Bug fix: #![feature(nll)] takes precedence over -Z borrowck=migrate.

(Includes test illustrating desired behavior; compare its diagnostic
output to that of the file `borrowck-migreate-to-nll.rs`.)

src/librustc/ty/context.rs

@@ -1405,9 +1405,11 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     pub fn borrowck_mode(&self) -> BorrowckMode {
         match self.sess.opts.borrowck_mode {
             mode @ BorrowckMode::Mir |
-            mode @ BorrowckMode::Migrate |
             mode @ BorrowckMode::Compare => mode,
 
+            // `BorrowckMode::Ast` is synonymous with no `-Z
+            // borrowck=...` flag at all. Therefore, we definitely
+            // want `#![feature(nll)]` to override it.
             mode @ BorrowckMode::Ast => {
                 if self.features().nll {
                     BorrowckMode::Mir
@@ -1416,6 +1418,19 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
                 }
             }
 
+            // `BorrowckMode::Migrate` is modelling the behavior one
+            // will eventually specify via `--edition 2018`. We want
+            // to allow developers on the Nightly channel to opt back
+            // into the "hard error" mode for NLL, which they can do
+            // via specifying `#![feature(nll)]` explicitly in their
+            // crate.
+            mode @ BorrowckMode::Migrate => {
+                if self.features().nll {
+                    BorrowckMode::Mir
+                } else {
+                    mode
+                }
+            }
         }
     }
 

src/test/ui/borrowck/borrowck-feature-nll-overrides-migrate.rs

@@ -0,0 +1,37 @@
+// 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.
+
+// This is a test that the `#![feature(nll)]` opt-in overrides the
+// migration mode. The intention here is to emulate the goal behavior
+// that `--edition 2018` effects on borrowck (modeled here by `-Z
+// borrowck=migrate`) are themselves overridden by the
+// `#![feature(nll)]` opt-in.
+//
+// Therefore, for developer convenience, under `#[feature(nll)]` the
+// NLL checks will be emitted as errors *even* in the presence of `-Z
+// borrowck=migrate`.
+
+// compile-flags: -Z borrowck=migrate
+
+#![feature(nll)]
+
+fn main() {
+    match Some(&4) {
+        None => {},
+        ref mut foo
+            if {
+                (|| { let bar = foo; bar.take() })();
+                //~^ ERROR cannot move out of borrowed content [E0507]
+                false
+            } => {},
+        Some(ref _s) => println!("Note this arm is bogus; the `Some` became `None` in the guard."),
+        _ => println!("Here is some supposedly unreachable code."),
+    }
+}

src/test/ui/borrowck/borrowck-feature-nll-overrides-migrate.stderr

@@ -0,0 +1,9 @@
+error[E0507]: cannot move out of borrowed content
+  --> $DIR/borrowck-feature-nll-overrides-migrate.rs:30:17
+   |
+LL |                 (|| { let bar = foo; bar.take() })();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot move out of borrowed content
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0507`.