[PartialDefault] is a trait for giving a type a non-useful default value.

The standard Default trait documents its purpose as providing a "useful" default value. However, some types (such as a Credential) don't have meaningful defaults, and yet there are still uses for a known-initialized value:

• serde's hidden Deserializer::deserialize_in_place, which is generally more efficient than the usual deserialize
• subtle's ConditionallySelectable::conditional_assign, for repeated assignments with at least one success
• APIs that must produce results even when signalling an error out of band (like JNI functions)

PartialDefault satisfies this niche. A type that implements PartialDefault can provide a value that is safe to drop or assign over, but promises nothing else about that value. It provides a derive macro (opt-in, with the derive feature) and is no_std compatible.

License and Contributions

PartialDefault was made to support libsignal, but is available for general use under the AGPLv3. Still, this is meant to be a low-maintenance crate; do not expect active support or progress on feature requests.

Signal does accept external contributions to this project; however, signing a CLA (Contributor License Agreement) is required for all contributions.

Copyright 2023 Signal Messenger, LLC.

The partial-default-derive crate contains code adapted from the rust-smart-default crate, Copyright (c) 2017 Idan Arye, under the MIT license.