Add regression test

tests/ui/traits/upcast_soundness_bug.rs

@@ -0,0 +1,69 @@
+#![feature(trait_upcasting)]
+// known-bug: #120222
+// check-pass
+//! This will segfault at runtime.
+
+pub trait SupSupA {
+    fn method(&self) {}
+}
+pub trait SupSupB {}
+impl<T> SupSupA for T {}
+impl<T> SupSupB for T {}
+
+pub trait Super<T>: SupSupA + SupSupB {}
+
+pub trait Unimplemented {}
+
+pub trait Trait<T1, T2>: Super<T1> + Super<T2> {
+    fn missing_method(&self)
+    where
+        T1: Unimplemented,
+    {
+    }
+}
+
+impl<S, T> Super<T> for S {}
+
+impl<S, T1, T2> Trait<T1, T2> for S {}
+
+#[inline(never)]
+pub fn user1() -> &'static dyn Trait<u8, u8> {
+    &()
+    /* VTABLE:
+    .L__unnamed_2:
+            .quad   core::ptr::drop_in_place<()>
+            .asciz  "\000\000\000\000\000\000\000\000\001\000\000\000\000\000\000"
+            .quad   example::SupSupA::method
+            .quad   .L__unnamed_4 // SupSupB vtable (pointer)
+            .zero   8             // null pointer for missing_method
+    */
+}
+
+#[inline(never)]
+pub fn user2() -> &'static dyn Trait<u8, u16> {
+    &()
+    /* VTABLE:
+    .L__unnamed_3:
+            .quad   core::ptr::drop_in_place<()>
+            .asciz  "\000\000\000\000\000\000\000\000\001\000\000\000\000\000\000"
+            .quad   example::SupSupA::method
+            .quad   .L__unnamed_4 // SupSupB vtable (pointer)
+            .quad   .L__unnamed_5 // Super<u16> vtable (pointer)
+            .zero   8             // null pointer for missing_method
+    */
+}
+
+fn main() {
+    let p: *const dyn Trait<u8, u8> = &();
+    let p = p as *const dyn Trait<u8, u16>; // <- this is bad!
+    let p = p as *const dyn Super<u16>; // <- this upcast accesses improper vtable entry
+    // accessing from L__unnamed_2 the position for the 'Super<u16> vtable (pointer)',
+    // thus reading 'null pointer for missing_method'
+
+    let p = p as *const dyn SupSupB; // <- this upcast dereferences (null) pointer from that entry
+    // to read the SupSupB vtable (pointer)
+
+    // SEGFAULT
+
+    println!("{:?}", p);
+}