add tests for '_' places

Author: RalfJung

tests/fail/pointer-deref-discard-let.rs

@@ -0,0 +1,9 @@
+#![feature(strict_provenance)]
+// Make sure places assigned to a `_` pattern are still checked for deref'ability.
+
+fn main() {
+    let ptr = std::ptr::invalid(32) as *const u8;
+    unsafe {
+        let _ = *ptr;
+    } //~ERROR: dangling pointer
+}

tests/fail/pointer-deref-discard-match.rs

@@ -0,0 +1,11 @@
+#![feature(strict_provenance)]
+// Make sure places assigned to a `_` pattern are still checked for deref'ability.
+
+fn main() {
+    let ptr = std::ptr::invalid(32) as *const u8;
+    unsafe {
+        match *ptr {
+            _ => {}
+        } //~ERROR: dangling pointer
+    }
+}