Support for "CREATE DOMAIN" types?

[ELLIOTTCABLE]

I'm experimenting with a learning project, and I'm a bit confused.

It looks like SQLx supports PostgreSQL's CREATE DOMAIN types; but I'm having trouble getting them to actually function in a sqlx::query! or sqlx::query_as! macro.

As a toy example, let's use this schema:

CREATE DOMAIN "hex_16" AS varchar(16)
CHECK ("value" ~ '^[0-9A-F]{16}$');

CREATE TABLE IF NOT EXISTS "twag_thingies" (
   "id" hex_16 UNIQUE NOT NULL PRIMARY KEY,
   "target_url" text NOT NULL,
   "access_count" integer DEFAULT 0
);

Then a Rust-side domain model,

#[derive(sqlx::Type, Debug, Clone, PartialEq, Eq, Deserialize, Serialize)]
#[sqlx(type_name = "hex_16", transparent)]
pub struct Hex16(String);

impl Hex16 {
   pub fn new(s: impl Into<String>) -> Result<Self, &'static str> { Ok(Hex16(s.into().to_uppercase())) }
}

#[derive(sqlx::FromRow)]
pub struct Thingie {
   pub id: Hex16,
   pub target_url: String,
   pub access_count: i32,
}

If I try and use query! or query_as! with this, though, I keep running into "unsupported type hex_16 for param #1":

error: unsupported type hex_16 for param #1
   --> src/main.rs:164:4
    |
164 | /    sqlx::query!(
165 | |       "INSERT INTO twag_thingies (id, target_url) VALUES ($1, $2)",
166 | |       &id,
167 | |       target_url
168 | |    );
    | |____^
    |
    = note: this error originates in the macro `$crate::sqlx_macros::expand_query` which comes from the expansion of the macro `sqlx::query` (in Nightly builds, run with -Z macro-backtrace for more info)

I've tried a few variations on this, without much luck.

Can someone help me understand how to map into/from Postgres-side newtypes and Rust-side newtypes, retaining type-safety as much as possible?

(I'm also new to Rust, which potentially isn't helping; but I'm trying to make a learning project out of this. I'm primarily familiar with OCaml, TypeScript, Swift, et al, if that's relevant.)

Edit:

I see #3856 which actually clarifies the exact error I'm getting (thanks, @dyc3!); so I think I've found the correct part of the documentation, at least. "Type Overrides: Bind Parameters (Postgres only)" says:

For typechecking of bind parameters, casts using as are treated as overrides for the inferred types of bind parameters and no typechecking is emitted …

Using expr as _ simply signals to the macro to not type-check that bind expression, and then that syntax is stripped from the expression so as to not trigger type errors.

However, when I try to use as instead of ::, I get a syntax-error back from PostgreSQL:

error: error returned from database: syntax error at or near "as"
   --> src/main.rs:163:4
    |
163 | /    sqlx::query!(
164 | |       r#"INSERT INTO twag_tags (id, target_url, access_count) VALUES ($1 as hex_14, ...
165 | |       &id,
166 | |       target_url,
167 | |       tap_count
168 | |    )
    | |____^
    |
    = note: this error originates in the macro `$crate::sqlx_macros::expand_query` which comes from the expansion of the macro `sqlx::query` (in Nightly builds, run with -Z macro-backtrace for more info)

[abonander]

See the example in the section you linked. The as goes in the bind expression, not the SQL itself.

[ELLIOTTCABLE]

Ah, it seems like I needed it in both - every attempt I'd made had either the Postgres-side cast or the Rust-side cast; this finally worked:

sqlx::query!(
   "INSERT INTO twag_thingies (id, target_url) VALUES ($1::hex_16, $2)",
   id as &Hex16,
   target_url
);