Auto merge of #23028 - Munksgaard:get_attrs_opt, r=eddyb

This is more flexible and less error-prone. `get_attrs` and
`get_attrs_opt` can be used on many more items than the old `get_attrs`
could.

This is all courtesy of @huonw, and directly taken from here:
https://github.com/rust-lang/rust/pull/22348/files#diff-0f85fcb07fb739876892e633fa0e2be6R5575

Also thanks to @Manishearth for pointing it out to me.

src/librustc/middle/ty.rs

@@ -5606,8 +5606,7 @@ pub fn predicates<'tcx>(
 pub fn get_attrs<'tcx>(tcx: &'tcx ctxt, did: DefId)
                        -> Cow<'tcx, [ast::Attribute]> {
     if is_local(did) {
-        let item = tcx.map.expect_item(did.node);
-        Cow::Borrowed(&item.attrs)
+        Cow::Borrowed(tcx.map.attrs(did.node))
     } else {
         Cow::Owned(csearch::get_item_attrs(&tcx.sess.cstore, did))
     }

src/libsyntax/ast_map/mod.rs

@@ -457,35 +457,32 @@ impl<'ast> Map<'ast> {
         }
     }
 
-    /// Given a node ID and a closure, apply the closure to the array
-    /// of attributes associated with the AST corresponding to the Node ID
-    pub fn with_attrs<T, F>(&self, id: NodeId, f: F) -> T where
-        F: FnOnce(Option<&[Attribute]>) -> T,
-    {
-        let attrs = match self.get(id) {
-            NodeItem(i) => Some(&i.attrs[..]),
-            NodeForeignItem(fi) => Some(&fi.attrs[..]),
-            NodeTraitItem(ref tm) => match **tm {
+    /// Given a node ID, get a list of of attributes associated with the AST
+    /// corresponding to the Node ID
+    pub fn attrs(&self, id: NodeId) -> &[Attribute] {
+        let attrs = match self.find(id) {
+            Some(NodeItem(i)) => Some(&i.attrs[..]),
+            Some(NodeForeignItem(fi)) => Some(&fi.attrs[..]),
+            Some(NodeTraitItem(ref tm)) => match **tm {
                 RequiredMethod(ref type_m) => Some(&type_m.attrs[..]),
                 ProvidedMethod(ref m) => Some(&m.attrs[..]),
                 TypeTraitItem(ref typ) => Some(&typ.attrs[..]),
             },
-            NodeImplItem(ref ii) => {
+            Some(NodeImplItem(ref ii)) => {
                 match **ii {
                     MethodImplItem(ref m) => Some(&m.attrs[..]),
                     TypeImplItem(ref t) => Some(&t.attrs[..]),
                 }
             }
-            NodeVariant(ref v) => Some(&v.node.attrs[..]),
+            Some(NodeVariant(ref v)) => Some(&v.node.attrs[..]),
             // unit/tuple structs take the attributes straight from
             // the struct definition.
-            // FIXME(eddyb) make this work again (requires access to the map).
-            NodeStructCtor(_) => {
-                return self.with_attrs(self.get_parent(id), f);
+            Some(NodeStructCtor(_)) => {
+                return self.attrs(self.get_parent(id));
             }
             _ => None
         };
-        f(attrs)
+        attrs.unwrap_or(&[])
     }
 
     /// Returns an iterator that yields the node id's with paths that