consolidate and extend testing for _ patterns discarding the place

Author: RalfJung

src/tools/miri/tests/pass/dangling_pointer_deref_match_underscore.rs

@@ -0,0 +1,76 @@
+// Various tests ensuring that underscore patterns really just construct the place, but don't check its contents.
+#![feature(strict_provenance)]
+use std::ptr;
+
+fn main() {
+    dangling_deref_match();
+    union_uninhabited_match();
+    dangling_let();
+    invalid_let();
+    dangling_let_type_annotation();
+    invalid_let_type_annotation();
+}
+
+fn dangling_deref_match() {
+    let p = {
+        let b = Box::new(42);
+        &*b as *const i32
+    };
+    unsafe {
+        match *p {
+            _ => {}
+        }
+    }
+}
+
+fn union_uninhabited_match() {
+    #[derive(Copy, Clone)]
+    enum Void {}
+    union Uninit<T: Copy> {
+        value: T,
+        uninit: (),
+    }
+    unsafe {
+        let x: Uninit<Void> = Uninit { uninit: () };
+        match x.value {
+            // rustc warns about un unreachable pattern,
+            // but is wrong in unsafe code.
+            #[allow(unreachable_patterns)]
+            _ => println!("hi from the void!"),
+        }
+    }
+}
+
+fn dangling_let() {
+    unsafe {
+        let ptr = ptr::invalid::<bool>(0x40);
+        let _ = *ptr;
+    }
+}
+
+fn invalid_let() {
+    unsafe {
+        let val = 3u8;
+        let ptr = ptr::addr_of!(val).cast::<bool>();
+        let _ = *ptr;
+    }
+}
+
+// Adding a type annotation used to change how MIR is generated, make sure we cover both cases.
+fn dangling_let_type_annotation() {
+    unsafe {
+        let ptr = ptr::invalid::<bool>(0x40);
+        let _: bool = *ptr;
+    }
+}
+
+fn invalid_let_type_annotation() {
+    unsafe {
+        let val = 3u8;
+        let ptr = ptr::addr_of!(val).cast::<bool>();
+        let _: bool = *ptr;
+    }
+}
+
+// FIXME: we should also test `!`, not just `bool` -- but that s currently buggy:
+// https://github.com/rust-lang/rust/issues/117288