Add an impl for Box<Error> from String.

Closes #30156.

src/test/run-pass/string-box-error.rs

@@ -0,0 +1,18 @@
+// Copyright 2015 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.
+
+// Ensure that both `Box<Error + Send + Sync>` and `Box<Error>` can be obtained from `String`.
+
+use std::error::Error;
+
+fn main() {
+    let _err1: Box<Error + Send + Sync> = From::from("test".to_string());
+    let _err2: Box<Error> = From::from("test".to_string());
+}