Add a suggestion to use a let binding on some borrowck errors.

Author: Cameron Zwarich

src/librustc/middle/borrowck/mod.rs

@@ -689,11 +689,16 @@ impl<'a> BorrowckCtxt<'a> {
                     "reference must be valid for ",
                     sub_scope,
                     "...");
+                let suggestion = if is_statement_scope(self.tcx, super_scope) {
+                    "; consider using a `let` binding to increase its lifetime"
+                } else {
+                    ""
+                };
                 note_and_explain_region(
                     self.tcx,
                     "...but borrowed value is only valid for ",
                     super_scope,
-                    "");
+                    suggestion);
             }
 
             err_borrowed_pointer_too_short(loan_scope, ptr_scope, _) => {
@@ -779,6 +784,18 @@ impl<'a> BorrowckCtxt<'a> {
     }
 }
 
+fn is_statement_scope(tcx: &ty::ctxt, region: ty::Region) -> bool {
+     match region {
+         ty::ReScope(node_id) => {
+             match tcx.map.find(node_id) {
+                 Some(ast_map::NodeStmt(_)) => true,
+                 _ => false
+             }
+         }
+         _ => false
+     }
+}
+
 impl DataFlowOperator for LoanDataFlowOperator {
     #[inline]
     fn initial_value(&self) -> bool {

src/test/compile-fail/borrowck-let-suggestion.rs

@@ -0,0 +1,20 @@
+// 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.
+
+fn f() {
+    let x = [1].iter(); //~ ERROR borrowed value does not live long enough
+                        //~^^ NOTE reference must be valid for the block
+                        //~^^ NOTE consider using a `let` binding to increase its lifetime
+}
+
+fn main() {
+    f();
+}
+