When using NLL, implicitly borrow match bindings for any guard,

deref'ing such borrows within that guard.

Review feedback: Add comment noting a point where we may or may not
need to add a cast when we finish the work on #27282.

Review feedback: Pass a newtype'd `ArmHasGuard` rather than a raw boolean.

Review feedback: toggle "ref binding in guards" semantics via specific
method. (This should ease a follow-up PR that just unconditionally
adopts the new semantics.)

Author: pnkfelix

src/librustc/ty/context.rs

@@ -1467,6 +1467,14 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.borrowck_mode().use_mir()
     }
 
+    /// If true, pattern variables for use in guards on match arms
+    /// will be bound as references to the data, and occurrences of
+    /// those variables in the guard expression will implicitly
+    /// dereference those bindings. (See rust-lang/rust#27282.)
+    pub fn all_pat_vars_are_implicit_refs_within_guards(self) -> bool {
+        self.borrowck_mode().use_mir()
+    }
+
     /// If true, we should enable two-phase borrows checks. This is
     /// done with either `-Ztwo-phase-borrows` or with
     /// `#![feature(nll)]`.

src/librustc_mir/build/block.rs

@@ -9,6 +9,8 @@
 // except according to those terms.
 
 use build::{BlockAnd, BlockAndExtension, Builder};
+use build::ForGuard::OutsideGuard;
+use build::matches::ArmHasGuard;
 use hair::*;
 use rustc::mir::*;
 use rustc::hir;
@@ -113,7 +115,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                     // Declare the bindings, which may create a visibility scope.
                     let remainder_span = remainder_scope.span(this.hir.tcx(),
                                                               &this.hir.region_scope_tree);
-                    let scope = this.declare_bindings(None, remainder_span, lint_level, &pattern);
+                    let scope = this.declare_bindings(None, remainder_span, lint_level, &pattern,
+                                                      ArmHasGuard(false));
 
                     // Evaluate the initializer, if present.
                     if let Some(init) = initializer {
@@ -135,8 +138,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                         }
 
                         this.visit_bindings(&pattern, &mut |this, _, _, node, span, _| {
-                            this.storage_live_binding(block, node, span);
-                            this.schedule_drop_for_binding(node, span);
+                            this.storage_live_binding(block, node, span, OutsideGuard);
+                            this.schedule_drop_for_binding(node, span, OutsideGuard);
                         })
                     }
 

src/librustc_mir/build/expr/as_place.rs

@@ -11,6 +11,7 @@
 //! See docs in build/expr/mod.rs
 
 use build::{BlockAnd, BlockAndExtension, Builder};
+use build::ForGuard::{OutsideGuard, WithinGuard};
 use build::expr::category::Category;
 use hair::*;
 use rustc::mir::*;
@@ -86,8 +87,18 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 block.and(Place::Local(Local::new(1)))
             }
             ExprKind::VarRef { id } => {
-                let index = this.var_indices[&id];
-                block.and(Place::Local(index))
+                let place = if this.is_bound_var_in_guard(id) {
+                    let index = this.var_local_id(id, WithinGuard);
+                    if this.hir.tcx().all_pat_vars_are_implicit_refs_within_guards() {
+                        Place::Local(index).deref()
+                    } else {
+                        Place::Local(index)
+                    }
+                } else {
+                    let index = this.var_local_id(id, OutsideGuard);
+                    Place::Local(index)
+                };
+                block.and(place)
             }
             ExprKind::StaticRef { id } => {
                 block.and(Place::Static(Box::new(Static { def_id: id, ty: expr.ty })))