diesel-rs/diesel · error · syn::Error

auto_type: tuple let assignment and its type ascription…

Error message

auto_type: tuple let assignment and its type ascription have different number of elements

What it means

When a tuple destructuring let has an explicit tuple type ascription, dsl_auto_type requires the element counts to match so it can map each pattern element to a type. If `let (a, b): (i32,) = ...` (or vice versa) is seen, expansion fails because element-wise inference is impossible.

Solutions

1. Make the tuple pattern and its type ascription have the same number of elements.
2. Drop the ascription and let the macro infer each element, or ascribe each element individually where inference is needed.
3. Use `..` rest patterns consistently and ensure the ascription still matches the listed elements.

Example fix

// before
let (a, b): (i32,) = (1, 2);
// after
let (a, b): (i32, i32) = (1, 2);

fn tuple_arity_matches<T>(pat_len: usize, ty: &[&str]) -> bool { pat_len == ty.len() }
// assert!(tuple_arity_matches(2, &"(i32, i32)".matches(',').collect::<Vec<_>>()) )

When it happens

Trigger: `#[auto_type]` function containing e.g. `let (x, y): (i32, i32, i32) = tuple_value;` — pattern elems.len() != ascription tuple elems.len().

Common situations: Refactoring a two-element tuple into three fields without updating the annotation, or using rest patterns / nested tuples where the ascription arity drifts from the pattern.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• auto_type: unexpected double type ascription
• references are not supported in `Queryable` types consider…
• invalid variadic argument count: not enough function…
• unsupported expression for auto_type, please provide a type…
• type_alias and no_type_alias are mutually exclusive

AI-assisted analysis of diesel-rs/diesel@6fa6ed01b2 (2026-09-07). Data as JSON: /api/errors/0dc4ffbc56050b80. Report an issue: GitHub.

Appendix: source

Thrown at dsl_auto_type/src/auto_type/local_variables_map.rs:88

                            errors: Vec::new(),
                        },
                        (None, None) => LetStatementInferredType {
                            type_: parse_quote_spanned!(pat_ident.span()=> _),
                            errors: vec![Rc::new(syn::Error::new_spanned(
                                pat_ident,
                                "auto_type: Let statement with no type ascription \
                                    and no initializer expression is not supported",
                            ))],
                        },
                    },
                );
            }
            syn::Pat::Tuple(pat_tuple) => {
                if let Some(type_ascription) = type_ascription
                    && let Type::Tuple(type_tuple) = type_ascription
                    && pat_tuple.elems.len() != type_tuple.elems.len()
                {
                    return Err(syn::Error::new(
                        type_ascription.span(),
                        "auto_type: tuple let assignment and its \
                                    type ascription have different number of elements",
                    ));
                }
                for (i, pat) in pat_tuple.elems.iter().enumerate() {
                    self.process_pat(
                        pat,
                        match type_ascription {
                            Some(Type::Tuple(type_tuple)) => Some(&type_tuple.elems[i]),
                            _ => None,
                        },
                        match local_init_expression {
                            Some(syn::Expr::Tuple(expr_tuple)) => Some(&expr_tuple.elems[i]),
                            _ => None,
                        },
                    )?;
                }

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