Struct ValueMissing 

pub struct ValueMissing { /* private fields */ }

Describes the storage state, requested type and source of a missing read.

The fields are private so callers cannot manufacture an incomplete fact. Use the constructors for storage states and the accessors for the optional facts. source_type and target_type are None only when the caller has no type information (for example, a generic empty iterator); source_index is set only when a collection item caused the failure. A preserved conversion_error is available only when the converter feature is on.

Strict fallback is allowed only for an unset value after type admission. Conversion fallback additionally accepts a policy-classified missing scalar but never a concrete empty collection or a missing collection item.

Examples

use qubit_datatype::DataType;
use qubit_value::{Value, ValueMissingReason};

let error = Value::new_unset(DataType::Int32).get::<i32>().unwrap_err();
let missing = error.missing().unwrap();
assert_eq!(missing.reason(), ValueMissingReason::UnsetScalar);
assert_eq!(missing.source_type(), Some(DataType::Int32));
assert_eq!(missing.target_type(), Some(DataType::Int32));
assert!(missing.is_defaultable_for_strict_read());

Strict reads record both source and target. Conversion failures additionally retain their original error and, for collection items, source index. Inspect Self::reason and the accessors instead of matching storage fields.

Implementations

impl ValueMissing

pub const fn unset_scalar(source: DataType, target: DataType) -> Self

Creates an unset scalar descriptor for a read from source to target.

Parameters

• source - Runtime type of the unset scalar storage.
• target - Type requested by the read operation.

Returns

A descriptor classified as ValueMissingReason::UnsetScalar.

pub const fn unset_collection(source: DataType, target: DataType) -> Self

Creates an unset collection descriptor for the requested element type.

Parameters

• source - Runtime element type of the unset collection storage.
• target - Element type requested by the read operation.

Returns

A descriptor classified as ValueMissingReason::UnsetCollection.

pub const fn empty_collection(source: DataType, target: DataType) -> Self

Creates a descriptor for a first-item read from a concrete empty collection.

Parameters

• source - Runtime element type of the empty collection.
• target - Element type requested by the read operation.

Returns

A descriptor classified as ValueMissingReason::EmptyCollection.

pub const fn reason(&self) -> ValueMissingReason

Returns the storage or policy reason for this missing result.

This value is always present, including when the source or target type is unknown.

Returns

The storage or conversion-policy classification.

pub const fn source_type(&self) -> Option<DataType>

Returns the known source type, or None when conversion did not retain one (for example, a generic empty iterator).

Returns

Some with the stored source type when known; otherwise None.

pub const fn target_type(&self) -> Option<DataType>

Returns the requested target type when known. Strict reads normally provide it; a low-level conversion failure may leave it absent.

Returns

Some with the requested type when known; otherwise None.

pub const fn source_index(&self) -> Option<usize>

Returns the original collection item index, or None for an outer failure or a scalar conversion.

Returns

Some with the zero-based source index for an item failure; otherwise None.

pub const fn conversion_error(&self) -> Option<&DataConversionError>

Returns the original conversion error, absent for a strict storage read. The error is the source for std::error::Error::source.

Returns

Some with the preserved conversion error for conversion failures; otherwise None.

pub const fn is_unset(&self) -> bool

Reports whether scalar or collection storage is unset.

This predicate is the condition used by strict fallback helpers.

Returns

true for unset scalar or collection storage; otherwise false.

pub const fn is_empty_collection(&self) -> bool

Reports whether a first-item read failed because a concrete collection is empty.

Returns

true only for a concrete empty collection; otherwise false.

pub const fn is_conversion(&self) -> bool

Reports whether conversion produced this failure, including enriched unset states.

Returns

true when a conversion error is represented or retained; otherwise false.

pub const fn is_defaultable_for_strict_read(&self) -> bool

Allows strict fallback only for unset storage after the type check passed. Concrete empty collections and type mismatches return false.

Returns

true when a strict read may use its caller-supplied default.

pub const fn is_defaultable_for_conversion(&self) -> bool

Allows conversion fallback for unset storage or a policy-missing scalar.

Empty collections and missing collection items never default.

Returns

true when a conversion read may use its caller-supplied default.

Trait Implementations

impl Clone for ValueMissing

fn clone(&self) -> ValueMissing

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for ValueMissing

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Display for ValueMissing

fn fmt(&self, formatter: &mut Formatter<'_>) -> Result

Formats diagnostic types and indices without exposing source payloads.

impl Eq for ValueMissing

impl Error for ValueMissing

fn source(&self) -> Option<&(dyn Error + 'static)>

Exposes the preserved conversion error, or terminates a strict-read chain.

fn description(&self) -> &str

👎Deprecated since 1.42.0:

use the Display impl or to_string()

fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

fn provide<'a>(&'a self, request: &mut Request<'a>)

🔬This is a nightly-only experimental API. (error_generic_member_access)

Provides type-based access to context intended for error reports.

impl PartialEq for ValueMissing

fn eq(&self, other: &ValueMissing) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for ValueMissing