Address review comments

src/librustc/hir/def.rs

@@ -333,6 +333,22 @@ impl NonMacroAttrKind {
             NonMacroAttrKind::LegacyPluginHelper => "legacy plugin helper attribute",
         }
     }
+
+    pub fn article(self) -> &'static str {
+        match self {
+            NonMacroAttrKind::Registered => "an",
+            _ => "a",
+        }
+    }
+
+    /// Users of some attributes cannot mark them as used, so they are considered always used.
+    pub fn is_used(self) -> bool {
+        match self {
+            NonMacroAttrKind::Tool | NonMacroAttrKind::DeriveHelper => true,
+            NonMacroAttrKind::Builtin | NonMacroAttrKind::Registered |
+            NonMacroAttrKind::LegacyPluginHelper => false,
+        }
+    }
 }
 
 impl<Id> Res<Id> {
@@ -389,6 +405,7 @@ impl<Id> Res<Id> {
     pub fn article(&self) -> &'static str {
         match *self {
             Res::Def(kind, _) => kind.article(),
+            Res::NonMacroAttr(kind) => kind.article(),
             Res::Err => "an",
             _ => "a",
         }

src/librustc_resolve/build_reduced_graph.rs

@@ -141,8 +141,7 @@ impl<'a> Resolver<'a> {
     crate fn get_macro(&mut self, res: Res) -> Option<Lrc<SyntaxExtension>> {
         match res {
             Res::Def(DefKind::Macro(..), def_id) => self.get_macro_by_def_id(def_id),
-            Res::NonMacroAttr(attr_kind) =>
-                Some(self.non_macro_attr(attr_kind == NonMacroAttrKind::Tool)),
+            Res::NonMacroAttr(attr_kind) => Some(self.non_macro_attr(attr_kind.is_used())),
             _ => None,
         }
     }

src/librustc_resolve/lib.rs

@@ -1472,7 +1472,7 @@ impl<'a> Resolver<'a> {
                 Scope::MacroRules(..) => true,
                 Scope::CrateRoot => true,
                 Scope::Module(..) => true,
-                Scope::RegisteredAttrs => true,
+                Scope::RegisteredAttrs => use_prelude,
                 Scope::MacroUsePrelude => use_prelude || rust_2015,
                 Scope::BuiltinAttrs => true,
                 Scope::LegacyPluginHelpers => use_prelude || rust_2015,

src/librustc_resolve/macros.rs

@@ -94,6 +94,7 @@ fn fast_print_path(path: &ast::Path) -> Symbol {
     }
 }
 
+/// The code common between processing `#![register_tool]` and `#![register_attr]`.
 fn registered_idents(
     sess: &Session,
     attrs: &[ast::Attribute],
@@ -832,7 +833,8 @@ impl<'a> Resolver<'a> {
                                          res: Option<Res>, span: Span) {
         if let Some(Res::NonMacroAttr(kind)) = res {
             if kind != NonMacroAttrKind::Tool && binding.map_or(true, |b| b.is_import()) {
-                let msg = format!("cannot use a {} through an import", kind.descr());
+                let msg =
+                    format!("cannot use {} {} through an import", kind.article(), kind.descr());
                 let mut err = self.session.struct_span_err(span, &msg);
                 if let Some(binding) = binding {
                     err.span_note(binding.span, &format!("the {} imported here", kind.descr()));

src/libsyntax/feature_gate/active.rs

@@ -524,10 +524,10 @@ declare_features! (
     (active, abi_efiapi, "1.40.0", Some(65815), None),
 
     /// Allows using the `#[register_attr]` attribute.
-    (active, register_attr, "1.41.0", Some(29642), None),
+    (active, register_attr, "1.41.0", Some(66080), None),
 
     /// Allows using the `#[register_attr]` attribute.
-    (active, register_tool, "1.41.0", Some(44690), None),
+    (active, register_tool, "1.41.0", Some(66079), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates

src/libsyntax/feature_gate/builtin_attrs.rs

@@ -330,11 +330,11 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     gated!(ffi_returns_twice, Whitelisted, template!(Word), experimental!(ffi_returns_twice)),
     gated!(track_caller, Whitelisted, template!(Word), experimental!(track_caller)),
     gated!(
-        register_attr, Whitelisted, template!(List: "attr1, attr2, ..."),
+        register_attr, CrateLevel, template!(List: "attr1, attr2, ..."),
         experimental!(register_attr),
     ),
     gated!(
-        register_tool, Whitelisted, template!(List: "tool1, tool2, ..."),
+        register_tool, CrateLevel, template!(List: "tool1, tool2, ..."),
         experimental!(register_tool),
     ),
 

