Lower closure binders to hir & properly check them

compiler/rustc_hir/src/hir.rs

@@ -1931,6 +1931,7 @@ pub enum ExprKind<'hir> {
     /// This may also be a generator literal or an `async block` as indicated by the
     /// `Option<Movability>`.
     Closure {
+        binder: &'hir ClosureBinder,
         capture_clause: CaptureBy,
         bound_generic_params: &'hir [GenericParam<'hir>],
         fn_decl: &'hir FnDecl<'hir>,
@@ -2715,6 +2716,17 @@ impl FnRetTy<'_> {
     }
 }
 
+/// Represents `for<...>` binder before a closure
+#[derive(Copy, Clone, Debug, HashStable_Generic)]
+pub enum ClosureBinder {
+    /// Binder is not specified.
+    Default,
+    /// Binder is specified.
+    ///
+    /// Span points to the whole `for<...>`.
+    For { span: Span },
+}
+
 #[derive(Encodable, Debug, HashStable_Generic)]
 pub struct Mod<'hir> {
     pub spans: ModSpans,

compiler/rustc_hir/src/intravisit.rs

@@ -925,7 +925,7 @@ pub fn walk_fn_kind<'v, V: Visitor<'v>>(visitor: &mut V, function_kind: FnKind<'
         FnKind::ItemFn(_, generics, ..) => {
             visitor.visit_generics(generics);
         }
-        FnKind::Method(..) | FnKind::Closure => {}
+        FnKind::Closure | FnKind::Method(..) => {}
     }
 }
 
@@ -1145,6 +1145,7 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
             walk_list!(visitor, visit_arm, arms);
         }
         ExprKind::Closure {
+            binder: _,
             bound_generic_params,
             ref fn_decl,
             body,
@@ -1153,7 +1154,13 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
             movability: _,
         } => {
             walk_list!(visitor, visit_generic_param, bound_generic_params);
-            visitor.visit_fn(FnKind::Closure, fn_decl, body, expression.span, expression.hir_id)
+            visitor.visit_fn(
+                FnKind::Closure,
+                fn_decl,
+                body,
+                expression.span,
+                expression.hir_id,
+            )
         }
         ExprKind::Block(ref block, ref opt_label) => {
             walk_list!(visitor, visit_label, opt_label);