On nightly with NLL, suggest #![feature(bind_by_move_pattern_guards)] when it might fix the code.

Author: pnkfelix

src/librustc_mir/hair/pattern/check_match.rs

@@ -538,10 +538,14 @@ fn check_legality_of_move_bindings(cx: &MatchVisitor,
                 .span_label(p.span, "binds an already bound by-move value by moving it")
                 .emit();
         } else if has_guard && !cx.tcx.allow_bind_by_move_patterns_with_guards() {
-            struct_span_err!(cx.tcx.sess, p.span, E0008,
-                      "cannot bind by-move into a pattern guard")
-                .span_label(p.span, "moves value into pattern guard")
-                .emit();
+            let mut err = struct_span_err!(cx.tcx.sess, p.span, E0008,
+                                       "cannot bind by-move into a pattern guard");
+            err.span_label(p.span, "moves value into pattern guard");
+            if cx.tcx.sess.opts.unstable_features.is_nightly_build() && cx.tcx.use_mir_borrowck() {
+                err.help("add #![feature(bind_by_move_pattern_guards)] to the \
+                          crate attributes to enable");
+            }
+            err.emit();
         } else if let Some(by_ref_span) = by_ref_span {
             struct_span_err!(
                 cx.tcx.sess,
@@ -613,10 +617,16 @@ impl<'a, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'tcx> {
               _: LoanCause) {
         match kind {
             ty::MutBorrow => {
-                struct_span_err!(self.cx.tcx.sess, span, E0301,
-                          "cannot mutably borrow in a pattern guard")
-                    .span_label(span, "borrowed mutably in pattern guard")
-                    .emit();
+                let mut err = struct_span_err!(self.cx.tcx.sess, span, E0301,
+                          "cannot mutably borrow in a pattern guard");
+                err.span_label(span, "borrowed mutably in pattern guard");
+                if self.cx.tcx.sess.opts.unstable_features.is_nightly_build() &&
+                    self.cx.tcx.use_mir_borrowck()
+                {
+                    err.help("add #![feature(bind_by_move_pattern_guards)] to the \
+                              crate attributes to enable");
+                }
+                err.emit();
             }
             ty::ImmBorrow | ty::UniqueImmBorrow => {}
         }

src/test/ui/bind-by-move/bind-by-move-no-guards.nll.stderr

@@ -0,0 +1,11 @@
+error[E0008]: cannot bind by-move into a pattern guard
+  --> $DIR/bind-by-move-no-guards.rs:8:14
+   |
+LL |         Some(z) if z.recv().unwrap() => { panic!() },
+   |              ^ moves value into pattern guard
+   |
+   = help: add #![feature(bind_by_move_pattern_guards)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0008`.

src/test/ui/borrowck/borrowck-mutate-in-guard.nll.stderr

@@ -0,0 +1,24 @@
+error[E0302]: cannot assign in a pattern guard
+  --> $DIR/borrowck-mutate-in-guard.rs:20:25
+   |
+LL |         Enum::A(_) if { x = Enum::B(false); false } => 1,
+   |                         ^^^^^^^^^^^^^^^^^^ assignment in pattern guard
+
+error[E0301]: cannot mutably borrow in a pattern guard
+  --> $DIR/borrowck-mutate-in-guard.rs:22:38
+   |
+LL |         Enum::A(_) if { let y = &mut x; *y = Enum::B(false); false } => 1,
+   |                                      ^ borrowed mutably in pattern guard
+   |
+   = help: add #![feature(bind_by_move_pattern_guards)] to the crate attributes to enable
+
+error[E0302]: cannot assign in a pattern guard
+  --> $DIR/borrowck-mutate-in-guard.rs:22:41
+   |
+LL |         Enum::A(_) if { let y = &mut x; *y = Enum::B(false); false } => 1,
+   |                                         ^^^^^^^^^^^^^^^^^^^ assignment in pattern guard
+
+error: aborting due to 3 previous errors
+
+Some errors occurred: E0301, E0302.
+For more information about an error, try `rustc --explain E0301`.

src/test/ui/error-codes/E0008.nll.stderr

@@ -0,0 +1,11 @@
+error[E0008]: cannot bind by-move into a pattern guard
+  --> $DIR/E0008.rs:13:14
+   |
+LL |         Some(s) if s.len() == 0 => {},
+   |              ^ moves value into pattern guard
+   |
+   = help: add #![feature(bind_by_move_pattern_guards)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0008`.

src/test/ui/error-codes/E0301.nll.stderr

@@ -0,0 +1,11 @@
+error[E0301]: cannot mutably borrow in a pattern guard
+  --> $DIR/E0301.rs:14:19
+   |
+LL |         option if option.take().is_none() => {}, //~ ERROR E0301
+   |                   ^^^^^^ borrowed mutably in pattern guard
+   |
+   = help: add #![feature(bind_by_move_pattern_guards)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0301`.