interpret: dyn trait metadata check: equate traits in a proper way

Author: RalfJung

tests/pass/issues/issue-miri-3541-dyn-vtable-trait-normalization.rs

@@ -0,0 +1,40 @@
+#![feature(ptr_metadata)]
+// This test is the result of minimizing the `emplacable` crate to reproduce
+// <https://github.com/rust-lang/miri/issues/3541>.
+
+use std::{ops::FnMut, ptr::Pointee};
+
+pub type EmplacerFn<'a, T> = dyn for<'b> FnMut(<T as Pointee>::Metadata) + 'a;
+
+#[repr(transparent)]
+pub struct Emplacer<'a, T>(EmplacerFn<'a, T>)
+where
+    T: ?Sized;
+
+impl<'a, T> Emplacer<'a, T>
+where
+    T: ?Sized,
+{
+    pub unsafe fn from_fn<'b>(emplacer_fn: &'b mut EmplacerFn<'a, T>) -> &'b mut Self {
+        // This used to trigger:
+        // constructing invalid value: wrong trait in wide pointer vtable: expected
+        // `std::ops::FnMut(<[std::boxed::Box<i32>] as std::ptr::Pointee>::Metadata)`, but encountered
+        // `std::ops::FnMut<(usize,)>`.
+        unsafe { &mut *((emplacer_fn as *mut EmplacerFn<'a, T>) as *mut Self) }
+    }
+}
+
+pub fn box_new_with<T>()
+where
+    T: ?Sized,
+{
+    let emplacer_closure = &mut |_meta| {
+        unreachable!();
+    };
+
+    unsafe { Emplacer::<T>::from_fn(emplacer_closure) };
+}
+
+fn main() {
+    box_new_with::<[Box<i32>]>();
+}