Modify `ModuleBuilder::declare_function` to Return Error on Conflicting Declarations

[Y-Nak]

Currently, the ModuleBuilder::declare_function method updates the existing function declaration when a function with the same name is declared again. This behavior can unintentionally overwrite previous declarations, potentially leading to inconsistencies or unexpected behaviors in the module.

We propose changing this behavior so that declare_function returns a Result. If a function with the same name is already registered and its signature differs from the new declaration, the method should return an error instead of updating the existing declaration.

Proposed Changes:

• Add BuilderError type

• Modify Return Type:

  • Change the return type of ModuleBuilder::declare_function to return a Result<Function, BuilderError>

• Conflict Detection:

  • Implement a check to determine if a function with the same name already exists.
  • If it exists and the signature matches, return the existing function.
  • If it exists and the signature differs, return an error indicating a conflicting declaration.

[syszery]

Just a quick follow-up question:

While working on the change, I noticed that both the parser and the linker already prevent conflicting function declarations by name before calling declare_function. So in those places, unwrapping the result seemed safe, and the new Result might not be used directly in practice. Also, both the parser and the linker also have their own dedicated error types for handling conflicting definitions, so propagating the new BuilderError felt out of place.

There was also a small helper module used in tests that called this method — I chose to unwrap there too, assuming it would be better to keep tests focused and avoid cluttering them with extra error handling unless necessary.

I was mostly wondering if the goal behind this change was primarily to make declare_function more self-defensive by design, or if there’s a longer-term plan to surface the error more explicitly elsewhere.

Just trying to better understand the intent and make sure I'm thinking in the right direction. Thanks!