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, 7 deletions

diff --git a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
index 61af4979e70..85499623f41 100644
--- a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
@@ -677,8 +677,6 @@ impl<'a> TraitDef<'a> {
         let self_type = cx.ty_path(path);
 
         let attr = cx.attribute(cx.meta_word(self.span, sym::automatically_derived));
-        // Just mark it now since we know that it'll end up used downstream
-        cx.sess.mark_attr_used(&attr);
         let opt_trait_ref = Some(trait_ref);
         let unused_qual = {
             let word = rustc_ast::attr::mk_nested_word_item(Ident::new(
diff --git a/compiler/rustc_builtin_macros/src/proc_macro_harness.rs b/compiler/rustc_builtin_macros/src/proc_macro_harness.rs
index f83329ecba8..7971c1fff42 100644
--- a/compiler/rustc_builtin_macros/src/proc_macro_harness.rs
+++ b/compiler/rustc_builtin_macros/src/proc_macro_harness.rs
@@ -260,11 +260,11 @@ impl<'a> Visitor<'a> for CollectProcMacros<'a> {
             return;
         }
 
-        if self.sess.check_name(attr, sym::proc_macro_derive) {
+        if attr.has_name(sym::proc_macro_derive) {
             self.collect_custom_derive(item, attr);
-        } else if self.sess.check_name(attr, sym::proc_macro_attribute) {
+        } else if attr.has_name(sym::proc_macro_attribute) {
             self.collect_attr_proc_macro(item);
-        } else if self.sess.check_name(attr, sym::proc_macro) {
+        } else if attr.has_name(sym::proc_macro) {
             self.collect_bang_proc_macro(item);
         };
 
diff --git a/compiler/rustc_builtin_macros/src/test_harness.rs b/compiler/rustc_builtin_macros/src/test_harness.rs
index be24b60294b..d791677cb8e 100644
--- a/compiler/rustc_builtin_macros/src/test_harness.rs
+++ b/compiler/rustc_builtin_macros/src/test_harness.rs
@@ -188,8 +188,7 @@ impl<'a> MutVisitor for EntryPointCleaner<'a> {
                     let attrs = attrs
                         .into_iter()
                         .filter(|attr| {
-                            !self.sess.check_name(attr, sym::rustc_main)
-                                && !self.sess.check_name(attr, sym::start)
+                            !attr.has_name(sym::rustc_main) && !attr.has_name(sym::start)
                         })
                         .chain(iter::once(allow_dead_code))
                         .collect();