Warn on repr without hints

src/librustc/diagnostics.rs

@@ -2139,6 +2139,7 @@ register_diagnostics! {
     E0657, // `impl Trait` can only capture lifetimes bound at the fn level
     E0687, // in-band lifetimes cannot be used in `fn`/`Fn` syntax
     E0688, // in-band lifetimes cannot be mixed with explicit lifetime binders
+    E0689, // `#[repr]` must have a hint
 
     E0906, // closures cannot be static
 }

src/librustc/hir/check_attr.rs

@@ -154,7 +154,22 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
         let hints: Vec<_> = item.attrs
             .iter()
             .filter(|attr| attr.name() == "repr")
-            .filter_map(|attr| attr.meta_item_list())
+            .filter_map(|attr| {
+                let list = attr.meta_item_list();
+                let mut has_hints = false;
+                if let Some(ref list) = list {
+                    has_hints = !list.is_empty();
+                }
+                if !has_hints {
+                    span_warn!(
+                        self.tcx.sess,
+                        item.span,
+                        E0689,
+                        "`repr` attribute cannot be empty",
+                    );
+                }
+                list
+            })
             .flat_map(|hints| hints)
             .collect();