A struct with only one field has the same layout as that field.
However this doesn't mean a reference to a struct with only one field has the same layout as a reference to that field, especially not DST. Length and pointer could be in theory swapped (even if there doesn't seem to be a reason to do so).
Also just because std does it doesn't mean outside code can do it: std is tied to compiler version so if compiler changes to exploit UB std can be changed with it to update the code.
I was unable to find any authoritative answer to this. Does anyone know?
However this doesn't mean a reference to a struct with only one field has the same layout as that field,
Note that the code you quoted is not transmuting a reference (or pointer); it is performing an as cast on the pointer itself. as is not a reinterpretation of the bytes of a value (except, arguably, for integer signedness casts and truncation), but does various things depending on the type of the value, so if the layout of the pointers were different, as would reasonably be expected to accommodate that.
So with this clarification, yeah, I do agree that we currently don't allow transmuting between fat references. as isn't a transmute, though, and is generally understood to be guaranteed (and will be if my reference PR lands).
That PR looks great! However I can't find any documentation that says the metadata of DST struct is the same as the metadata of the DST field inside that struct. If they weren't the code would still be UB.