Add some test cases

Author: Nadrieril

src/test/ui/pattern/usefulness/slice-patterns.rs

@@ -93,4 +93,18 @@ fn main() {
         [false] => {}
         [..] => {}
     }
+    match s {
+    //~^ ERROR `&[_, _, true]` not covered
+        [] => {}
+        [_] => {}
+        [_, _] => {}
+        [.., false] => {}
+    }
+    match s {
+    //~^ ERROR `&[true, _, _]` not covered
+        [] => {}
+        [_] => {}
+        [_, _] => {}
+        [false, .., false] => {}
+    }
 }

src/test/ui/pattern/usefulness/slice-patterns.stderr

@@ -112,6 +112,22 @@ error: unreachable pattern
 LL |         [false, true] => {}
    |         ^^^^^^^^^^^^^
 
-error: aborting due to 15 previous errors
+error[E0004]: non-exhaustive patterns: `&[_, _, true]` not covered
+  --> $DIR/slice-patterns.rs:96:11
+   |
+LL |     match s {
+   |           ^ pattern `&[_, _, true]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[true, _, _]` not covered
+  --> $DIR/slice-patterns.rs:103:11
+   |
+LL |     match s {
+   |           ^ pattern `&[true, _, _]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error: aborting due to 17 previous errors
 
 For more information about this error, try `rustc --explain E0004`.