The documentation for std::convert::Into contains some types from the syntax crate that should not be there:
impl Into<Option<P<PathParameters>>> for AngleBracketedParameterData
impl Into<Vec<Annotatable>> for Annotatable
impl<T> Into<Vec<T>> for ThinVec<T>
impl<T> Into<Vec<T>> for P<[T]>
all of those are dead links (eg. for syntax::ptr::P).