t27b: an untyped list local that the code indexes is a Zig tuple: constant indices, and run-time ones only at compile time #8050

Part of #6063 (t27b coverage), which is item C10 of #6488. Split off from the notes on #7735.

What blocks

Two specs pass in the reference, but t27b blocks them on ExprArrayLiteral with the detail "array literal with no result type":

ternary_gates' second blocker, ExprCall(@setEvalBranchQuota), is gone on master since #7459. On master 2dfe978, ExprArrayLiteral is the only construct t27b reports for either file.

What the reference does

Measured with t27c test-report (zig 0.16.0, t27c lab):

• t27c gen drops the literal's type and prints const vals = .{ ... };. That is a Zig tuple whose fields have the element values' types, and which are comptime fields where the values are compile-time known.
• A compile-time index works: vals[0], ts[1].v. An assert on such a field is folded, so it is vacuous, unless a call reads it.
• A run-time index in run-time code does not compile: "unable to resolve comptime value". This holds for typed consts, literals, struct elements, and inside a fn.
• A run-time index inside a comptime {} block, which is how an invariant is printed, works, because the whole block runs at compile time.
• A bench is an uncalled fn, so Zig never analyzes it.

The step

1. A conformance spec first, specs/tri/t27b/conformance/tuple_local_index.t27. It covers typed-const elements and struct elements with a str field, both at constant indices with calls reading them, and an invariant that indexes in a loop. Reference: 2/2, 6 runtime asserts, 0 vacuous.
2. The decisions go in a t27 plan through t27c gen-rust:
   • Which locals are taken: the literal names its element type T, and every element is a struct literal of T or a module const declared as T.
   • Where they live: read-only data, which folds constant reads as Zig's comptime fields do, or a constant aggregate where T holds a str.
   • A run-time index outside an invariant, a bench or a fn no test reaches is refused by name as ExprIndex(tuple), as Zig refuses it.
3. Ledger: both specs move to pass.

User Scenarios

• Given a spec declares a const local as an untyped list literal with struct elements, and code reads it at a constant index inside a called function, When t27b compiles the spec, Then the read succeeds and the value is available at runtime.
• Given a spec declares a const local as an untyped list literal and an invariant indexes it with a loop variable inside a comptime {} block, When t27b compiles the spec, Then the invariant runs at compile time and the indexing succeeds.
• Given a spec declares a const local as an untyped list literal and a bench function indexes it with a runtime variable, When t27b compiles the spec, Then compilation fails with a clear error naming ExprIndex(tuple) because runtime indexing of a tuple is not allowed.

Requirements

• FR-001: t27b MUST recognize an untyped list local as a tuple candidate when every element is a struct literal of the same declared type T or a module const declared as T.
• FR-002: t27b MUST lower such a local to read-only data that folds constant-index reads at compile time, or to a constant aggregate when T contains a str field.
• FR-003: t27b MUST accept constant-index reads of the tuple in called functions and in comptime {} blocks (invariants).
• FR-004: t27b MUST reject runtime-index reads outside comptime {}, benches, or unreached functions with an error that names ExprIndex(tuple).
• FR-005: The new conformance spec specs/tri/t27b/conformance/tuple_local_index.t27 MUST cover typed-const elements, struct elements with a str field, constant-index calls, and a loop-indexed invariant.

Success Criteria

• t27c test specs/tri/t27b/conformance/tuple_local_index.t27 exits with exit 0 and reports 2/2 tests passing, 6 runtime asserts, 0 vacuous.
• t27c test specs/isa/ternary_gates.t27 exits with exit 0 (previously blocked on ExprArrayLiteral).
• t27c test specs/port/scripts/ultra_engine_v70_ultimate.t27 exits with exit 0 (previously blocked on ExprArrayLiteral).
• t27c gen-rust specs/tri/t27b/conformance/tuple_local_index.t27 produces a plan that identifies the tuple locals and their read-only data placement.

Boundary

• specs/tri/t27b/conformance/tuple_local_index.t27
• specs/isa/ternary_gates.t27
• specs/port/scripts/ultra_engine_v70_ultimate.t27
• cli/t27b/src/lower/tuple.rs
• scripts/tri_loop/t27b.py