check that the path from which we borrow is readable

The old code incorrectly assumed that the borrow would always generate a
loan. In that case, we will check if that loan conflicts with other
loans and hence get an error if the path we borrowed is mutably
borrowed. But this is not true if we are borrowing the referent of an
`&T` reference.

Fixes rust-lang#38899.

Author: nikomatsakis

src/libgetopts/lib.rs

@@ -101,6 +101,7 @@ use self::SplitWithinState::*;
 use self::Whitespace::*;
 use self::LengthLimit::*;
 
+use std::cell::Cell;
 use std::fmt;
 use std::iter::repeat;
 use std::result;
@@ -858,7 +859,7 @@ fn each_split_within<F>(ss: &str, lim: usize, mut it: F) -> bool
     let mut slice_start = 0;
     let mut last_start = 0;
     let mut last_end = 0;
-    let mut state = A;
+    let state = Cell::new(A);
     let mut fake_i = ss.len();
     let mut lim = lim;
 
@@ -881,7 +882,7 @@ fn each_split_within<F>(ss: &str, lim: usize, mut it: F) -> bool
             OverLim
         };
 
-        state = match (state, whitespace, limit) {
+        state.set(match (state.get(), whitespace, limit) {
             (A, Ws, _) => A,
             (A, Cr, _) => {
                 slice_start = i;
@@ -925,7 +926,7 @@ fn each_split_within<F>(ss: &str, lim: usize, mut it: F) -> bool
                 A
             }
             (C, Ws, UnderLim) => C,
-        };
+        });
 
         *cont
     };
@@ -934,7 +935,7 @@ fn each_split_within<F>(ss: &str, lim: usize, mut it: F) -> bool
 
     // Let the automaton 'run out' by supplying trailing whitespace
     while cont &&
-          match state {
+          match state.get() {
         B | C => true,
         A => false,
     } {

src/librustc_borrowck/borrowck/check_loans.rs

@@ -141,6 +141,7 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for CheckLoanCtxt<'a, 'tcx> {
                 _ => MovedInUse,
             };
             self.check_if_path_is_moved(borrow_id, borrow_span, moved_value_use_kind, &lp);
+            self.check_for_copy_of_frozen_path(borrow_id, borrow_span, &lp);
         }
 
         self.check_for_conflicting_loans(borrow_id);

src/test/compile-fail/borrowck/borrowck-reborrow-shared-referent-issue-38899.rs

@@ -0,0 +1,27 @@
+// 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.
+
+// Test that reborrowing the referent of an `&T` reference is not
+// possible if the path we use to reach that reference is itself
+// borrowed. Regression test for #38899.
+
+#![allow(dead_code)]
+
+pub struct Block<'a> {
+    current: &'a u8,
+    unrelated: &'a u8,
+}
+
+fn bump<'a>(mut block: &mut Block<'a>) {
+    let x = &mut block;
+    let p: &'a u8 = &*block.current; //~ ERROR E0503
+}
+
+fn main() {}