Support concatenation in query*! macros

[NyxCode]

I would like to use macros to generate calls to query!, e.g:

macro_rules! get_by_id {
    ($s:ident, $t:literal) => {
        impl $s {
            async fn get_by_id(db: &mut PgConnection, id: i32) -> Result<Self> {
                sqlx::query_as!(Self, "SELECT * FROM " + $t + " WHERE id = $1", id)
                    .fetch_one(db)
                    .await
            }
        }
    }
}

struct User {
    id: i32,
    name: String
}
get_by_id!(User, "registered_users");

For this, string concatination in the query_as! macro is necessarry. While the rust stdlib supports macro calls inside println! (playground), @abonander suggested using + for string concatenation, like in the example above.

I believe this would tremendously reduce boilerplate code for many users, while still having all queries checked at compile time.

[mehcode]

I don't dislike + but I think we should also support concat! / stringify! like println!. No real need to be scared of SQL injection as its all compile-time anyway.

println!(concat!("Hello, ", stringify!(10)));

[abonander]

There's a certain logical extreme where if we support concat!() and stringify!() then maybe people come to expect us to support include_str!() as well, which we currently can't reproduce on stable (#11); I think that might end up surprising people.

[mikeplus64]

Rust newbie here; I thought I could have a go at implementing this by patching query{_as,_as_unchecked,_unchecked}! to accept a tt for the query parameter, then patching QueryMacroInput::parse as in ...

            if key == "source" {
                enum SourceFragment { Str(LitStr), Ident(Ident) };
                let mut fragments: Vec<SourceFragment> = Vec::new();
                'source_parser: while !input.is_empty() {
                    fragments.push(match input.lookahead1() {
                        la if la.peek(Ident) => SourceFragment::Ident(input.parse()?),
                        la if la.peek(LitStr) => SourceFragment::LitStr(input.parse()?),
                        _ => break 'source_parser,
                    });
                    if !input.lookahead1().peek(Token![+]) {
                        break 'source_parser;
                    }
                }
                if fragments.len() == 0 {
                    return Err(syn::Error::new_spanned(key, "no source given"));
                }
                query_src = ???;

But, as you can see, I have no idea how to "unquote" the fragments vec into a concrete String, and fear this approach is simply impossible. In fact, I have no idea how println! can do it either =D... Eventually a single String must be available (at compile-time) in order to do compile-time checking against the DB.

Maybe https://github.com/Emoun/eager can be used to make this a bit easier?

Edit: I realised what I thought was necessary was not, really, since just supporting Vec<LitStr> is enough to at least make queries parameterisable by an outer macro_rules as in the issue. Made a PR here #474