add some tests making sure we get the alias checking right

tests/compile-fail/validation_aliasing_mut1.rs

+#![allow(unused_variables)]
+
+mod safe {
+    pub fn safe(x: &mut i32, y: &mut i32) {} //~ ERROR: in conflict with lock WriteLock
+}
+
+fn main() {
+    let x = &mut 0 as *mut _;
+    unsafe { safe::safe(&mut *x, &mut *x) };
+}

tests/compile-fail/validation_aliasing_mut2.rs

+#![allow(unused_variables)]
+
+mod safe {
+    pub fn safe(x: &i32, y: &mut i32) {} //~ ERROR: in conflict with lock ReadLock
+}
+
+fn main() {
+    let x = &mut 0 as *mut _;
+    unsafe { safe::safe(&*x, &mut *x) };
+}

tests/compile-fail/validation_aliasing_mut3.rs

+#![allow(unused_variables)]
+
+mod safe {
+    pub fn safe(x: &mut i32, y: &i32) {} //~ ERROR: in conflict with lock WriteLock
+}
+
+fn main() {
+    let x = &mut 0 as *mut _;
+    unsafe { safe::safe(&mut *x, &*x) };
+}