test against passing invalid shared refs around

tests/compile-fail/stacked_borrows/load_invalid_shr.rs

@@ -0,0 +1,9 @@
+// Make sure that we cannot load from memory a `&` that got already invalidated.
+fn main() {
+    let x = &mut 42;
+    let xraw = x as *mut _;
+    let xref = unsafe { &*xraw };
+    let xref_in_mem = Box::new(xref);
+    let _val = *x; // invalidate xraw
+    let _val = *xref_in_mem; //~ ERROR Shr reference with non-reactivatable tag: Location should be frozen
+}

tests/compile-fail/stacked_borrows/pass_invalid_shr.rs

@@ -0,0 +1,10 @@
+// Make sure that we cannot pass by argument a `&` that got already invalidated.
+fn foo(_: &i32) {}
+
+fn main() {
+    let x = &mut 42;
+    let xraw = &*x as *const _;
+    let xref = unsafe { &*xraw };
+    let _val = *x; // invalidate xraw
+    foo(xref); //~ ERROR Shr reference with non-reactivatable tag: Location should be frozen
+}

tests/compile-fail/stacked_borrows/return_invalid_shr.rs

@@ -0,0 +1,11 @@
+// Make sure that we cannot return a `&` that got already invalidated.
+fn foo(x: &mut (i32, i32)) -> &i32 {
+    let xraw = x as *mut (i32, i32);
+    let ret = unsafe { &(*xraw).1 };
+    let _val = *x; // invalidate xraw and its children
+    ret //~ ERROR Shr reference with non-reactivatable tag: Location should be frozen
+}
+
+fn main() {
+    foo(&mut (1, 2));
+}