Add miri tests.

Author: cjgillot

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_deref_match_never.rs

@@ -0,0 +1,17 @@
+// Make sure we find these even with many checks disabled.
+//@compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
+
+#![allow(unreachable_code)]
+#![feature(never_type)]
+
+fn main() {
+    let p = {
+        let b = Box::new(42);
+        &*b as *const i32 as *const !
+    };
+    unsafe {
+        match *p {} //~ ERROR: entering unreachable code
+    }
+    panic!("this should never print");
+}
+

src/tools/miri/tests/fail/dangling_pointers/dangling_pointer_deref_match_never.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: entering unreachable code
+  --> $DIR/dangling_pointer_deref_match_never.rs:LL:CC
+   |
+LL |         match *p {}
+   |               ^^ entering unreachable code
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/dangling_pointer_deref_match_never.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

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

@@ -0,0 +1,14 @@
+// A `_` binding in a match is a nop, so we do not detect that the pointer is dangling.
+//@compile-flags: -Zmiri-disable-alignment-check -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
+
+fn main() {
+    let p = {
+        let b = Box::new(42);
+        &*b as *const i32
+    };
+    unsafe {
+        match *p {
+            _ => {}
+        }
+    }
+}