allow NOP-casts with mismatching vtables

Author: RalfJung

tests/fail/dyn-upcast-trait-mismatch.rs

@@ -51,8 +51,7 @@ impl Baz for i32 {
 
 fn main() {
     let baz: &dyn Baz = &1;
-    // We already fail on the implicit upcast inserted here.
     let baz_fake: &dyn Bar = unsafe { std::mem::transmute(baz) };
-    //~^ERROR: upcast on a pointer whose vtable does not match its type
     let _err = baz_fake as &dyn Foo;
+    //~^ERROR: upcast on a pointer whose vtable does not match its type
 }

tests/fail/dyn-upcast-trait-mismatch.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: upcast on a pointer whose vtable does not match its type
   --> $DIR/dyn-upcast-trait-mismatch.rs:LL:CC
    |
-LL |     let baz_fake: &dyn Bar = unsafe { std::mem::transmute(baz) };
-   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^ upcast on a pointer whose vtable does not match its type
+LL |     let _err = baz_fake as &dyn Foo;
+   |                ^^^^^^^^ upcast on a pointer whose vtable does not match its type
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

tests/pass/dyn-upcast.rs

@@ -6,6 +6,15 @@ fn main() {
     diamond();
     struct_();
     replace_vptr();
+    vtable_mismatch_nop_cast();
+}
+
+fn vtable_mismatch_nop_cast() {
+    let ptr: &dyn std::fmt::Display = &0;
+    // Even though the vtable is for the wrong trait, this cast doesn't actually change the needed
+    // vtable so it should still be allowed.
+    let ptr: *const (dyn std::fmt::Debug + Send + Sync) = unsafe { std::mem::transmute(ptr) };
+    let _ptr2 = ptr as *const dyn std::fmt::Debug;
 }
 
 fn basic() {