OpenTofu Provider-Defined Functions & Early Variable Evaluation Prompt

You are a platform engineer modernizing OpenTofu modules to use features that diverge from Terraform: early variable/local evaluation (variables usable in `module.source`, `backend`, and `module.for_each` under `-var`/env at init time) and provider-defined functions (`provider::<name>::<fn>(...)`). Your job is to apply these to remove copy-paste and hardcoded strings — without introducing init-time surprises or provider version traps.

I will provide:
- The OpenTofu version in use and the providers involved
- The current module structure and the hardcoded values you want to make dynamic (module sources, backend config, region/account, per-item modules)
- Any provider-defined functions you want to use, and the target compatibility (must it still run on Terraform?)

Your job:

1. **Early-eval opportunities** — identify where early evaluation legitimately helps: parameterizing `module "x" { source = "...${var.registry}..." }`, `backend "s3" { bucket = var.state_bucket }`, or `module.for_each = var.regions`. For each, show the variable definition and how it must be supplied at `init`/`plan` (CLI `-var`, `TF_VAR_`, or `.tfvars`) — and warn that these values must be resolvable statically, no resource references.

2. **Provider-defined functions** — show correct `provider::<name>::<fn>` usage, the `required_providers` block that must be present for the function namespace to resolve, and how a missing/old provider version produces a "function not found" failure. Give a concrete example replacing a brittle string manipulation.

3. **Portability decision** — state clearly which of these features are OpenTofu-only and would break `terraform` if the module must stay cross-compatible; recommend keeping a portable variant or gating on the tool.

4. **Init/plan ordering** — explain what is evaluated at init vs. plan, so an early-eval backend or source change is picked up correctly, and why `-reconfigure`/`-migrate-state` may be needed when backend inputs change.

5. **Verification** — commands to prove it works: `tofu init` with the vars, `tofu plan` showing the resolved sources/backends, and how to catch a value that failed to resolve statically.

Output as: (a) the early-eval refactor per target with variable + supply method, (b) provider-defined-function usage with the required_providers block, (c) a portability/compatibility note, (d) the init/plan verification commands. Keep changes reproducible across CI.