Enum KeyHolderNesting 

pub enum KeyHolderNesting {
    Recursive,
    KeylessNestedWhole,
}

How far Step 1’s member removal reaches into a key holder.

XTypes 1.3 §7.6.8 Step 1 keeps the key members of the holder —

If there are any key members, then remove the non-key members from FooKeyHolder. Otherwise do not remove any members.

— and Step 3 says to “apply steps 1 and 2 recursively to the members of FooKeyHolder if they are themselves AggregatedTypes”. FooKeyHolder at that point is whatever Step 1 left, so for a keyless aggregate it is every member, and Step 3 applies to all of them. The recursion has no depth limit written into it.

What the vendors do

Measured on 2026-08-09 through keyhash-probe/ — Cyclone DDS 11.0.1, RTI Connext 7.7.0, Fast DDS from source. Three shapes:

@final struct NestedNoKey        { long a; long b; };
@final struct OuterKeylessNested { @key NestedNoKey inner; long tail; };

@final struct DeepLeaf  { @key long p; long q; };
@final struct DeepMid   { DeepLeaf leaf; long r; };   // no key of its own
@final struct DeepOuter { @key DeepMid mid; long tail; };

@final struct ChainLeaf  { @key long p; long q; };
@final struct ChainMid   { @key ChainLeaf inner; long r; };
@final struct ChainOuter { @key ChainMid mid; long tail; };

All three vendors, under every encoding, give OuterKeylessNested both of NestedNoKey’s members and ChainOuter only p — so the “otherwise do not remove any members” clause holds, and the recursion through keyed levels is unlimited everywhere.

They part over DeepOuter, where a keyless level sits above a keyed one:

	DeepOuter holder
Cyclone, XCDR1 and XCDR2	0d0d0d0d 0f0f0f0f — p and r
Fast DDS	the same
RTI, XCDR2	the same
RTI, XCDR1	0d0d0d0d 0e0e0e0e 0f0f0f0f — p, q and r

RTI’s XCDR1 holder stops filtering at the first keyless aggregate and takes what is under it whole: maxSizeSerializedKey V1 / V2 reports 12 / 8 for that type, so the two holders are genuinely different types and not one type serialized twice.

Variants

Recursive

Step 1 applies at every level, however deep. The clause, and what four of the five measured combinations do.

KeylessNestedWhole

The first keyless aggregate is taken whole; nothing below it is filtered.

RTI Connext 7.7 under XCDR1. Under XCDR2 RTI recurses like everyone else, which is why Dialect::key_holder_nesting takes the encoding.

Implementations

impl KeyHolderNesting

pub const ALL: [Self; 2]

Both.

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 falling back to the default: a typo here changes every instance handle.

Trait Implementations

impl Clone for KeyHolderNesting

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 KeyHolderNesting

impl Debug for KeyHolderNesting

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

Formats the value using the given formatter.

impl Default for KeyHolderNesting

fn default() -> Self

Returns the “default value” for a type.

impl Eq for KeyHolderNesting

impl Hash for KeyHolderNesting

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 KeyHolderNesting

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 KeyHolderNesting

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

Equality operator ==.

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

Inequality operator !=.

impl PartialOrd for KeyHolderNesting

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 KeyHolderNesting