Restore legacy provenance behavior

Author: carbotaniuman

src/intptrcast.rs

@@ -122,10 +122,18 @@ impl<'mir, 'tcx> GlobalStateInner {
 
         // Special-case NULL, it is always an invalid pointer.
         if addr == 0 || global_state.provenance_mode == ProvenanceMode::Strict {
-            return Pointer::new(None, Size::from_bytes(addr));
+            Pointer::new(None, Size::from_bytes(addr))
+        } else if global_state.provenance_mode == ProvenanceMode::Legacy {
+            let alloc_id = Self::alloc_id_from_addr(ecx, addr);
+
+            Pointer::new(
+                alloc_id
+                    .map(|alloc_id| Tag::Concrete(ConcreteTag { alloc_id, sb: SbTag::Untagged })),
+                Size::from_bytes(addr),
+            )
+        } else {
+            Pointer::new(Some(Tag::Wildcard), Size::from_bytes(addr))
         }
-
-        Pointer::new(Some(Tag::Wildcard), Size::from_bytes(addr))
     }
 
     fn alloc_base_addr(ecx: &MiriEvalContext<'mir, 'tcx>, alloc_id: AllocId) -> u64 {

tests/compile-fail/ptr_legacy_provenance.rs

@@ -0,0 +1,21 @@
+// compile-flags: -Zmiri-disable-stacked-borrows
+#![feature(strict_provenance)]
+
+use std::ptr;
+
+// Make sure that with legacy provenance, the allocation id of
+// a casted pointer is determined at cast-time
+fn main() {
+    let x: i32 = 0;
+    let y: i32 = 1;
+
+    let x_ptr = &x as *const i32;
+    let y_ptr = &y as *const i32;
+
+    let x_usize = x_ptr.expose_addr();
+    let y_usize = y_ptr.expose_addr();
+
+    let ptr = ptr::from_exposed_addr::<i32>(y_usize);
+    let ptr = ptr.with_addr(x_usize);
+    assert_eq!(unsafe { *ptr }, 0); //~ ERROR pointer to 4 bytes starting at offset -20 is out-of-bounds
+}