Uninhabited while-let pattern fix

src/librustc_const_eval/check_match.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::ptr;
 use _match::{MatchCheckCtxt, Matrix, expand_pattern, is_useful};
 use _match::Usefulness::*;
 use _match::WitnessPreference::*;
@@ -302,10 +303,20 @@ fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
                             let &(ref first_arm_pats, _) = &arms[0];
                             let first_pat = &first_arm_pats[0];
                             let span = first_pat.0.span;
-                            struct_span_err!(cx.tcx.sess, span, E0165,
-                                             "irrefutable while-let pattern")
-                                .span_label(span, &format!("irrefutable pattern"))
-                                .emit();
+
+                            // check which arm we're on.
+                            if ptr::eq(first_arm_pats, pats) {
+                                let mut diagnostic = Diagnostic::new(Level::Warning,
+                                                                     "unreachable pattern");
+                                diagnostic.set_span(pat.span);
+                                cx.tcx.sess.add_lint_diagnostic(lint::builtin::UNREACHABLE_PATTERNS,
+                                                                hir_pat.id, diagnostic);
+                            } else {
+                                struct_span_err!(cx.tcx.sess, span, E0165,
+                                                 "irrefutable while-let pattern")
+                                    .span_label(span, &format!("irrefutable pattern"))
+                                    .emit();
+                            }
                         },
 
                         hir::MatchSource::ForLoopDesugar |

src/librustc_const_eval/lib.rs

@@ -30,6 +30,7 @@
 #![feature(box_syntax)]
 #![feature(const_fn)]
 #![feature(i128_type)]
+#![feature(ptr_eq)]
 
 extern crate arena;
 #[macro_use] extern crate syntax;

src/test/compile-fail/uninhabited-patterns.rs

@@ -24,6 +24,10 @@ struct NotSoSecretlyEmpty {
     _priv: !,
 }
 
+fn foo() -> Option<NotSoSecretlyEmpty> {
+    None
+}
+
 fn main() {
     let x: &[!] = &[];
 
@@ -45,5 +49,9 @@ fn main() {
         Err(Err(_y)) => (),
         Err(Ok(_y)) => (),  //~ ERROR unreachable pattern
     }
+
+    while let Some(_y) = foo() {
+        //~^ ERROR unreachable pattern
+    }
 }