finally we can actually have adjacent allocations :)

src/intptrcast.rs

@@ -191,11 +191,9 @@ impl<'mir, 'tcx> GlobalStateInner {
                     slack,
                 );
 
-                // Remember next base address.  Leave a gap of at least 1 to avoid two zero-sized allocations
-                // having the same base address, and to avoid ambiguous provenance for the address between two
-                // allocations (also see https://github.com/rust-lang/unsafe-code-guidelines/issues/313).
-                let size_plus_1 = size.bytes().checked_add(1).unwrap();
-                global_state.next_base_addr = base_addr.checked_add(size_plus_1).unwrap();
+                // Remember next base address. We *do* allow allocations to touch each other,
+                // and ZST allocations to have the same address.
+                global_state.next_base_addr = base_addr.checked_add(size.bytes()).unwrap();
                 // Given that `next_base_addr` increases in each allocation, pushing the
                 // corresponding tuple keeps `int_to_ptr_map` sorted
                 global_state.int_to_ptr_map.push((base_addr, alloc_id));

tests/pass/adjacent-allocs.rs

@@ -1,5 +1,35 @@
 // compile-flags: -Zmiri-permissive-provenance
 
+fn ensure_allocs_can_be_adjacent() {
+    for _ in 0..512 {
+        let n = 0u64;
+        let ptr: *const u64 = &n;
+        let ptr2 = {
+            let m = 0u64;
+            &m as *const u64
+        };
+        if ptr.wrapping_add(1) == ptr2 {
+            return;
+        }
+    }
+    panic!("never saw adjacent stack variables?");
+}
+
+fn ensure_zst_allocs_can_be_adjacent() {
+    for _ in 0..512 {
+        let n = ();
+        let ptr: *const () = &n;
+        let ptr2 = {
+            let m = ();
+            &m as *const ()
+        };
+        if ptr == ptr2 {
+            return;
+        }
+    }
+    panic!("never saw adjacent zero-sized stack variables?");
+}
+
 fn test1() {
     // The slack between allocations is random.
     // Loop a few times to hit the zero-slack case.
@@ -42,6 +72,8 @@ fn test2() {
 }
 
 fn main() {
+    ensure_allocs_can_be_adjacent();
+    ensure_zst_allocs_can_be_adjacent();
     test1();
     test2();
 }