ensure we allow zero-sized references to functions and vtables

Author: RalfJung

Diff for: src/tools/miri/src/borrow_tracker/tree_borrows/mod.rs

@@ -248,6 +248,13 @@ trait EvalContextPrivExt<'mir: 'ecx, 'tcx: 'mir, 'ecx>: crate::MiriInterpCxExt<'
                 .insert(new_tag, protect);
         }
 
+        let alloc_kind = this.get_alloc_info(alloc_id).2;
+        if !matches!(alloc_kind, AllocKind::LiveData) {
+            // There's not actually any bytes here where accesses could even be tracked.
+            // Just produce the new provenance, nothing else to do.
+            return Ok(Some(Provenance::Concrete { alloc_id, tag: new_tag }));
+        }
+
         let span = this.machine.current_span();
         let alloc_extra = this.get_alloc_extra(alloc_id)?;
         let range = alloc_range(base_offset, ptr_size);

Diff for: src/tools/miri/tests/pass/strange_references.rs

@@ -0,0 +1,25 @@
+//@revisions: stack tree
+//@[tree]compile-flags: -Zmiri-tree-borrows
+
+// Create zero-sized references to vtables and function data.
+// Just make sure nothing explodes.
+
+use std::{mem, ptr};
+
+fn check_ref(x: &()) {
+    let _ptr = ptr::addr_of!(*x);
+}
+
+fn main() {
+    check_ref({
+        // Create reference to a function.
+        let fnptr: fn(&()) = check_ref;
+        unsafe { mem::transmute(fnptr) }
+    });
+    check_ref({
+        // Create reference to a vtable.
+        let wideptr: &dyn Send = &0;
+        let fields: (&i32, &()) = unsafe { mem::transmute(wideptr) };
+        fields.1
+    })
+}