Minimal repro for temperlang/temper#328

What this is

This is a stripped-down reproduction of the Rust backend codegen bug I hit in a larger project (safe_httpeex).

The first version of this repo didn't actually trigger the bug (thanks Tom for catching that). Turns out it matters where the var is declared — it has to be inside a when branch, not at the function body level. The original repro had it at function body level so it compiled fine and didn't reproduce anything lol.

The actual pattern that breaks

// DOES trigger the bug — var inside when branch
let f(item: Item, extras: List<String>): String {
  when (item) {
    is FooItem -> do {
      var first = true;          // <-- inside the when (NestingStatement)
      for (let e of extras) {    // <-- for loop becomes a closure, captures first
        first = false;
      }
    };
  }
}

// Does NOT trigger — var at function body level
let g(items: List<Item>): String {
  var first = true;              // <-- this one is fine, varsDeclared finds it
  for (let item of items) {
    first = false;
    when (item) { ... }
  }
}

How to see it

# these all pass fine (bug is Rust-specific):
temper test -b js
temper test -b py
temper test -b lua

# this fails on unfixed temper, passes on fixed:
temper test -b rust

The errors you get

Two flavors, both from the same root cause:

E0434 — the generated Rust has a fn inside a ClosureGroup struct that tries to assign to first__27 using the bare var name instead of self.first__27. Rust fn items can't capture from enclosing scope:

error[E0434]: can't capture dynamic environment in a fn item
   --> src/mod.rs:107:17
    |
107 |                 first__27 = false;
    |                 ^^^^^^^^^
    |
    = help: use the `|| { ... }` closure form instead

E0308 — type mismatch because first got declared as plain bool but the closure generates Arc<RwLock<bool>> access patterns:

error[E0308]: mismatched types
   --> src/mod.rs:104:48
    |
104 |                 if ! temper_core::read_locked( & self.first__27) {
    |                                                ^^^^^^^^^^^^^^^^ expected `&Arc<RwLock<bool>>`, found `&bool`

See also

• temperlang/temper#328 — the bug report
• safe_httpeex — the larger project where I originally hit this