Unsize rules issues

[compiler-errors]

https://github.com/ferrocene/specification/blob/496ef38bcfe074d88580021b5b3fadc0239c85b2/src/types-and-traits.rst?plain=1#L2011-L2012

This rule is not required. This code compiles even though i32: Unsize<u32> is not true.

https://github.com/ferrocene/specification/blob/496ef38bcfe074d88580021b5b3fadc0239c85b2/src/types-and-traits.rst?plain=1#L2017-L2020

This rule should be generalized to allow multiple parameters to be changed. See the example committed in rust-lang/rust#117495 (tests/ui/unsized/unsize-coerce-multiple-adt-params.rs).

Also, this X field isn't referenced later in the document.