#[test]
fn test_downcast_mut_context_message() {
    let (mut err, dropped) = make_chain();

    // Memory-safety issue: `context_chain_downcast` finds context messages by
    // taking a shared reference to the field, while `Report::downcast_mut`
    // turns the returned erased pointer into `&mut D`. Miri with Tree Borrows
    // rejects the write through this safe mutable downcast because the pointer
    // is derived from a frozen shared tag.
    let high = err.downcast_mut::<HighLevel>().unwrap();
    high.message = "mutated high-level context";
    assert_eq!(high.to_string(), "mutated high-level context");

    assert!(dropped.none());
    drop(err);
    assert!(dropped.all());
}

#[test]
fn test_downcast_mut_inner_context_message() {
    let (mut err, dropped) = make_chain();

    // Memory-safety issue: this asks for a mutable reference to a context
    // message below the outermost wrapper. The lookup recurses through
    // `context_chain_downcast` into `context_downcast`, which returns a pointer
    // derived from a shared field reference before `Report::downcast_mut`
    // creates `&mut MidLevel`. Miri flags the subsequent safe write as UB.
    let mid = err.downcast_mut::<MidLevel>().unwrap();
    mid.message = "mutated mid-level context";
    assert_eq!(mid.to_string(), "mutated mid-level context");

    assert!(dropped.none());
    drop(err);
    assert!(dropped.all());
}

#[test]
fn test_downcast_mut_inner_error() {
    let (mut err, dropped) = make_chain();

    // Memory-safety issue: mutable downcast to the innermost diagnostic also
    // reuses the recursive context-chain lookup. `context_downcast` returns a
    // pointer derived from a shared field reference, then `Report::downcast_mut`
    // creates `&mut LowLevel`; Miri flags the safe write through it as UB.
    let low = err.downcast_mut::<LowLevel>().unwrap();
    low.message = "mutated low-level error";
    assert_eq!(low.to_string(), "mutated low-level error");

    assert!(dropped.none());
    drop(err);
    assert!(dropped.all());
}