How to get query! to pick up sqlx::Type/sqlx::Encode information

[eureka-cpu]

I have a type which implements Type and Encode for &[u8] with Postgres. When I go to try and compile, query! fails because I gave it my type, and not &[u8], but I had thought these trait impls where how you would get around that.

        impl sqlx::Type<sqlx::Postgres> for Md5Hash {
            fn type_info() -> sqlx::postgres::PgTypeInfo {
                <&[u8] as sqlx::Type<sqlx::Postgres>>::type_info()
            }
        }

        impl<'q> sqlx::Encode<'q, sqlx::Postgres> for Md5Hash {
            fn encode_by_ref(
                &self,
                buf: &mut <sqlx::Postgres as sqlx::Database>::ArgumentBuffer<'q>,
            ) -> Result<sqlx::encode::IsNull, sqlx::error::BoxDynError> {
                <&[u8] as sqlx::Encode<sqlx::Postgres>>::encode_by_ref(&self.0.as_ref(), buf)
            }
        }

CREATE TABLE project_files (
    project_id      UUID NOT NULL REFERENCES projects (project_id),
    relative_path   TEXT NOT NULL,
    content_hash    BYTEA NOT NULL, -- <- this field
    lamport_clock   BIGINT NOT NULL,
    created_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
    deleted_at      TIMESTAMPTZ,
    PRIMARY KEY (project_id, relative_path)
);

        for key in payload.updated.iter() {
            sqlx::query!(
                "UPDATE project_files
                 SET content_hash = $3,
                     lamport_clock = $4,
                     deleted_at = NULL
                 WHERE project_id = $1 AND relative_path = $2",
                project_id.as_ref(),
                &key.relative_blob_name(),
                key.as_ref(), // <- used here
                lamport_clock,
            )
            .execute(&mut *tx)
            .await?;
        }

error[E0308]: mismatched types
  --> src/stitches/src/database/project.rs:60:17
   |
60 |                 key.as_ref(),
   |                 ^^^^^^^^^^^^
   |                 |
   |                 expected `&[u8]`, found `&Md5Hash`
   |                 expected due to the type of this binding
   |
   = note: expected reference `&[u8]`
              found reference `&Md5Hash`

[ryux1]

Your Type/Encode implementations are used when the value is actually bound, but query! separately asks Postgres for the parameter type. Postgres reports $3 as BYTEA, so the macro generates a compile-time check against SQLx's canonical Rust mapping (&[u8]). It cannot infer that your Md5Hash is another valid representation of BYTEA.

For Postgres bind parameters, use the macro's supported type-override syntax:

sqlx::query!(
    "UPDATE project_files
     SET content_hash = $3,
         lamport_clock = $4,
         deleted_at = NULL
     WHERE project_id = $1 AND relative_path = $2",
    project_id.as_ref(),
    &key.relative_blob_name(),
    key.as_ref() as _,
    lamport_clock,
)
.execute(&mut *tx)
.await?;

expr as _ tells query! to skip only its generated Rust-type comparison for that bind expression. The macro removes the wildcard cast before generating the actual bind, so your existing Encode<Postgres> and Type<Postgres> implementations still handle the value. SQL syntax, parameter count, and result columns remain checked.

An explicit override such as key.as_ref() as &Md5Hash can also work, but as _ is the documented form when the expression already has the desired custom type.

I verified this in SQLx 0.8.6's macro implementation and its PostgreSQL test_bind_arg_override_wildcard test, which exercises the same custom-wrapper pattern. Disclosure: I used OpenAI Codex to inspect the relevant macro expansion and test coverage.

[eureka-cpu]

Thanks @ryux1 I'll try this!