{"record":{"id":"0dc4ffbc56050b80","repo":"diesel-rs/diesel","slug":"auto-type-tuple-let-assignment-and-its-type-ascri","errorCode":null,"errorMessage":"auto_type: tuple let assignment and its type ascription have different number of elements","messagePattern":"auto_type: tuple let assignment and its type ascription have different number of elements","errorType":"exception","errorClass":"syn::Error","httpStatus":null,"severity":"error","filePath":"dsl_auto_type/src/auto_type/local_variables_map.rs","lineNumber":88,"sourceCode":"                            errors: Vec::new(),\n                        },\n                        (None, None) => LetStatementInferredType {\n                            type_: parse_quote_spanned!(pat_ident.span()=> _),\n                            errors: vec![Rc::new(syn::Error::new_spanned(\n                                pat_ident,\n                                \"auto_type: Let statement with no type ascription \\\n                                    and no initializer expression is not supported\",\n                            ))],\n                        },\n                    },\n                );\n            }\n            syn::Pat::Tuple(pat_tuple) => {\n                if let Some(type_ascription) = type_ascription\n                    && let Type::Tuple(type_tuple) = type_ascription\n                    && pat_tuple.elems.len() != type_tuple.elems.len()\n                {\n                    return Err(syn::Error::new(\n                        type_ascription.span(),\n                        \"auto_type: tuple let assignment and its \\\n                                    type ascription have different number of elements\",\n                    ));\n                }\n                for (i, pat) in pat_tuple.elems.iter().enumerate() {\n                    self.process_pat(\n                        pat,\n                        match type_ascription {\n                            Some(Type::Tuple(type_tuple)) => Some(&type_tuple.elems[i]),\n                            _ => None,\n                        },\n                        match local_init_expression {\n                            Some(syn::Expr::Tuple(expr_tuple)) => Some(&expr_tuple.elems[i]),\n                            _ => None,\n                        },\n                    )?;\n                }","sourceCodeStart":70,"sourceCodeEnd":106,"githubUrl":"https://github.com/diesel-rs/diesel/blob/6fa6ed01b24b24248ab2a611698d0a7c6a2e9120/dsl_auto_type/src/auto_type/local_variables_map.rs#L70-L106","documentation":"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.","triggerScenarios":"`#[auto_type]` function containing e.g. `let (x, y): (i32, i32, i32) = tuple_value;` — pattern elems.len() != ascription tuple elems.len().","commonSituations":"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.","solutions":["Make the tuple pattern and its type ascription have the same number of elements.","Drop the ascription and let the macro infer each element, or ascribe each element individually where inference is needed.","Use `..` rest patterns consistently and ensure the ascription still matches the listed elements."],"exampleFix":"// before\nlet (a, b): (i32,) = (1, 2);\n// after\nlet (a, b): (i32, i32) = (1, 2);","handlingStrategy":"validation","validationCode":"fn tuple_arity_matches<T>(pat_len: usize, ty: &[&str]) -> bool { pat_len == ty.len() }\n// assert!(tuple_arity_matches(2, &\"(i32, i32)\".matches(',').collect::<Vec<_>>()) )","typeGuard":null,"tryCatchPattern":null,"preventionTips":["Keep tuple patterns and ascriptions in sync; count elements on both sides after edits.","Prefer inference (no ascription) when the tuple comes from a typed function."],"tags":["rust","proc-macro","patterns","compile-time"],"backgroundTag":"type-mismatch","analyzedSha":"6fa6ed01b24b24248ab2a611698d0a7c6a2e9120","analyzedAt":"2026-09-07T01:50:13.074Z","contentChangedAt":"2026-09-07T01:50:13.074Z","schemaVersion":2},"datasetVersion":"2026-09-16T09:17:16.951Z"}