What causes "no `#[diesel(sql_type = ...)]` attribute provided" in diesel-rs/diesel?

Add `#[diesel(sql_type = Int4)]` (or the appropriate type) to the enum.

no `#[diesel(sql_type = ...)]` attribute provided

Error message

no `#[diesel(sql_type = ...)]` attribute provided

What it means

The `AsExpression`/`FromSqlRow` derive for enums (or newtype usage) requires at least one `#[diesel(sql_type = ...)]` attribute so it knows which SQL types to implement for. If none are found on the item, the derive fails at enum_.rs:245 with this error.

Solutions

Add `#[diesel(sql_type = Int4)]` (or the appropriate type) to the enum.
If you need several SQL representations, add multiple `#[diesel(sql_type = ...)]` attributes.
Update old 1.x syntax `#[sql_type = "Int4"]` to the Diesel 2.x `#[diesel(sql_type = ...)]` form.

Example fix

// before
#[derive(AsExpression, FromSqlRow)]
pub enum Status { Active, Retired }

// after
#[derive(AsExpression, FromSqlRow)]
#[diesel(sql_type = Int4)]
pub enum Status { Active, Retired }

Defensive patterns

Strategy: validation

Validate before calling

// Verify the enum carries at least one diesel sql_type attribute:
// #[derive(AsExpression, FromSqlRow)]
// #[diesel(sql_type = Int4)]
// pub enum MyEnum { ... }

Prevention

Always pair AsExpression/FromSqlRow enum derives with `#[diesel(sql_type = ...)]`.
Migrate Diesel 1.x `#[sql_type = "..."]` syntax to 2.x `#[diesel(sql_type = ...)]`.
Check for typos: the attribute must be `sql_type` inside the `diesel` namespace.

When it happens

Trigger: Deriving diesel traits on an enum without any `#[diesel(sql_type = X)]` attribute at the enum level, or misspelling/renesting the attribute (e.g. `#[diesel(sql_type=X)]` outside the `#[diesel(...)]` group or writing `#[sql_type(...)]`, the old pre-2.0 form).

Common situations: Migrating from Diesel 1.x where `#[sql_type = "..."]` was top-level; forgetting the attribute after copying an enum example without it; typos like `sqltype`.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of diesel-rs/diesel@6fa6ed01b2 (2026-09-07). Data as JSON: /api/errors/7dce3c879cf51d1b. Report an issue: GitHub.

Appendix: source

Thrown at diesel_derives/src/enum_.rs:245

        let sql_type_attrs = attrs
            .into_iter()
            .filter_map(
                |a: AttributeSpanWrapper<diesel_attribute_parser::StructAttr>| {
                    if let diesel_attribute_parser::StructAttr::SqlType(_, path) = a.item {
                        Some(diesel_attribute_parser::AttributeSpanWrapper {
                            item: path,
                            attribute_span: a.attribute_span,
                            ident_span: a.ident_span,
                        })
                    } else {
                        None
                    }
                },
            )
            .collect::<Vec<_>>();
        if sql_type_attrs.is_empty() {
            return Err(syn::Error::new(
                input_span,
                "no `#[diesel(sql_type = ...)]` attribute provided",
            ));
        }

        Ok(Self {
            sql_type_attrs,
            ident: input.ident,
            has_explicit_discriminants,
            variants,
        })
    }
}

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