From 7c94fa7d01f264e5268ab5f57519f071c00a6579 Mon Sep 17 00:00:00 2001
From: Mikail Bagishov <bagishov.mikail@yandex.ru>
Date: Wed, 6 May 2020 17:45:47 +0300
Subject: [PATCH] Fix usefulness check for never type

---
 crates/ra_hir_ty/src/_match.rs | 29 +++++++++++++++++++++++------
 1 file changed, 23 insertions(+), 6 deletions(-)

diff --git a/crates/ra_hir_ty/src/_match.rs b/crates/ra_hir_ty/src/_match.rs
index 779e7857458..149f6504241 100644
--- a/crates/ra_hir_ty/src/_match.rs
+++ b/crates/ra_hir_ty/src/_match.rs
@@ -573,14 +573,20 @@ pub(crate) fn is_useful(
     matrix: &Matrix,
     v: &PatStack,
 ) -> MatchCheckResult<Usefulness> {
-    // Handle the special case of enums with no variants. In that case, no match
-    // arm is useful.
-    if let Ty::Apply(ApplicationTy { ctor: TypeCtor::Adt(AdtId::EnumId(enum_id)), .. }) =
-        cx.infer[cx.match_expr].strip_references()
-    {
-        if cx.db.enum_data(*enum_id).variants.is_empty() {
+    // Handle two special cases:
+    // - enum with no variants
+    // - `!` type
+    // In those cases, no match arm is useful.
+    match cx.infer[cx.match_expr].strip_references() {
+        Ty::Apply(ApplicationTy { ctor: TypeCtor::Adt(AdtId::EnumId(enum_id)), .. }) => {
+            if cx.db.enum_data(*enum_id).variants.is_empty() {
+                return Ok(Usefulness::NotUseful);
+            }
+        }
+        Ty::Apply(ApplicationTy { ctor: TypeCtor::Never, .. }) => {
             return Ok(Usefulness::NotUseful);
         }
+        _ => (),
     }
 
     if v.is_empty() {
@@ -1917,6 +1923,17 @@ fn test_fn(never: Never) {
         check_no_diagnostic(content);
     }
 
+    #[test]
+    fn type_never() {
+        let content = r"
+            fn test_fn(never: !) {
+                match never {}
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
     #[test]
     fn enum_never_ref() {
         let content = r"