Handle region errors where the actual scope is a statement

Prevents a case of "unknown scope, this is a bug".

I judged this too small to review. Closes #4500

Author: catamorphism

src/librustc/middle/region.rs

@@ -622,7 +622,7 @@ fn determine_rp_in_ty(ty: @ast::Ty,
                       &&cx: determine_rp_ctxt,
                       visitor: visit::vt<determine_rp_ctxt>) {
 
-    // we are only interesting in types that will require an item to
+    // we are only interested in types that will require an item to
     // be region-parameterized.  if cx.item_id is zero, then this type
     // is not a member of a type defn nor is it a constitutent of an
     // impl etc.  So we can ignore it and its components.

src/librustc/util/ppaux.rs

@@ -82,6 +82,9 @@ fn explain_region_and_span(cx: ctxt, region: ty::Region)
               _ => explain_span(cx, "expression", expr.span)
             }
           }
+          Some(ast_map::node_stmt(stmt)) => {
+              explain_span(cx, "statement", stmt.span)
+          }
           Some(_) | None => {
             // this really should not happen
             (fmt!("unknown scope: %d.  Please report a bug.", node_id),

src/test/compile-fail/issue-4500.rs

@@ -0,0 +1,15 @@
+// Copyright 2012 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 main () {
+    let mut _p: & int = & 4;
+    _p = ~3; //~ ERROR illegal borrow: borrowed value does not live long enough
+    //~^ NOTE ...but borrowed value is only valid for the statement
+}