fix: Add `FastVectorCopy` again but avoid using already-deleted `data()` for fast-copying primitive vectors

[mrousavy]

Previously we passed the std::vector<double> to void* get_data_vector_double(const std::vector<double&).

As a C++ developer, you might expect this to return a valid void* to the given std::vector<double>.

However because Swift does not really support const-ref types, it copies the std::vector<double> into the function arguments slot of get_data_vector_double, which then returns the void* to not our vector, but the temporary copy that was made. This is super stupid because obviously that temporary copy is no longer valid after that method returns, so we were reading corrupted memory!

There is now a test that covered exactly this case (array of numbers (primitives) inside a struct, which was again packaged into an array).

The solution to this is now ContiguousArray+fastCopy(vector: ...) which is written entirely in Swift.

I think it's also worth noting that accessing any prop on a struct runs the getter each time, so e.g. this:

let someNumbers = myStructFromJS.someNumbers
let someNumbers2 = myStructFromJS.someNumbers

This creates 2 array copies, and someNumbers != someNumbers2.

I'm not sure if we want to cache this, we'll see.

• Fixes https://github.com/mrousavy/nitro/issues/977

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Project	Deployment	Preview	Comments	Updated (UTC)
nitro-docs	Skipped			Oct 24, 2025 10:08am