check_attrs: Warn when #[macro_export] is used on macros 2.0

The compiler should emit a more specific error when the `#[macro_export]`
attribute is present on a decl macro, instead of silently ignoring it.

This commit adds the required error message in rustc_passes/messages.ftl,
 as well as a note. A new variant is added to the `errors::MacroExport`
enum, specifically for the case where the attribute is added to a macro
2.0.

compiler/rustc_passes/messages.ftl

@@ -428,6 +428,10 @@ passes_link_section =
 passes_macro_export =
     `#[macro_export]` only has an effect on macro definitions
 
+passes_macro_export_on_decl_macro =
+    `#[macro_export]` has no effect on declarative macro definitions
+    .note = declarative macros follow the same exporting rules as regular items
+
 passes_macro_use =
     `#[{$name}]` only has an effect on `extern crate` and modules
 

compiler/rustc_passes/src/check_attr.rs

@@ -2133,6 +2133,20 @@ fn check_macro_export(&self, hir_id: HirId, attr: &Attribute, target: Target) {
                     );
                 }
             }
+        } else {
+            // special case when `#[macro_export]` is applied to a macro 2.0
+            let (macro_definition, _) =
+                self.tcx.hir().find(hir_id).unwrap().expect_item().expect_macro();
+            let is_decl_macro = !macro_definition.macro_rules;
+
+            if is_decl_macro {
+                self.tcx.emit_spanned_lint(
+                    UNUSED_ATTRIBUTES,
+                    hir_id,
+                    attr.span,
+                    errors::MacroExport::OnDeclMacro,
+                );
+            }
         }
     }
 

compiler/rustc_passes/src/errors.rs

@@ -690,6 +690,10 @@ pub enum MacroExport {
     #[diag(passes_macro_export)]
     Normal,
 
+    #[diag(passes_macro_export_on_decl_macro)]
+    #[note]
+    OnDeclMacro,
+
     #[diag(passes_invalid_macro_export_arguments)]
     UnknownItem { name: Symbol },
 

tests/ui/attributes/macro_export_on_decl_macro.rs

+// Using #[macro_export] on a decl macro has no effect and should warn
+
+#![feature(decl_macro)]
+#![deny(unused)]
+
+#[macro_export] //~ ERROR `#[macro_export]` has no effect on declarative macro definitions
+pub macro foo() {}
+
+fn main() {}

tests/ui/attributes/macro_export_on_decl_macro.stderr

+error: `#[macro_export]` has no effect on declarative macro definitions
+  --> $DIR/macro_export_on_decl_macro.rs:6:1
+   |
+LL | #[macro_export]
+   | ^^^^^^^^^^^^^^^
+   |
+   = note: declarative macros follow the same exporting rules as regular items
+note: the lint level is defined here
+  --> $DIR/macro_export_on_decl_macro.rs:4:9
+   |
+LL | #![deny(unused)]
+   |         ^^^^^^
+   = note: `#[deny(unused_attributes)]` implied by `#[deny(unused)]`
+
+error: aborting due to previous error
+