Re: make check-functional failures with --enable-rust: unsafe precondition(s) violated: NonNull::new_unchecked

[Peter Maydell <peter.maydell@linaro.org>]

On Fri, 7 Mar 2025 at 18:24, Peter Maydell <peter.maydell@linaro.org> wrote:
> thread '<unnamed>' panicked at core/src/panicking.rs:221:5:
> unsafe precondition(s) violated: NonNull::new_unchecked requires that
> the pointer is non-null
> stack backtrace:
>    0: rust_begin_unwind
>              at
> /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/std/src/panicking.rs:662:5
>    1: core::panicking::panic_nounwind_fmt::runtime
>              at
> /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/panicking.rs:112:18
>    2: core::panicking::panic_nounwind_fmt
>              at
> /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/panicking.rs:122:5
>    3: core::panicking::panic_nounwind
>              at
> /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/panicking.rs:221:5
>    4: core::ptr::non_null::NonNull<T>::new_unchecked::precondition_check
>              at
> /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/ub_checks.rs:70:21
>    5: core::ptr::non_null::NonNull<T>::new_unchecked
>              at
> /rustc/f6e511eec7342f59a25f7c0534f1dbea00d01b14/library/core/src/ub_checks.rs:77:17
>    6: qemu_api::qom::Owned<T>::from
>              at ./rust/qemu-api/libqemu_api.rlib.p/structured/qom.rs:741:19
>    7: pl011_create
>              at ./../../rust/hw/char/pl011/src/device.rs:716:24
>    8: vexpress_common_init
>              at ./../../hw/arm/vexpress.c:647:5

The backtrace suggests that the rust version of pl011_create()
isn't able to handle being passed in a NULL chr. This is
permitted and means "don't connect up a chardev to the device's
chr property" which in turn means "you don't have a chardev
backend" (and all the C qemu_chr_fe_* functions happily accept
NULL and interpret it as "just do nothing").

-- PMM