Keep track of the start of the argument block of a closure

compiler/rustc_hir/src/hir.rs

@@ -928,7 +928,10 @@ pub struct Closure<'hir> {
     pub bound_generic_params: &'hir [GenericParam<'hir>],
     pub fn_decl: &'hir FnDecl<'hir>,
     pub body: BodyId,
+    /// The span of the declaration block: 'move |...| -> ...'
     pub fn_decl_span: Span,
+    /// The span of the argument block `|...|`
+    pub fn_arg_span: Option<Span>,
     pub movability: Option<Movability>,
 }
 

compiler/rustc_hir/src/intravisit.rs

@@ -740,6 +740,7 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr<'v>)
             body,
             capture_clause: _,
             fn_decl_span: _,
+            fn_arg_span: _,
             movability: _,
         }) => {
             walk_list!(visitor, visit_generic_param, bound_generic_params);