Have the HIR borrowck force the MIR borrowck

Fixes rust-lang#48697

Author: nikomatsakis

src/librustc_borrowck/borrowck/mod.rs

@@ -120,15 +120,12 @@ fn borrowck<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, owner_def_id: DefId)
         used_mut_nodes: RefCell::new(FxHashSet()),
     };
 
-    // Eventually, borrowck will always read the MIR, but at the
-    // moment we do not. So, for now, we always force MIR to be
-    // constructed for a given fn, since this may result in errors
-    // being reported and we want that to happen.
-    //
-    // Note that `mir_validated` is a "stealable" result; the
-    // thief, `optimized_mir()`, forces borrowck, so we know that
-    // is not yet stolen.
-    tcx.mir_validated(owner_def_id).borrow();
+    // Force the MIR-based borrow checker to run; this also forces the
+    // MIR to be built. This may lead to errors being reported,
+    // particularly in NLL mode, where some ordinary region errors are
+    // suppressed. It's important that those errors get reported to
+    // avoid ICEs like #48697.
+    let _ = tcx.mir_borrowck(owner_def_id);
 
     // option dance because you can't capture an uninitialized variable
     // by mut-ref.

src/test/ui/nll/issue-48697.rs

@@ -0,0 +1,26 @@
+// Copyright 2018 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 we do not ICE when we have suppressed a lexical region
+// error that would otherwise cause AST borrowck to die.
+//
+// Regression test for #48697.
+
+#![feature(nll)]
+#![allow(warnings)]
+
+fn foo(x: &i32) -> &i32 {
+    let z = 4;
+    let f = &|y| { y };
+    let k = f(&z);
+    f(x)
+}
+
+fn main() {}