comptime · phase 2: structs in the evaluator

Phase 2 of compile-time values, after #562 (fixed-size arrays of scalars).

Scope

A struct as a compile-time value: build one, read a field, write a field.

In scope

• A struct variant in Value (src/hir/env.rs), holding the field names and their values.
• Construction from a struct literal, S { f : 1, g : 2 }.
• Field read, s.f.
• Field write, s.f = v.
• Fields may hold anything the evaluator already holds, so a struct with an array field and
  an array of structs both follow from this plus #562.

Deferred

• Methods on the struct. A method call is not evaluated at all today (#560), so nothing here
  depends on it and nothing here fixes it.
• Enums and their variants.

What exists now

eval_expr (src/hir/stmt.rs) has no arm for a struct literal or for field access, and
eval_statement and check_assign_stmt only handle an identifier or an index on the left of
an assignment. A s.f = v therefore falls through both.

The rule this has to keep

The same one #562 established. A field write the evaluator cannot carry out drops the whole
struct rather than leaving the old field values readable. check_assign_stmt already has a
place_root helper that walks a place expression down to the variable it writes through;
extending it to cover a field access is how a s.f = v and a s.a[i] = v reach the right
variable.

Two related holes are worth reading before starting, because both were wrong values rather
than missing ones: #558 and #569.

Tests to add

• Construct, field read, field write, each with a false assert so the fixture can go red.
• A field write the evaluator cannot compute, checked with a pair of contradictory asserts
  that are both expected to be accepted. That pair is the oracle for "the evaluator holds no
  value here".
• A struct holding an array, and an array holding structs, once #562 has landed.

Related: #562, #558, #560, #569.

Start with src/hir/env.rs, src/hir/stmt.rs, eval_expr, eval_statement, and check_assign_stmt, including its place_root helper. Read #562, #558, and #569 before adding coverage for struct construction, field reads and writes, and unsupported writes. Done means the requested false-assert fixtures expose each behavior, including contradictory asserts for dropped values and nested struct/array cases after #562.