Compiler should catch when your #[lang = ""] items are not equal to what it expects

[strega-nil]

By making the lang-item index not equal what it is in core:

#[lang = "index"]
trait Index {
    // ...
    fn index<'a>(&'a self) -> &'a Self::Output;
}

This causes the assertion:

assertion failed: `left == right` (left: `1`, right: `2`)

If instead you change it to a *const instead of an &'a, you get

src/lib.rs:11:5: 11:9 error: internal compiler error: cat_expr Errd
src/lib.rs:11     a[0]

However, if you change from &'a to &'a mut, nothing happens.

[nagisa]

dupe of #9307