wherein the status of empty and reason-only lint attributes is clarified

We avoid an ICE by checking for an empty meta-item list before we
index into the meta-items, and leave commentary about where we'd like
to issue unused-attributes lints in the future. Note that empty lint
attributes are already accepted by the stable compiler; generalizing
this to weird reason-only lint attributes seems like the
conservative/consilient generalization.

src/librustc/lint/levels.rs

@@ -216,9 +216,14 @@ impl<'a> LintLevelsBuilder<'a> {
             } else {
                 let mut err = bad_attr(meta.span);
                 err.emit();
-                continue
+                continue;
             };
 
+            if metas.is_empty() {
+                // FIXME (#55112): issue unused-attributes lint for `#[level()]`
+                continue;
+            }
+
             // Before processing the lint names, look for a reason (RFC 2383)
             // at the end.
             let mut reason = None;
@@ -231,6 +236,8 @@ impl<'a> LintLevelsBuilder<'a> {
                         if item.ident == "reason" {
                             // found reason, reslice meta list to exclude it
                             metas = &metas[0..metas.len()-1];
+                            // FIXME (#55112): issue unused-attributes lint if we thereby
+                            // don't have any lint names (`#[level(reason = "foo")]`)
                             if let ast::LitKind::Str(rationale, _) = name_value.node {
                                 if gate_reasons {
                                     feature_gate::emit_feature_err(

src/test/ui/lint/empty-lint-attributes.rs

@@ -0,0 +1,17 @@
+#![feature(lint_reasons)]
+
+// run-pass
+
+// Empty (and reason-only) lint attributes are legal—although we may want to
+// lint them in the future (Issue #55112).
+
+#![allow()]
+#![warn(reason = "observationalism")]
+
+#[forbid()]
+fn devoir() {}
+
+#[deny(reason = "ultion")]
+fn waldgrave() {}
+
+fn main() {}