Emit map_unit_fn lint in closure case

compiler/rustc_lint/src/map_unit_fn.rs

@@ -72,6 +72,22 @@ impl<'tcx> LateLintPass<'tcx> for MapUnitFn {
                                 },
                             )
                         }
+                    } else if let ty::Closure(id, subs) = arg_ty.kind() {
+                        let cl_ty = subs.as_closure().sig();
+                        let ret_ty = cl_ty.output().skip_binder();
+                        if is_unit_type(ret_ty) {
+                            cx.emit_spanned_lint(
+                                MAP_UNIT_FN,
+                                span,
+                                MappingToUnit {
+                                    function_label: cx.tcx.span_of_impl(*id).unwrap(),
+                                    argument_label: args[0].span,
+                                    map_label: arg_ty.default_span(cx.tcx),
+                                    suggestion: path.ident.span,
+                                    replace: "for_each".to_string(),
+                                },
+                            )
+                        }
                     }
                 }
             }