,
+        }
+
+        impl<'ast, 'a> Visitor<'ast> for GateProcMacroInput<'a> {
+            fn visit_item(&mut self, item: &'ast ast::Item) {
+                match &item.kind {
+                    ast::ItemKind::Mod(module) if !module.inline => {
+                        emit_feature_err(
+                            self.parse_sess,
+                            sym::proc_macro_hygiene,
+                            item.span,
+                            GateIssue::Language,
+                            "non-inline modules in proc macro input are unstable",
+                        );
+                    }
+                    _ => {}
+                }
+
+                visit::walk_item(self, item);
+            }
+
+            fn visit_mac(&mut self, _: &'ast ast::Mac) {}
+        }
+
+        if !self.cx.ecfg.proc_macro_hygiene() {
+            annotatable.visit_with(&mut GateProcMacroInput { parse_sess: self.cx.parse_sess });
+        }
+    }
+
     fn gate_proc_macro_expansion_kind(&self, span: Span, kind: AstFragmentKind) {
         let kind = match kind {
             AstFragmentKind::Expr |

src/test/ui/proc-macro/attributes-on-modules-fail.rs

@@ -0,0 +1,29 @@
+// aux-build:test-macros.rs
+
+#[macro_use]
+extern crate test_macros;
+
+#[identity_attr] //~ ERROR custom attributes cannot be applied to modules
+mod m {
+    pub struct X;
+
+    type A = Y; //~ ERROR cannot find type `Y` in this scope
+}
+
+struct Y;
+type A = X; //~ ERROR cannot find type `X` in this scope
+
+#[derive(Copy)] //~ ERROR `derive` may only be applied to structs, enums and unions
+mod n {}
+
+#[empty_attr]
+mod module; //~ ERROR non-inline modules in proc macro input are unstable
+
+#[empty_attr] //~ ERROR custom attributes cannot be applied to modules
+mod outer {
+    mod inner; //~ ERROR non-inline modules in proc macro input are unstable
+
+    mod inner_inline {} // OK
+}
+
+fn main() {}

src/test/ui/proc-macro/attributes-on-modules-fail.stderr

@@ -0,0 +1,76 @@
+error[E0658]: custom attributes cannot be applied to modules
+  --> $DIR/attributes-on-modules-fail.rs:6:1
+   |
+LL | #[identity_attr]
+   | ^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54727
+   = help: add `#![feature(proc_macro_hygiene)]` to the crate attributes to enable
+
+error: `derive` may only be applied to structs, enums and unions
+  --> $DIR/attributes-on-modules-fail.rs:16:1
+   |
+LL | #[derive(Copy)]
+   | ^^^^^^^^^^^^^^^
+
+error[E0658]: non-inline modules in proc macro input are unstable
+  --> $DIR/attributes-on-modules-fail.rs:20:1
+   |
+LL | mod module;
+   | ^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54727
+   = help: add `#![feature(proc_macro_hygiene)]` to the crate attributes to enable
+
+error[E0658]: non-inline modules in proc macro input are unstable
+  --> $DIR/attributes-on-modules-fail.rs:24:5
+   |
+LL |     mod inner;
+   |     ^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54727
+   = help: add `#![feature(proc_macro_hygiene)]` to the crate attributes to enable
+
+error[E0658]: custom attributes cannot be applied to modules
+  --> $DIR/attributes-on-modules-fail.rs:22:1
+   |
+LL | #[empty_attr]
+   | ^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54727
+   = help: add `#![feature(proc_macro_hygiene)]` to the crate attributes to enable
+
+error[E0412]: cannot find type `Y` in this scope
+  --> $DIR/attributes-on-modules-fail.rs:10:14
+   |
+LL |     type A = Y;
+   |     ---------^- similarly named type alias `A` defined here
+   |
+help: a type alias with a similar name exists
+   |
+LL |     type A = A;
+   |              ^
+help: possible candidate is found in another module, you can import it into scope
+   |
+LL |     use Y;
+   |
+
+error[E0412]: cannot find type `X` in this scope
+  --> $DIR/attributes-on-modules-fail.rs:14:10
+   |
+LL | type A = X;
+   | ---------^- similarly named type alias `A` defined here
+   |
+help: a type alias with a similar name exists
+   |
+LL | type A = A;
+   |          ^
+help: possible candidate is found in another module, you can import it into scope
+   |
+LL | use m::X;
+   |
+
+error: aborting due to 7 previous errors
+
+Some errors have detailed explanations: E0412, E0658.
+For more information about an error, try `rustc --explain E0412`.

src/test/ui/proc-macro/attributes-on-modules.rs

@@ -0,0 +1,13 @@
+// aux-build:test-macros.rs
+
+#[macro_use]
+extern crate test_macros;
+
+#[identity_attr] //~ ERROR custom attributes cannot be applied to modules
+mod m {
+    pub struct S;
+}
+
+fn main() {
+    let s = m::S;
+}

src/test/ui/proc-macro/attributes-on-modules.stderr

@@ -0,0 +1,12 @@
+error[E0658]: custom attributes cannot be applied to modules
+  --> $DIR/attributes-on-modules.rs:6:1
+   |
+LL | #[identity_attr]
+   | ^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/54727
+   = help: add `#![feature(proc_macro_hygiene)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/proc-macro/module.rs

@@ -0,0 +1 @@
+// ignore-test

src/test/ui/proc-macro/outer/inner.rs

@@ -0,0 +1 @@
+// ignore-test