Remove `Session.used_attrs` and move logic to `CheckAttrVisitor`

Instead of updating global state to mark attributes as used,
we now explicitly emit a warning when an attribute is used in
an unsupported position. As a side effect, we are to emit more
detailed warning messages (instead of just a generic "unused" message).

`Session.check_name` is removed, since its only purpose was to mark
the attribute as used. All of the callers are modified to use
`Attribute.has_name`

Additionally, `AttributeType::AssumedUsed` is removed - an 'assumed
used' attribute is implemented by simply not performing any checks
in `CheckAttrVisitor` for a particular attribute.

We no longer emit unused attribute warnings for the `#[rustc_dummy]`
attribute - it's an internal attribute used for tests, so it doesn't
mark sense to treat it as 'unused'.

With this commit, a large source of global untracked state is removed.

3 files changed, 4 insertions, 8 deletions

diff --git a/compiler/rustc_mir/src/dataflow/framework/engine.rs b/compiler/rustc_mir/src/dataflow/framework/engine.rs
index 3f9f558223b..7ff7c860591 100644
--- a/compiler/rustc_mir/src/dataflow/framework/engine.rs
+++ b/compiler/rustc_mir/src/dataflow/framework/engine.rs
@@ -338,7 +338,7 @@ impl RustcMirAttrs {
 
         let rustc_mir_attrs = attrs
             .iter()
-            .filter(|attr| tcx.sess.check_name(attr, sym::rustc_mir))
+            .filter(|attr| attr.has_name(sym::rustc_mir))
             .flat_map(|attr| attr.meta_item_list().into_iter().flat_map(|v| v.into_iter()));
 
         for attr in rustc_mir_attrs {
diff --git a/compiler/rustc_mir/src/dataflow/mod.rs b/compiler/rustc_mir/src/dataflow/mod.rs
index 4ca757cf269..8a426cc1015 100644
--- a/compiler/rustc_mir/src/dataflow/mod.rs
+++ b/compiler/rustc_mir/src/dataflow/mod.rs
@@ -30,12 +30,12 @@ pub struct MoveDataParamEnv<'tcx> {
 }
 
 pub(crate) fn has_rustc_mir_with(
-    sess: &Session,
+    _sess: &Session,
     attrs: &[ast::Attribute],
     name: Symbol,
 ) -> Option<MetaItem> {
     for attr in attrs {
-        if sess.check_name(attr, sym::rustc_mir) {
+        if attr.has_name(sym::rustc_mir) {
             let items = attr.meta_item_list();
             for item in items.iter().flat_map(|l| l.iter()) {
                 match item.meta_item() {
diff --git a/compiler/rustc_mir/src/monomorphize/polymorphize.rs b/compiler/rustc_mir/src/monomorphize/polymorphize.rs
index 30e758c7fdf..fea1adfa3d5 100644
--- a/compiler/rustc_mir/src/monomorphize/polymorphize.rs
+++ b/compiler/rustc_mir/src/monomorphize/polymorphize.rs
@@ -204,11 +204,7 @@ fn emit_unused_generic_params_error<'tcx>(
     unused_parameters: &FiniteBitSet<u32>,
 ) {
     let base_def_id = tcx.closure_base_def_id(def_id);
-    if !tcx
-        .get_attrs(base_def_id)
-        .iter()
-        .any(|a| tcx.sess.check_name(a, sym::rustc_polymorphize_error))
-    {
+    if !tcx.get_attrs(base_def_id).iter().any(|a| a.has_name(sym::rustc_polymorphize_error)) {
         return;
     }