BIP394: rawtr() Output Script Descriptors

[jeanpablojp]

This is a BIP draft for the rawtr() script expression, following the style of BIPs 381-387. Bitcoin Core has implemented it since 24.0 and it has never had a BIP; sipa noted that on #2050 in March.

While writing this I did some digging and found rawtr in more places than I expected. BIP 390 allows musig() inside it, and two of the BIP 390 test vectors are rawtr() descriptors, so a published BIP already points at an expression no BIP defines. Outside Core it's a gap people route around: rust-miniscript prints rawtr_not_supported_yet where a descriptor should be, and bdk-ffi's tests cite rawtr as BIP-386.

Mail list discussion: https://groups.google.com/g/bitcoindev/c/CCZN_qQ5C1s

[achow101]

Concept ACK

[danielabrozzoni]

This was a very good read, thanks for submitting! It's coincise and easy to understand.
Looks good to me! Though I think someone who knows more about descriptors than I do should also take a look :)

EDIT: I just realized, you should change BIP380's Appendix B to point to this page.

[danielabrozzoni]

I think the type should be "Informational", similarly to other descriptor BIPs:

BIP	Description	Type
380	Output Script Descriptors (general)	Informational
381	pk(), pkh(), multi() etc.	Informational
382	wpkh(), wsh()	Informational
383	sh()	Informational
384	combo()	Informational
385	raw(), addr()	Informational
386	tr()	Informational
387	multi_a(), sortedmulti_a()	Informational
389	Multipath descriptors	Informational
390	musig() (used inside rawtr())	Informational

[jeanpablojp]

You are right, I just pushed the change.
Thanks.

[murchandamus]

Actually, I think all of these should be Specification BIPs. The criteria for BIP types were amended by BIP3 compared to BIP2, but we haven’t updated all of the affected BIPs, yet.

[jeanpablojp]

@danielabrozzoni about the Appendix B, BIP 3 says only the editor assigns the number, and until then the BIP is referred to by name. I can add the row now pointing to bip-rawtr.mediawiki, or wait for the editor to assign the number in this PR and register it with the number. Which do you prefer?

[murchandamus]

Thanks for your submission, this looks great already. I noticed that there was no Rationale section, yet. Please include one if you get any questions or comments about design decisions or related work, or you feel that content in that regard is missing.

Perhaps @achow101 could take a more thorough look at this, too.

Let’s call this BIP394. Please update the file name of your document, the corresponding preamble headers and add the README entry you were asking about with that number.

[jeanpablojp]

Thanks for the number, @murchandamus.

Renamed to bip-0394.mediawiki, Rationale added, preamble updated, the Type: Specification suggestion taken, and the README row added.

I also added the rawtr(KEY) row to BIP 380's Appendix B, which is what @danielabrozzoni asked for and what BIP 392 and BIP 393 did in their own PRs.