[redis] Avoid rvalue struct assign

[ntrel]

I'm trying to make assigning to a struct rvalue a compiler error, because often the assignment is silently thrown away:
dlang/dmd#21717 (comment)

That PR breaks some buildkite projects, including this one - so it will probably be a deprecation/error in an edition. This PR makes this project compatible.

Call opAssign directly instead, which is allowed by the spec:

an instance method can still be called on an rvalue struct instance,
even if the method is not const

https://dlang.org/spec/struct.html#member-functions

Disclaimer: I'm not sure if this is a suitable solution for this project, as I'm not familiar with it. From running dub test, it appears to work without triggering the new dmd error.

[s-ludwig]

I think in order to not break other code that uses the same pattern, it is necessary to implement opIndexAssign instead of fixing the place where the assignment is made:

void opIndexAssign(U)(U value, IDS id) { auto entry = this[id]; entry = value; }

[ntrel]

Thanks, sorry for the late response. I closed the original PR and the replacement one doesn't need to change vibe.d, because it allows assignment to an rvalue if the struct has a field with a pointer.