add some tests for retagging inside tuples and options

tests/compile-fail-fullmir/stacked_borrows/return_invalid_mut_option.rs

@@ -0,0 +1,11 @@
+// Make sure that we cannot return a `&mut` that got already invalidated, not even in an `Option`.
+fn foo(x: &mut (i32, i32)) -> Option<&mut i32> {
+    let xraw = x as *mut (i32, i32);
+    let ret = Some(unsafe { &mut (*xraw).1 });
+    let _val = unsafe { *xraw }; // invalidate xref
+    ret //~ ERROR does not exist on the stack
+}
+
+fn main() {
+    foo(&mut (1, 2));
+}

tests/compile-fail-fullmir/stacked_borrows/return_invalid_mut_tuple.rs

@@ -0,0 +1,11 @@
+// Make sure that we cannot return a `&mut` that got already invalidated, not even in a tuple.
+fn foo(x: &mut (i32, i32)) -> (&mut i32,) {
+    let xraw = x as *mut (i32, i32);
+    let ret = (unsafe { &mut (*xraw).1 },);
+    let _val = unsafe { *xraw }; // invalidate xref
+    ret //~ ERROR does not exist on the stack
+}
+
+fn main() {
+    foo(&mut (1, 2));
+}

tests/compile-fail-fullmir/stacked_borrows/return_invalid_shr_option.rs

@@ -0,0 +1,11 @@
+// Make sure that we cannot return a `&` that got already invalidated, not even in an `Option`.
+fn foo(x: &mut (i32, i32)) -> Option<&i32> {
+    let xraw = x as *mut (i32, i32);
+    let ret = Some(unsafe { &(*xraw).1 });
+    unsafe { *xraw = (42, 23) }; // unfreeze
+    ret //~ ERROR is not frozen
+}
+
+fn main() {
+    foo(&mut (1, 2));
+}

tests/compile-fail-fullmir/stacked_borrows/return_invalid_shr_tuple.rs

@@ -0,0 +1,11 @@
+// Make sure that we cannot return a `&` that got already invalidated, not even in a tuple.
+fn foo(x: &mut (i32, i32)) -> (&i32,) {
+    let xraw = x as *mut (i32, i32);
+    let ret = (unsafe { &(*xraw).1 },);
+    unsafe { *xraw = (42, 23) }; // unfreeze
+    ret //~ ERROR is not frozen
+}
+
+fn main() {
+    foo(&mut (1, 2));
+}