Fix NLL migration mode so that reports region errors when necessary.

src/librustc/infer/error_reporting/mod.rs

@@ -303,7 +303,14 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     ) {
         debug!("report_region_errors(): {} errors to start", errors.len());
 
-        if will_later_be_reported_by_nll && self.tcx.use_mir_borrowck() {
+        if will_later_be_reported_by_nll &&
+            // FIXME: `use_mir_borrowck` seems wrong here...
+            self.tcx.use_mir_borrowck() &&
+            // ... this is a band-aid; may be better to explicitly
+            // match on every borrowck_mode variant to guide decision
+            // here.
+            !self.tcx.migrate_borrowck() {
+
             // With `#![feature(nll)]`, we want to present a nice user
             // experience, so don't even mention the errors from the
             // AST checker.

src/test/ui/borrowck/issue-45983.stderr → src/test/ui/borrowck/issue-45983.ast.stderr

@@ -1,5 +1,5 @@
 error: borrowed data cannot be stored outside of its closure
-  --> $DIR/issue-45983.rs:17:27
+  --> $DIR/issue-45983.rs:36:27
    |
 LL |     let x = None;
    |         - borrowed data cannot be stored into here...

src/test/ui/borrowck/issue-45983.migrate.stderr

@@ -0,0 +1,12 @@
+error: borrowed data cannot be stored outside of its closure
+  --> $DIR/issue-45983.rs:36:27
+   |
+LL |     let x = None;
+   |         - borrowed data cannot be stored into here...
+LL |     give_any(|y| x = Some(y));
+   |              ---          ^ cannot be stored outside of its closure
+   |              |
+   |              ...because it cannot outlive this closure
+
+error: aborting due to previous error
+

src/test/ui/borrowck/issue-45983.nll.stderr

@@ -1,11 +1,11 @@
 warning: not reporting region error due to nll
-  --> $DIR/issue-45983.rs:17:27
+  --> $DIR/issue-45983.rs:36:27
    |
 LL |     give_any(|y| x = Some(y));
    |                           ^
 
 error: borrowed data escapes outside of closure
-  --> $DIR/issue-45983.rs:17:18
+  --> $DIR/issue-45983.rs:36:18
    |
 LL |     let x = None;
    |         - `x` is declared here, outside of the closure body
@@ -15,7 +15,7 @@ LL |     give_any(|y| x = Some(y));
    |               `y` is a reference that is only valid in the closure body
 
 error[E0594]: cannot assign to `x`, as it is not declared as mutable
-  --> $DIR/issue-45983.rs:17:18
+  --> $DIR/issue-45983.rs:36:18
    |
 LL |     let x = None;
    |         - help: consider changing this to be mutable: `mut x`

src/test/ui/borrowck/issue-45983.rs

@@ -8,12 +8,35 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// As documented in Issue #45983, this test is evaluating the quality
+// of our diagnostics on erroneous code using higher-ranked closures.
+//
+// However, as documented on Issue #53026, this test also became a
+// prime example of our need to test the NLL migration mode
+// *separately* from the existing test suites that focus solely on
+// AST-borrwock and NLL.
+
+// revisions: ast migrate nll
+
+// Since we are testing nll (and migration) explicitly as a separate
+// revisions, dont worry about the --compare-mode=nll on this test.
+
+// ignore-compare-mode-nll
+
+//[ast]compile-flags: -Z borrowck=ast
+//[migrate]compile-flags: -Z borrowck=migrate -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+
 fn give_any<F: for<'r> FnOnce(&'r ())>(f: F) {
     f(&());
 }
 
 fn main() {
     let x = None;
     give_any(|y| x = Some(y));
-    //~^ ERROR borrowed data cannot be stored outside of its closure
+    //[ast]~^ ERROR borrowed data cannot be stored outside of its closure
+    //[migrate]~^^ ERROR borrowed data cannot be stored outside of its closure
+    //[nll]~^^^ WARN not reporting region error due to nll
+    //[nll]~| ERROR borrowed data escapes outside of closure
+    //[nll]~| ERROR cannot assign to `x`, as it is not declared as mutable
 }