No need to build the match body if there are no generics. This should hopefully
save some work too.
Depends on D26289
Differential Revision: https://phabricator.services.mozilla.com/D26290
Otherwise, deriving ToComputedValue and ToAnimatedValue in structs or enums with
other where clauses just doesn't work.
Differential Revision: https://phabricator.services.mozilla.com/D21859
We assume that types such as `<Self as ToComputedValue>::ToComputedValue == Self`
just construct a new value that is just a clone of the original one without any
additional code.
For the traits we derive which methods don't depend on associated types (i.e.
all of them but ToAnimatedValue and ToComputedValue), we now add trait bounds
for the actual field types directly, instead of bounding the type parameters.
For now, only impls for types like in style::values::generics can be derived.
This also needed a few ToComputedValueAsSpecified impls that I would like to
replace by some #[to_computed_value(clone)] attribute, but I think it is ok
to keep it like this for now.
