toolchain: Update to Rust 1.31.0

We updated the production toolchain from 1.30 to 1.31 in CL:1366446.
This CL does the same upgrade for the local developer toolchain and
Kokoro.

The relevant changes are in rust-toolchain and kokoro/Dockerfile.
The rest are from rustfmt.

TEST=cargo fmt --all -- --check
TEST=as described in kokoro/README.md

Change-Id: I3b4913f3e237baa36c664b4953be360c09efffd4
Reviewed-on: https://chromium-review.googlesource.com/1374376
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: David Tolnay <dtolnay@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>

2 files changed, 4 insertions, 2 deletions

diff --git a/msg_socket/tests/enum.rs b/msg_socket/tests/enum.rs
index d63cfc1..9134f6e 100644
--- a/msg_socket/tests/enum.rs
+++ b/msg_socket/tests/enum.rs
@@ -55,7 +55,8 @@ fn sock_send_recv_enum() {
     res.send(&Response::E {
         f0: 0x12,
         f1: 0x0f0f,
-    }).unwrap();
+    })
+    .unwrap();
     match req.recv().unwrap() {
         Response::E { f0, f1 } => {
             assert_eq!(f0, 0x12);
diff --git a/msg_socket/tests/struct.rs b/msg_socket/tests/struct.rs
index ab97a6f..07cde3e 100644
--- a/msg_socket/tests/struct.rs
+++ b/msg_socket/tests/struct.rs
@@ -25,7 +25,8 @@ fn sock_send_recv_struct() {
         field0: 2,
         field1: e0,
         field2: 0xf0f0,
-    }).unwrap();
+    })
+    .unwrap();
     let r = res.recv().unwrap();
     assert_eq!(r.field0, 2);
     assert_eq!(r.field2, 0xf0f0);