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/src/check_attr.rs

@@ -2133,6 +2133,20 @@ impl CheckAttrVisitor<'_> {
                     );
                 }
             }
+        } 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 },