From 516a177273b1f6943f63abd2a74ffa5f53542fdb Mon Sep 17 00:00:00 2001
From: Cameron Zwarich <zwarich@mozilla.com>
Date: Wed, 21 May 2014 21:44:49 -0700
Subject: [PATCH] Add a suggestion to use a `let` binding on some borrowck
 errors.

---
 src/librustc/middle/borrowck/mod.rs           | 19 +++++++++++++++++-
 .../compile-fail/borrowck-let-suggestion.rs   | 20 +++++++++++++++++++
 2 files changed, 38 insertions(+), 1 deletion(-)
 create mode 100644 src/test/compile-fail/borrowck-let-suggestion.rs

diff --git a/src/librustc/middle/borrowck/mod.rs b/src/librustc/middle/borrowck/mod.rs
index 0f67520d8d3..eaa39ee231d 100644
--- a/src/librustc/middle/borrowck/mod.rs
+++ b/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 {
diff --git a/src/test/compile-fail/borrowck-let-suggestion.rs b/src/test/compile-fail/borrowck-let-suggestion.rs
new file mode 100644
index 00000000000..a03087f9b2d
--- /dev/null
+++ b/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();
+}
+