CTransaction: Add constructor taking inputs and outputs #36696

Part of #36692. Please view this PR in context of changes like #36694.

CTransaction gains a constructor that allows direct declarative construction as an alternative to building through CMutableTransaction. CTxIn's constructors gain an optional argument to pass the script witness.

It is a conscious decision that vin/vout are passed as generic input ranges and that only their elements are copied/moved. Passing the std::vector by value and then moving the container itself into place would allow the following:

std::vector<CTxIn> inputs = ...;
CTxIn& backdoor = inputs.back();

const CTransaction tx{std::move(inputs), ...};

backdoor = ...; // <- modify `tx`, potentially breaking its invariant

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36696
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36696.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36696#author-ralph

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #36543 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36543.svg">} (validation: check the most recent AssumeUTXO commitment during IBD by l0rinc)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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LLM Linter (✨ experimental)

Possible places where named args for integral literals may be used (e.g. func(x, /*named_arg=*/0) in C++, and func(x, named_arg=0) in Python):

• src/test/sigopcount_tests.cpp: CTransaction{..., ..., 1}
• src/test/sigopcount_tests.cpp: CTransaction{..., ..., 1}
• src/test/sigopcount_tests.cpp: AddCoins(coins, creationTx, 0)

^{2026-10-30 07:17:58}

The CTxIn witness argument is a nice addition. It would remove the “assign the witness after replacing vin” workaround I needed in several places in #36606 and #36694.

I have replaced the commits that exemplify the constructor in action with one that shows multiple things in a single place:

• It shows the new constructor of CTransaction in action.
• It shows the new constructor of CTxIn in action.
• It shows how CMutableTransaction is not necessary in many places, not just for building, but also as function argument.
• It reveals GetTxSigOpCost as an example test that contains multiple blocks where each should become its own unit test.
• It reveals potential test dependencies: Each test setup modifies the CCoinsViewCache. It is not clear whether a test might depend on the modifications of tests run earlier or not.
• It shows that CTransaction could benefit from applying best practices (never use const data members, maintain immutability by not providing write access): In return {std::move(creationTx), std::move(spendingTx)};, the compiler provided move constructor of CTransaction invokes the move constructors of all its data members, which are const, so they fall back to copying (this does not fire performance-move-const-arg, because the individual data members fall back, not the tx object).
• It reveals the annoyance that CTransaction does not provide regular semantics: It is not possible to replace the existing values with std::tie(creationTx, spendingTx) = BuildTxs(...);, because CTransaction is only sort-of copyable (due to not following ... I repeat myself. This is not a big deal here, as tests should be refactored for independence anyway).
• It also provides an example where code could be much less verbose if the constructor accepted std::vector and explicit types were not required at the call site, and why I claim this to be less readable:

CTransaction creationTx{{{{}}}, {{1, scriptPubKey}}, 1};
CTransaction spendingTx{{{{creationTx.GetHash(), 0}, scriptSig, CTxIn::SEQUENCE_FINAL, witness}}, {{1, {}}}, 1};