diesel-rs/diesel · error · syn::Error

auto_type: tuple type and its expression have different…

Error message

auto_type: tuple type and its expression have different number of elements

What it means

auto_type compared a tuple expression with its tuple type hint and found different element counts. The offending input is the tuple expression or the type annotation; one side has more or fewer elements than the other.

Solutions

1. Make the tuple expression and type hint have the same arity
2. Remove or correct the tuple type annotation
3. Split the expression into separately typed parts

When it happens

Trigger: Thrown at dsl_auto_type/src/auto_type/expression_type_inference.rs:108 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of diesel-rs/diesel@6fa6ed01b2 (2026-09-07). Data as JSON: /api/errors/547569d0a2d6533b. Report an issue: GitHub.

Appendix: source

Thrown at dsl_auto_type/src/auto_type/expression_type_inference.rs:108

        ) {
            (syn::Expr::Group(syn::ExprGroup { expr, .. }), type_hint) => {
                return self.try_infer_expression_type(expr, type_hint);
            }
            (syn::Expr::Paren(syn::ExprParen { expr, .. }), type_hint) => {
                return self.try_infer_expression_type(expr, type_hint);
            }
            (
                syn::Expr::Tuple(syn::ExprTuple {
                    elems: expr_elems, ..
                }),
                Some(syn::Type::Tuple(
                    type_tuple @ syn::TypeTuple {
                        elems: type_elems, ..
                    },
                )),
            ) => {
                if type_elems.len() != expr_elems.len() {
                    return Err(syn::Error::new(
                        type_tuple.span(),
                        "auto_type: tuple type and its \
                            expression have different number of elements",
                    ));
                }
                syn::Type::Tuple(syn::TypeTuple {
                    elems: type_elems
                        .iter()
                        .zip(expr_elems.iter())
                        .map(|(type_, expr)| self.infer_expression_type(expr, Some(type_)))
                        .collect(),
                    ..type_tuple.clone()
                })
            }
            (syn::Expr::Tuple(syn::ExprTuple { elems, .. }), None) => {
                syn::Type::Tuple(syn::TypeTuple {
                    elems: elems
                        .iter()

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