Enum MapLengthPolicy 

pub enum MapLengthPolicy {
    SequenceEquivalent,
    Rule15Omits,
}

Whether a delimited map carries an entry count.

This policy exists because XTypes 1.3 contradicts itself, not because two implementations were measured disagreeing. Nothing in crates/ast-layout/tests/fixtures/vendor_bytes.md has a map in it, so neither shape below is verified against a vendor.

§7.4.1.1.4 defines a map by an equivalence: it “shall be serialized as if” it were sequence<MapEntry_<key_type>_<value_type>>. A sequence under XCDR2 is rule (12) of §7.4.3.5.3, DHEADER then O.length then the elements. So the equivalence yields both words.

Rule (15) of the same clause, which is the rule written for MAP_TYPE itself under XCDR2, writes DHEADER and then the pairs — with no O.length. Rules (14) and (16), the primitive-map and the XCDR1 cases, both write O.length and no delimiter, so the omission is only in (15).

The two readings differ by four bytes. Both are decodable — with a delimiter a reader can consume entries until the byte extent is used — which is why neither can be ruled out from the stream alone.

Both are in the field

Measured on 2026-08-09. Neither C generator parses map, so the implementations that can be asked are eProsima Fast DDS and OpenDDS 3.34.0, and they took one reading each. @final struct { map<long, Cell> entries; long tail; } with one entry 5 -> {1, 2}, XCDR2 little endian: Fast DDS writes 10000000 01000000 05000000 01000000 02000000 07000000 and OpenDDS writes 0c000000 05000000 01000000 02000000 07000000. Under XCDR1, where neither delimits, the two are identical. crates/ast-layout/tests/fixtures/vendor_bytes.md, “Where the three cannot answer”, has the run and a second shape.

Variants

SequenceEquivalent

Write the entry count whether or not a delimiter precedes it.

The default, because it is the reading under which §7.4.1.1.4’s equivalence holds, because it is the only one of the two that makes a map’s bytes the same shape as the sequence the clause says it is, and because it is what Fast DDS writes.

Rule15Omits

Omit the entry count when the map is delimited, as rule (15) is written.

OpenDDS’s reading: its generated operator<< calls write_delimiter under XCDR2 and the << length in the else branch under XCDR1, so the count and the delimiter are alternatives rather than both.

Implementations

impl MapLengthPolicy

pub const ALL: [Self; 2]

Both readings.

pub const fn is_default(self) -> bool

Whether this is the default.

pub const fn as_str(self) -> &'static str

Short name, for configuration and diagnostics.

pub fn parse(value: &str) -> Option<Self>

Parse a configuration value.

An unknown value returns None rather than the default: a typo in a wire-affecting setting must be loud.

Trait Implementations

impl Clone for MapLengthPolicy

fn clone(&self) -> Self

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Copy for MapLengthPolicy

impl Debug for MapLengthPolicy

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

Formats the value using the given formatter.

impl Default for MapLengthPolicy

fn default() -> Self

Returns the “default value” for a type.

impl Eq for MapLengthPolicy

impl Hash for MapLengthPolicy

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl Ord for MapLengthPolicy

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

fn clamp_to<R>(self, range: R) -> Self

where Self: Sized, R: ClampBounds<Self>,

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

Restrict a value to a certain range.

impl PartialEq for MapLengthPolicy

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

Equality operator ==.

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

Inequality operator !=.

impl PartialOrd for MapLengthPolicy

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

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

Tests less than (for self and other) and is used by the < operator.

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

Tests less than or equal to (for self and other) and is used by the <= operator.

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

Tests greater than (for self and other) and is used by the > operator.

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

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl StructuralPartialEq for MapLengthPolicy