test describing a currently unsupported corner case.

src/test/compile-fail/borrowck/two-phase-reservation-sharing-interference.rs

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+// Copyright 2017 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.
+
+// revisions: lxl nll
+//[lxl]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows -Z nll
+
+// This is a corner case that the current implementation is (probably)
+// treating more conservatively than is necessary. But it also does
+// not seem like a terribly important use case to cover.
+//
+// So this test is just making a note of the current behavior, with
+// the caveat that in the future, the rules may be loosened, at which
+// point this test might be thrown out.
+
+fn main() {
+    let mut vec = vec![0, 1];
+    let delay: &mut Vec<_>;
+    {
+        let shared = &vec;
+
+        // we reserve here, which could (on its own) be compatible
+        // with the shared borrow. But in the current implementation,
+        // its an error.
+        delay = &mut vec;
+        //[lxl]~^ ERROR cannot borrow `vec` as mutable because it is also borrowed as immutable
+        //[nll]~^^   ERROR cannot borrow `vec` as mutable because it is also borrowed as immutable
+
+        shared[0];
+    }
+
+    // the &mut-borrow only becomes active way down here.
+    //
+    // (At least in theory; part of the reason this test fails is that
+    // the constructed MIR throws in extra &mut reborrows which
+    // flummoxes our attmpt to delay the activation point here.)
+    delay.push(2);
+}
+