Rewrite empty attribute lint

Signed-off-by: Jonathan Brouwer <jonathantbrouwer@gmail.com>

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -67,8 +67,6 @@ pub enum ReprAttr {
     ReprSimd,
     ReprTransparent,
     ReprAlign(Align),
-    // this one is just so we can emit a lint for it
-    ReprEmpty,
 }
 pub use ReprAttr::*;
 
@@ -304,7 +302,7 @@ pub enum AttributeKind {
     PubTransparent(Span),
 
     /// Represents [`#[repr]`](https://doc.rust-lang.org/stable/reference/type-layout.html#representations).
-    Repr(ThinVec<(ReprAttr, Span)>),
+    Repr { reprs: ThinVec<(ReprAttr, Span)>, first_span: Span },
 
     /// Represents `#[rustc_layout_scalar_valid_range_end]`.
     RustcLayoutScalarValidRangeEnd(Box<u128>, Span),

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -41,7 +41,7 @@ impl AttributeKind {
             Optimize(..) => No,
             PassByValue(..) => Yes,
             PubTransparent(..) => Yes,
-            Repr(..) => No,
+            Repr { .. } => No,
             RustcLayoutScalarValidRangeEnd(..) => Yes,
             RustcLayoutScalarValidRangeStart(..) => Yes,
             RustcObjectLifetimeDefault => No,

compiler/rustc_attr_data_structures/src/lints.rs

@@ -12,4 +12,5 @@ pub struct AttributeLint<Id> {
 pub enum AttributeLintKind {
     UnusedDuplicate { this: Span, other: Span, warning: bool },
     IllFormedAttributeInput { suggestions: Vec<String> },
+    EmptyAttribute { first_span: Span },
 }