fix: don't replace default members' body

[lowr]

cc #12779, #12821
addresses #12821 (comment)

gen_trait_fn_body() only attempts to implement required trait member functions, so we shouldn't call it for Implement default members assist.

This patch also documents the precondition of gen_trait_fn_body() and inserts debug_assert!, but I'm not entirely sure if the assertions are appropriate.

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit 7e0bd37 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7e0bd37 with merge 894d6a2...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 894d6a2 to master...

[lnicola]

but I'm not entirely sure if the assertions are appropriate.

I think we'd generally use stdx::always! here.

[lowr]

I'll keep that in mind, thanks! Is it worth another PR to replace the assertions?

[lnicola]

I think so, at least for consistency (just in case another contributor wants to do something the same way), but I can do it later today if you don't get a chance bu then.