Derive ToSql and FromSql implementations for generic data types

[alexwl]

At the moment, it is impossible to derive FromSql or ToSql for a data type with generic parameters.

#[derive(FromSql)]
struct Data<'a> {
    id: i32,
    description: &'a str,
}

Error: missing lifetime specifier

#[derive(ToSql)]
struct Data<T: ToSql> {
    id: i32,
    description: String,
    links: Vec<T>,
}

Error: wrong number of type arguments

The generated code for FromSql and ToSql doesn't take into account the fact that a structure may have lifetime or type parameters:

rust-postgres/postgres-derive/src/fromsql.rs

Lines 62 to 64 in cc9b823

	impl<'a> postgres_types::FromSql<'a> for #ident {
	fn from_sql(_type: &postgres_types::Type, buf: &'a [u8])
	-> std::result::Result<#ident,

rust-postgres/postgres-derive/src/tosql.rs

Line 59 in cc9b823

impl postgres_types::ToSql for #ident {

Adding support for lifetimes should be relatively straightforward, not sure about type parameters though.

[LukasKalbertodt]

I am also interested in that. In my case, I'm reading a text[] column with many elements. Since I have to post-process these texts anyway and discard many elements, it would be a shame having to allocate all those intermediate Strings. Of course, this is already possible since FromSql<'a> is implemented for &'a str. Nice! But unfortunately, my array is actually a custom composite type with a bigint field and a text field. And now it gets too complicated. It would be really nice to just be able to write the first example in the top comment. FWIW: I am using that impl manually inside another FromSql impl, and not via Row::get.

The linked code snippets are not quite up to date anymore, but there is no support for this yet.

rust-postgres/postgres-derive/src/fromsql.rs

Lines 118 to 122 in 98f5a11

	let (generics, lifetime) = build_generics(&input.generics);
	let (impl_generics, _, _) = generics.split_for_impl();
	let (_, ty_generics, where_clause) = input.generics.split_for_impl();
	let out = quote! {
	impl #impl_generics postgres_types::FromSql<#lifetime> for #ident #ty_generics #where_clause {

rust-postgres/postgres-derive/src/fromsql.rs

Lines 236 to 246 in 98f5a11

	fn build_generics(source: &Generics) -> (Generics, Lifetime) {
	// don't worry about lifetime name collisions, it doesn't make sense to derive FromSql on a struct with a lifetime
	let lifetime = Lifetime::new("'a", Span::call_site());
	let mut out = append_generic_bound(source.to_owned(), &new_fromsql_bound(&lifetime));
	out.params.insert(
	0,
	GenericParam::Lifetime(LifetimeParam::new(lifetime.to_owned())),
	);
	(out, lifetime)

I wonder what this comment means: why would it not make sense? Is there anything I'm missing?