Add an impl for Box<Error> from String.

Closes #30156.

src/libstd/error.rs

@@ -119,6 +119,15 @@ impl From<String> for Box<Error + Send + Sync> {
     }
 }
 
+#[stable(feature = "string_box_error", since = "1.7.0")]
+impl From<String> for Box<Error> {
+    fn from(str_err: String) -> Box<Error> {
+        let err1: Box<Error + Send + Sync> = From::from(str_err);
+        let err2: Box<Error> = err1;
+        err2
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, 'b> From<&'b str> for Box<Error + Send + Sync + 'a> {
     fn from(err: &'b str) -> Box<Error + Send + Sync + 'a> {

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());
+}