WIP lint reasons with unused-attributes for empty lint attrs

I can't figure out why the first empty lint attr. warning is getting emitted
twice; I could hack around it with one-time-diagnostics, but I'd rather not
duct-tape over my lack of understanding

 INFO 2018-10-16T05:40:37Z: rustc::lint::levels: ZMD C empty-attr being called on Deny Node(unused_attributes, Span { lo: BytePos(35), hi: BytePos(52), ctxt: #0 }, None) Some(MultiSpan { primary_spans: [Span { lo: BytePos(56), hi: BytePos(66), ctxt: #0 }], span_labels: [] })
  INFO 2018-10-16T05:40:37Z: rustc::lint::levels: ZMD C empty-attr being called on Deny Node(unused_attributes, Span { lo: BytePos(35), hi: BytePos(52), ctxt: #0 }, None) Some(MultiSpan { primary_spans: [Span { lo: BytePos(121), hi: BytePos(167), ctxt: #0 }], span_labels: [] })
   INFO 2018-10-16T05:40:37Z: rustc::lint::levels: ZMD C empty-attr being called on Deny Node(unused_attributes, Span { lo: BytePos(35), hi: BytePos(52), ctxt: #0 }, None) Some(MultiSpan { primary_spans: [Span { lo: BytePos(222), hi: BytePos(231), ctxt: #0 }], span_labels: [] })
    INFO 2018-10-16T05:40:37Z: rustc::lint::levels: ZMD C empty-attr being called on Deny Node(unused_attributes, Span { lo: BytePos(35), hi: BytePos(52), ctxt: #0 }, None) Some(MultiSpan { primary_spans: [Span { lo: BytePos(56), hi: BytePos(66), ctxt: #0 }], span_labels: [] })
     INFO 2018-10-16T05:40:37Z: rustc::lint::levels: ZMD C empty-attr being called on Deny Node(unused_attributes, Span { lo: BytePos(35), hi: BytePos(52), ctxt: #0 }, None) Some(MultiSpan { primary_spans: [Span { lo: BytePos(121), hi: BytePos(167), ctxt: #0 }], span_labels: [] })
      INFO 2018-10-16T05:40:37Z: rustc::lint::levels: ZMD C empty-attr being called on Deny Node(unused_attributes, Span { lo: BytePos(35), hi: BytePos(52), ctxt: #0 }, None) Some(MultiSpan { primary_spans: [Span { lo: BytePos(222), hi: BytePos(231), ctxt: #0 }], span_labels: [] })

src/librustc/lint/builtin.rs

@@ -68,6 +68,12 @@ declare_lint! {
     "detect assignments that will never be read"
 }
 
+declare_lint! {
+    pub UNUSED_ATTRIBUTES,
+    Warn,
+    "detects attributes that were not used by the compiler"
+}
+
 declare_lint! {
     pub DEAD_CODE,
     Warn,

src/librustc/lint/context.rs

@@ -707,6 +707,7 @@ impl<'a> LintContext<'a> for EarlyContext<'a> {
                           f: F)
         where F: FnOnce(&mut Self)
     {
+        info!("ZMD A with_lint_attrs {:?} ", attrs.iter().map(|a| (a.id, a.path.clone())).collect::<Vec<_>>());
         let push = self.builder.push(attrs);
         self.check_id(id);
         self.enter_attrs(attrs);

src/librustc/lint/levels.rs

@@ -24,6 +24,7 @@ use syntax::attr;
 use syntax::feature_gate;
 use syntax::source_map::MultiSpan;
 use syntax::symbol::Symbol;
+use syntax_pos::Span;
 use util::nodemap::FxHashMap;
 
 pub struct LintLevelSets {
@@ -196,12 +197,22 @@ impl<'a> LintLevelsBuilder<'a> {
     ///
     /// Don't forget to call `pop`!
     pub fn push(&mut self, attrs: &[ast::Attribute]) -> BuilderPush {
+        info!("ZMD B push called with {:?}", attrs);
         let mut specs = FxHashMap();
         let store = self.sess.lint_store.borrow();
         let sess = self.sess;
         let bad_attr = |span| {
             struct_span_err!(sess, span, E0452, "malformed lint attribute")
         };
+        let empty_attr = |span: Span, specs| {
+            let lint = builtin::UNUSED_ATTRIBUTES;
+            let (level, src) = self.sets.get_lint_level(lint, self.cur, Some(&specs), &sess);
+            let sp = Some(span.into());
+            let msg = "empty lint attribute is unused";
+            info!("ZMD C empty-attr being called on {:?} {:?} {:?}", level, src, sp);
+            lint::struct_lint_level(sess, lint, level, src, sp, msg)
+        };
+
         for attr in attrs {
             let level = match Level::from_str(&attr.name().as_str()) {
                 None => continue,
@@ -216,9 +227,15 @@ impl<'a> LintLevelsBuilder<'a> {
             } else {
                 let mut err = bad_attr(meta.span);
                 err.emit();
-                continue
+                continue;
             };
 
+            if metas.is_empty() {
+                let mut err = empty_attr(attr.span, specs.clone());
+                err.emit();
+                continue;
+            }
+
             // Before processing the lint names, look for a reason (RFC 2383)
             // at the end.
             let mut reason = None;
@@ -231,6 +248,13 @@ impl<'a> LintLevelsBuilder<'a> {
                         if item.ident == "reason" {
                             // found reason, reslice meta list to exclude it
                             metas = &metas[0..metas.len()-1];
+                            // ... but also notice if we thereby don't have any lint names
+                            // left (attribute was `#[level(reason = "foo")]`)
+                            if metas.is_empty() {
+                                let mut err = empty_attr(attr.span, specs.clone());
+                                err.emit();
+                                continue;
+                            }
                             if let ast::LitKind::Str(rationale, _) = name_value.node {
                                 if gate_reasons {
                                     feature_gate::emit_feature_err(

src/librustc/lint/mod.rs

@@ -463,7 +463,7 @@ impl Level {
 }
 
 /// How a lint level was set.
-#[derive(Clone, Copy, PartialEq, Eq)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
 pub enum LintSource {
     /// Lint is at the default level as declared
     /// in rustc or a plugin.

src/librustc_lint/unused.rs

@@ -10,10 +10,13 @@
 
 use rustc::hir::def::Def;
 use rustc::hir::def_id::DefId;
+
 use rustc::ty;
 use rustc::ty::adjustment;
-use lint::{LateContext, EarlyContext, LintContext, LintArray};
-use lint::{LintPass, EarlyLintPass, LateLintPass};
+use lint::{
+    LateContext, EarlyContext, LintContext, LintArray, LintPass, EarlyLintPass, LateLintPass,
+    builtin::UNUSED_ATTRIBUTES
+};
 
 use syntax::ast;
 use syntax::attr;
@@ -195,12 +198,6 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for PathStatements {
     }
 }
 
-declare_lint! {
-    pub UNUSED_ATTRIBUTES,
-    Warn,
-    "detects attributes that were not used by the compiler"
-}
-
 #[derive(Copy, Clone)]
 pub struct UnusedAttributes;