Rollup merge of #127856 - RalfJung:interpret-cast-sanity, r=oli-obk

interpret: add sanity check in dyn upcast to double-check what codegen does

For dyn receiver calls, we already have two codepaths: look up the function to call by indexing into the vtable, or alternatively resolve the DefId given the dynamic type of the receiver. With debug assertions enabled, the interpreter does both and compares the results. (Without debug assertions we always use the vtable as it is simpler.)

This PR does the same for dyn trait upcasts. However, for casts *not* using the vtable is the easier thing to do, so now the vtable path is the debug-assertion-only path. In particular, there are cases where the vtable does not contain a pointer for upcasts but instead reuses the old pointer: when the supertrait vtable is a prefix of the larger vtable. We don't want to expose this optimization and detect UB if people do a transmute assuming this optimization, so we cannot in general use the vtable indexing path.

r? ``@oli-obk``

Author: matthiaskrgr

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

@@ -59,8 +59,10 @@ impl Baz for i32 {
 }
 
 fn main() {
-    let baz: &dyn Baz = &1;
-    let baz_fake: *const dyn Bar = unsafe { std::mem::transmute(baz) };
-    let _err = baz_fake as *const dyn Foo;
-    //~^ERROR: using vtable for trait `Baz` but trait `Bar` was expected
+    unsafe {
+        let baz: &dyn Baz = &1;
+        let baz_fake: *const dyn Bar = std::mem::transmute(baz);
+        let _err = baz_fake as *const dyn Foo;
+        //~^ERROR: using vtable for trait `Baz` but trait `Bar` was expected
+    }
 }

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

@@ -1,8 +1,8 @@
 error: Undefined Behavior: using vtable for trait `Baz` but trait `Bar` was expected
   --> $DIR/dyn-upcast-trait-mismatch.rs:LL:CC
    |
-LL |     let _err = baz_fake as *const dyn Foo;
-   |                ^^^^^^^^ using vtable for trait `Baz` but trait `Bar` was expected
+LL |         let _err = baz_fake as *const dyn Foo;
+   |                    ^^^^^^^^ using vtable for trait `Baz` but trait `Bar` was expected
    |
    = 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