Rollup merge of #70806 - RalfJung:miri-assignment-check, r=eddyb

fix Miri assignment sanity check

Thanks @eddyb for pointing me to the right APIs!

r? @eddyb
Fixes https://github.com/rust-lang/rust/issues/70804

src/test/ui/consts/const-eval/issue-70804-fn-subtyping.rs

@@ -0,0 +1,10 @@
+// check-pass
+#![feature(const_fn)]
+
+const fn nested(x: (for<'a> fn(&'a ()), String)) -> (fn(&'static ()), String) {
+    x
+}
+
+pub const TEST: (fn(&'static ()), String) = nested((|_x| (), String::new()));
+
+fn main() {}