Require braces when a closure has an explicit return type. This is a

[breaking-change]: instead of a closure like `|| -> i32 22`, prefer `||
-> i32 { 22 }`.

Fixes #23420.

src/test/compile-fail/fn-trait-formatting.rs

@@ -26,7 +26,7 @@ fn main() {
     //~| found `Box<core::ops::Fn(isize, isize)>`
     //~| expected ()
     //~| found box
-    let _: () = (box || -> isize unimplemented!()) as Box<FnMut() -> isize>;
+    let _: () = (box || -> isize { unimplemented!() }) as Box<FnMut() -> isize>;
     //~^ ERROR mismatched types
     //~| expected `()`
     //~| found `Box<core::ops::FnMut() -> isize>`

src/test/compile-fail/liveness-issue-2163.rs

@@ -13,6 +13,6 @@ use std::vec::Vec;
 fn main() {
     let a: Vec<isize> = Vec::new();
     a.iter().all(|_| -> bool {
-        //~^ ERROR mismatched types
+        //~^ ERROR not all control paths return a value
     });
 }

src/test/parse-fail/closure-return-syntax.rs

@@ -0,0 +1,16 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// 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.
+
+// Test that we cannot parse a closure with an explicit return type
+// unless it uses braces.
+
+fn main() {
+    let x = || -> i32 22; //~ ERROR expected `{`, found `22`
+}

src/test/run-pass/block-explicit-types.rs

@@ -10,5 +10,5 @@
 
 pub fn main() {
     fn as_buf<T, F>(s: String, f: F) -> T where F: FnOnce(String) -> T { f(s) }
-    as_buf("foo".to_string(), |foo: String| -> () println!("{}", foo) );
+    as_buf("foo".to_string(), |foo: String| -> () { println!("{}", foo) });
 }

src/test/run-pass/borrowck-move-by-capture-ok.rs

@@ -14,6 +14,6 @@
 
 pub fn main() {
     let bar: Box<_> = box 3;
-    let h = || -> int *bar;
+    let h = || -> int { *bar };
     assert_eq!(h(), 3);
 }

src/test/run-pass/issue-17816.rs

@@ -14,7 +14,7 @@ use std::marker::PhantomData;
 
 fn main() {
     struct Symbol<'a, F: Fn(Vec<&'a str>) -> &'a str> { function: F, marker: PhantomData<&'a ()> }
-    let f = |x: Vec<&str>| -> &str "foobar";
+    let f = |x: Vec<&str>| -> &str { "foobar" };
     let sym = Symbol { function: f, marker: PhantomData };
     (sym.function)(vec![]);
 }