I am the author of this crate. As such, I obviously trust it and believe it is useful.
So this review will instead point at the trickiest parts and try to rationalize them.

First and foremost, a caution: YOLO_RC_HEAP_LAYOUT_KNOWN is unsound, don't use it.
It isn't public API nor documented API, but it does create "working" code that only
happens to work for the current compiler and relies on too many implementation details.

This crate provides a completely safe API through SliceWithHeader.
Prefer using it to the unsafe APIs that power it.

It should be noted that this crate's unsafety is purely on slice dst construction.
Once the slice dst has been manually allocated into a Box, everything becomes safe again.
Rust actually supports these trailing-slice DSTs perfectly well,
it is only construction that is unsafe.

Triomphe also provides a HeaderSlice<HeaderWithLength, [Item]> equivalent to slice-dst's
SliceWithHeader<Header, Item>. Slice-dst's is usable with std smart pointers, wheras triomphe's
is only usable with triomphe::Arc (and variants). Triomphe, however, is more battle tested.

Smoke tests are run under miri, but the crate could potentially do with some more examples
as well as some more tests to ensure that all of the functionality works as advertised.

I've only put this review as positive rather than strong because of a lack of real-world use.
As of yet, the library is just of theoretical use, and has yet to be stress tested for real.