Struct SizeCache 

pub struct SizeCache { /* private fields */ }

Transient pre-order cache of nested-message sizes for the two-pass serialization model (compute_size populates, write_to consumes).

Message::encode and friends construct and discard a SizeCache internally — most callers never name this type. It appears in the compute_size / write_to signatures so that manual Message implementations can thread it through nested-message recursion.

Storage is a small inline [u32; 16] array with a Vec<u32> spill for the (uncommon) case of more than 16 nested length-delimited sub-messages, so a fresh cache is allocation-free for typical messages.

Reusable across encodes: call clear between uses to retain the spill allocation. SizeCache is intentionally not Clone — it is transient encode state, not data. Reuse via clear().

Implementations

impl SizeCache

pub const fn new() -> SizeCache

Create an empty cache. No heap allocation.

pub fn clear(&mut self)

Clear the cache for reuse. Retains the spill allocation’s capacity.

pub fn reserve(&mut self) -> usize

Reserve a slot for a nested message’s size. Call immediately before recursing into child.compute_size(cache), then fill the slot with set after the recursion returns. This reserves the slot in pre-order even though the size is known in post-order.

Used by generated compute_size implementations.

pub fn set(&mut self, idx: usize, size: u32)

Fill a previously-reserved slot.

Used by generated compute_size implementations.

Panics

Panics if idx was not returned by a prior reserve on this cache (i.e. idx >= len).

pub fn consume_next(&mut self) -> u32

Consume the next cached size in pre-order.

Used by generated write_to implementations for length-delimited nested message headers.

Panics

Panics if the cursor runs past the end of the cache — i.e. if write_to traversal diverges from compute_size traversal. For generated code this indicates a codegen bug; for manual Message implementations it indicates a traversal-order mismatch.

Trait Implementations

impl Debug for SizeCache

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

Formats the value using the given formatter.

impl Default for SizeCache

fn default() -> SizeCache

Returns the “default value” for a type.