add some tests specifically for validity checks arising from match binders

Author: RalfJung

tests/fail/validity/match_binder_checks_validity1.rs

@@ -0,0 +1,15 @@
+fn main() {
+    #[derive(Copy, Clone)]
+    enum Void {}
+    union Uninit<T: Copy> {
+        value: T,
+        uninit: (),
+    }
+    unsafe {
+        let x: Uninit<Void> = Uninit { uninit: () };
+        match x.value {
+            #[allow(unreachable_patterns)]
+            _x => println!("hi from the void!"), //~ERROR: invalid value
+        }
+    }
+}

tests/fail/validity/match_binder_checks_validity1.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: constructing invalid value: encountered a value of uninhabited type `main::Void`
+  --> $DIR/match_binder_checks_validity1.rs:LL:CC
+   |
+LL |             _x => println!("hi from the void!"),
+   |             ^^ constructing invalid value: encountered a value of uninhabited type `main::Void`
+   |
+   = 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/match_binder_checks_validity1.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

tests/fail/validity/match_binder_checks_validity2.rs

@@ -0,0 +1,14 @@
+fn main() {
+    #[derive(Copy, Clone)]
+    union Uninit<T: Copy> {
+        value: T,
+        uninit: u8,
+    }
+    unsafe {
+        let x: Uninit<bool> = Uninit { uninit: 3 };
+        match x.value {
+            #[allow(unreachable_patterns)]
+            _x => println!("hi from the void!"), //~ERROR: invalid value
+        }
+    }
+}

tests/fail/validity/match_binder_checks_validity2.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: constructing invalid value: encountered 0x03, but expected a boolean
+  --> $DIR/match_binder_checks_validity2.rs:LL:CC
+   |
+LL |             _x => println!("hi from the void!"),
+   |             ^^ constructing invalid value: encountered 0x03, but expected a boolean
+   |
+   = 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/match_binder_checks_validity2.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+