util: add default_value_vec for defaults without LengthReadable#4507
util: add default_value_vec for defaults without LengthReadable#4507carlaKC wants to merge 1 commit intolightningdevkit:mainfrom
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Right now, use of `default_value` requires that the struct implements `LengthReadable` itself. When trying to use `default_value` outside of LDK for `Vec<T>`, your code will run into the orphan rule because it does not own the trait `LengthReadable` or the type `Vec`. There are various ugly workarounds for this (like using `custom`), but wanting to persist a vec with a default value seems like a common enough use case to justify the change.
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👋 Hi! I see this is a draft PR. |
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| items: Vec<u32>, | ||
| } | ||
| impl_writeable_tlv_based!(DefaultValueVecStruct, { | ||
| (1, items, (default_value_vec, Vec::new())), |
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nit: Might make sense to use a dummy default value here rather than the empty Vec to check this is actually what's used rather than Vec::default() in general?
Hmm, I guess we could use I'm not a huge fan of yet more write variants, ideally we'd drop some, really, but I do see why this is useful, and its not much of an extension on what we already have, so if |
Yeah we can, it's just pretty ugly, worth having the option IMO. |
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In lightningdevkit/ldk-node#825, I ran into the orphan rule when trying to migrate from a single incoming/outgoing HTLC to a vec of
HTLCLocatortypes to allow expressing multiple in/out HTLCs. This issue stems from the following:HTLCLocatortype in LDK-Node, so that we can have niceties like a typeduser_channel_id.impl_writeable_tlv_based_enum!macro for LDK-Node'sEventtype.default_valueto fill the old fields when the new ones aren't present.However:
default_valueusesrequiredunder the hood, which expects the field to beLengthReadable.LengthReadableforVec<HTLCLocator>in LDK-Node due to the orphan rule (we don't ownVec<T>, even ifTis our type).impl_for_vec!to implement this.This change introduces a new
default_value_vecto our macro which usesrequired_vec(and thusWithoutLength) under the hood, so that we don't needLengthReadable. There are a few ugly workarounds (like usingcustom), but wanting to write aVecwith a default seems like a common enough one to justify a change to our macros.