src/test/ui/attributes/register-attr-tool-import.rs

@@ -0,0 +1,14 @@
+// edition:2018
+
+#![feature(register_attr)]
+#![feature(register_tool)]
+
+#![register_attr(attr)]
+#![register_tool(tool)]
+
+use attr as renamed_attr; // OK
+use tool as renamed_tool; // OK
+
+#[renamed_attr] //~ ERROR cannot use an explicitly registered attribute through an import
+#[renamed_tool::attr] //~ ERROR cannot use a tool module through an import
+fn main() {}

src/test/ui/attributes/register-attr-tool-import.stderr

@@ -0,0 +1,26 @@
+error: cannot use an explicitly registered attribute through an import
+  --> $DIR/register-attr-tool-import.rs:12:3
+   |
+LL | #[renamed_attr]
+   |   ^^^^^^^^^^^^
+   |
+note: the explicitly registered attribute imported here
+  --> $DIR/register-attr-tool-import.rs:9:5
+   |
+LL | use attr as renamed_attr; // OK
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: cannot use a tool module through an import
+  --> $DIR/register-attr-tool-import.rs:13:3
+   |
+LL | #[renamed_tool::attr]
+   |   ^^^^^^^^^^^^
+   |
+note: the tool module imported here
+  --> $DIR/register-attr-tool-import.rs:10:5
+   |
+LL | use tool as renamed_tool; // OK
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/attributes/register-attr-tool-prelude.rs

@@ -0,0 +1,14 @@
+#![feature(register_attr)]
+#![feature(register_tool)]
+
+#![register_attr(attr)]
+#![register_tool(tool)]
+
+#[no_implicit_prelude]
+mod m {
+    #[attr] //~ ERROR cannot find attribute `attr` in this scope
+    #[tool::attr] //~ ERROR failed to resolve: use of undeclared type or module `tool`
+    fn check() {}
+}
+
+fn main() {}

src/test/ui/attributes/register-attr-tool-prelude.stderr

@@ -0,0 +1,15 @@
+error[E0433]: failed to resolve: use of undeclared type or module `tool`
+  --> $DIR/register-attr-tool-prelude.rs:10:7
+   |
+LL |     #[tool::attr]
+   |       ^^^^ use of undeclared type or module `tool`
+
+error: cannot find attribute `attr` in this scope
+  --> $DIR/register-attr-tool-prelude.rs:9:7
+   |
+LL |     #[attr]
+   |       ^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0433`.

src/test/ui/attributes/register-attr-tool-unused.rs

@@ -0,0 +1,10 @@
+#![deny(unused)]
+
+#![feature(register_attr)]
+#![feature(register_tool)]
+
+#[register_attr(attr)] //~ ERROR crate-level attribute should be an inner attribute
+                       //~| ERROR unused attribute
+#[register_tool(tool)] //~ ERROR crate-level attribute should be an inner attribute
+                       //~| ERROR unused attribute
+fn main() {}

src/test/ui/attributes/register-attr-tool-unused.stderr

@@ -0,0 +1,33 @@
+error: unused attribute
+  --> $DIR/register-attr-tool-unused.rs:6:1
+   |
+LL | #[register_attr(attr)]
+   | ^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/register-attr-tool-unused.rs:1:9
+   |
+LL | #![deny(unused)]
+   |         ^^^^^^
+   = note: `#[deny(unused_attributes)]` implied by `#[deny(unused)]`
+
+error: crate-level attribute should be an inner attribute: add an exclamation mark: `#![foo]`
+  --> $DIR/register-attr-tool-unused.rs:6:1
+   |
+LL | #[register_attr(attr)]
+   | ^^^^^^^^^^^^^^^^^^^^^^
+
+error: unused attribute
+  --> $DIR/register-attr-tool-unused.rs:8:1
+   |
+LL | #[register_tool(tool)]
+   | ^^^^^^^^^^^^^^^^^^^^^^
+
+error: crate-level attribute should be an inner attribute: add an exclamation mark: `#![foo]`
+  --> $DIR/register-attr-tool-unused.rs:8:1
+   |
+LL | #[register_tool(tool)]
+   | ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+

src/test/ui/feature-gates/feature-gate-register_attr.stderr

@@ -4,7 +4,7 @@ error[E0658]: the `#[register_attr]` attribute is an experimental feature
 LL | #![register_attr(attr)]
    | ^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = note: for more information, see https://github.com/rust-lang/rust/issues/66080
    = help: add `#![feature(register_attr)]` to the crate attributes to enable
 
 error: aborting due to previous error

src/test/ui/feature-gates/feature-gate-register_tool.stderr

@@ -4,7 +4,7 @@ error[E0658]: the `#[register_tool]` attribute is an experimental feature
 LL | #![register_tool(tool)]
    | ^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/44690
+   = note: for more information, see https://github.com/rust-lang/rust/issues/66079
    = help: add `#![feature(register_tool)]` to the crate attributes to enable
 
 error: aborting due to previous error