linter to check omitted fields when constructing aggregate types? #25631

It would be nice to have a linter/tidy/whatever to enforce all non-defaulted fields in an aggregate type are initialized.

Assuming the code:

struct A {
    struct Options {
        unsigned cache_size;
        signed delta;
        bool in_memory = false;
    };
    A(const Options&) {} // unsafe (may have fields uninitialized)
    A(unsigned cache_size, signed delta, bool in_memory = false) {}  // safe, all non-default args are initialized
};

int main() {
    (void)A{
        A::Options{
            .delta = -1,
        },
    };
}

This leaves cache_size uninitialized when using designated initializers. (The problem exists even without designated initializers, but with designated initializers recently enabled it seems likely that we'll increase the use of aggregate types)

Probably another good candidate for a clang-tidy plugin. cc @theuni

Upstream might be happy about this as well, so one could consider adding it there.

Ooh, fun one! I'll give it a go.

Here's a quick and working (but probably under-specified) plugin using @MarcoFalke's sample above as a test: https://github.com/theuni/bitcoin-tidy/commit/4cbd5bf639ea7d56e4d4fe516494c6ac025a3034

Example usage: clang-tidy --load=`pwd`/libbitcoin-tidy.so -checks='-*,bitcoin-init-list' ../example_init.cc -- -std=c++17

1 warning generated.
/home/cory/dev/bitcoin-tidy/build/../example_init.cc:13:19: warning: Designated initializer with uninitialized member of type: unsigned int [bitcoin-init-list]
        A::Options{
                  ^

Edit: Updated to print the offending uninitialized type.

I think this might end up being a good "hello world" check for us to start with for a clang-tidy plugin infrastructure. Ping @fanquake.

I hacked together a quick local test, and the current bitcoind sources pass this check with no warnings: https://github.com/theuni/bitcoin/tree/tidy-init-list

When an initialization error is introduced it is detected as expected:

$ make bitcoin-tidy-lint

/home/cory/dev/bitcoin2/src/init.cpp:1417:37: warning: Designated initializer with uninitialized member of type: int64_t [bitcoin-init-list]
    CTxMemPool::Options mempool_opts{

I think this might end up being a good "hello world" check for us to start with for a clang-tidy plugin infrastructure. Ping @fanquake.

I have a WIP branch that integrates compiling the bitcoin-tidy plugin and using it in our run-clang-tody invocation into the CI. We can clean up that branch a bit, and open a PR at some point.

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