use a more conservative inhabitableness rule

[rust.git] / src / librustc_const_eval / check_match.rs

diff --git a/src/librustc_const_eval/check_match.rs b/src/librustc_const_eval/check_match.rs
index 6f33b4fad769f20d9e7d06b053a0a014fafbb5c3..9b30946c0bebbc1d834cd9cdd045352dbd76be28 100644 (file)
--- a/src/librustc_const_eval/check_match.rs
+++ b/src/librustc_const_eval/check_match.rs
@@ -177,6 +177,31 @@ fn check_match(
             // Fourth, check for unreachable arms.
             check_arms(cx, &inlined_arms, source);
 
+            // Then, if the match has no arms, check whether the scrutinee
+            // is uninhabited.
+            let pat_ty = self.tables.node_id_to_type(scrut.id);
+            let module = self.tcx.hir.local_def_id(self.tcx.hir.get_module_parent(scrut.id));
+            if inlined_arms.is_empty() {
+                let scrutinee_is_uninhabited = if self.tcx.sess.features.borrow().never_type {
+                    pat_ty.is_uninhabited_from(module, self.tcx)
+                } else {
+                    self.conservative_is_uninhabited(pat_ty)
+                };
+                if !scrutinee_is_uninhabited {
+                    // We know the type is inhabited, so this must be wrong
+                    let mut err = create_e0004(self.tcx.sess, scrut.span,
+                                               format!("non-exhaustive patterns: type {} \
+                                                        is non-empty",
+                                                       pat_ty));
+                    span_help!(&mut err, scrut.span,
+                               "Please ensure that all possible cases are being handled; \
+                                possibly adding wildcards or more match arms.");
+                    err.emit();
+                }
+                // If the type *is* uninhabited, it's vacuously exhaustive
+                return;
+            }
+
             let matrix: Matrix = inlined_arms
                 .iter()
                 .filter(|&&(_, guard)| guard.is_none())
@@ -188,6 +213,15 @@ fn check_match(
         })
     }
 
+    fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
+        // "rustc-1.0-style" uncontentious uninhabitableness check
+        match scrutinee_ty.sty {
+            ty::TyNever => true,
+            ty::TyAdt(def, _) => def.variants.is_empty(),
+            _ => false
+        }
+    }
+
     fn check_irrefutable(&self, pat: &Pat, is_fn_arg: bool) {
         let origin = if is_fn_arg {
             "function argument"