Auto merge of #3570 - muth:master, r=phansch

panic at map_unit_fn.rs:202 for map() without args

map_unit_fn.rs accessed map() arguments before type check which ensures type is Option or Result.

Boiled it down to a simple test case.

FWIW: Found this panic when running clippy against code which used a gtk::Window's map() fn inherited from gtk::WidgetExt   http://gtk-rs.org/docs/gtk/trait.WidgetExt.html#tymethod.map

Author: bors

clippy_lints/src/map_unit_fn.rs

@@ -199,7 +199,6 @@ fn suggestion_msg(function_type: &str, map_type: &str) -> String {
 
 fn lint_map_unit_fn(cx: &LateContext<'_, '_>, stmt: &hir::Stmt, expr: &hir::Expr, map_args: &[hir::Expr]) {
     let var_arg = &map_args[0];
-    let fn_arg = &map_args[1];
 
     let (map_type, variant, lint) = if match_type(cx, cx.tables.expr_ty(var_arg), &paths::OPTION) {
         ("Option", "Some", OPTION_MAP_UNIT_FN)
@@ -208,6 +207,7 @@ fn lint_map_unit_fn(cx: &LateContext<'_, '_>, stmt: &hir::Stmt, expr: &hir::Expr
     } else {
         return;
     };
+    let fn_arg = &map_args[1];
 
     if is_unit_function(cx, fn_arg) {
         let msg = suggestion_msg("function", map_type);

tests/ui/map_unit_fn.rs

@@ -0,0 +1,20 @@
+// Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(unused)]
+struct Mappable {}
+
+impl Mappable {
+    pub fn map(&self) {}
+}
+
+fn main() {
+    let m = Mappable {};
+    m.map();
+}