t27b: an array literal of strings for a slice field or a returned slice (wrapup-auto) #7833

Part of #6063 (t27b coverage), item C10 of #6488. Follows #7680, #7470 and #7765.

Blocked spec

specs/automation/wrapup-auto.t27 passes the reference (1/1, since #7745). t27b refuses it with ExprArrayLiteral(to slice field) (an array literal whose elements hold a str, for a slice (a t27b static cannot hold a string's address)) at line 44: files_modified: ["specs/automation/wrapup-auto.t27"], a [str] field of a named struct literal.

#7470's description parked this shape until its logic could be written in t27.

What the reference does

t27c prints @constCast(&[_][]const u8{ "specs/automation/wrapup-auto.t27" }), the same form as for scalar elements (slice_lit_plan.t27). The array is a comptime constant of slices that point at string literals:

• reading it is defined;
• writing it is undefined behaviour, which t27b: a write through an array literal printed @constCast(&[_]E{ ... }) is undefined behaviour in Zig; refuse it by name instead of encoding x86_64 Debug's result #7765 refuses by name.

The step

• The plan. In specs/tri/t27b/slice_lit_plan.t27, elements that hold a str become STATIC like any other constant elements, and the refusal is dropped.
• Where the object is written. A string's address is not known before the program runs, so a new answer, written_at_entry, says the object is written at the start of every test, invariant and bench. That is t27b: module vars that hold a string or start undefined (Closes #7448) #7470's mechanism for module vars that hold a string.
• The write check. t27b: a write through an array literal printed @constCast(&[_]E{ ... }) is undefined behaviour in Zig; refuse it by name instead of encoding x86_64 Debug's result #7765's check covers the new statics as it covers any STATIC.
• The conformance spec comes first: specs/tri/t27b/conformance/string_slice_literal.t27.

User Scenarios

• Given the conformance spec specs/tri/t27b/conformance/string_slice_literal.t27 exists and specs/tri/t27b/slice_lit_plan.t27 allows string elements as STATIC, when t27b compiles specs/automation/wrapup-auto.t27, then it succeeds without ExprArrayLiteral(to slice field) refusal.
• Given a struct literal with a [str] field initialized from an array literal of string literals, when t27b emits the backend representation, then the array becomes a comptime constant of slices pointing at string literals with written_at_entry semantics.
• Given the write check from t27b: a write through an array literal printed @constCast(&[_]E{ ... }) is undefined behaviour in Zig; refuse it by name instead of encoding x86_64 Debug's result #7765, when a STATIC array of string slices is declared, then the check covers it and refuses writes as it does for any STATIC.

Requirements

• FR-001: The conformance spec specs/tri/t27b/conformance/string_slice_literal.t27 MUST be created and define the expected behavior for array literals of strings assigned to slice fields.
• FR-002: specs/tri/t27b/slice_lit_plan.t27 MUST be updated so that elements holding a str become STATIC like other constant elements, and the refusal for ExprArrayLiteral(to slice field) is dropped.
• FR-003: The t27b compiler MUST implement written_at_entry semantics for module variables that hold string slices, writing the object at the start of every test, invariant, and bench.
• FR-004: The write check from t27b: a write through an array literal printed @constCast(&[_]E{ ... }) is undefined behaviour in Zig; refuse it by name instead of encoding x86_64 Debug's result #7765 MUST cover the new STATIC arrays of string slices and refuse writes to them.
• FR-005: specs/automation/wrapup-auto.t27 MUST compile successfully with t27b after the changes.

Success Criteria

• t27b specs/tri/t27b/conformance/string_slice_literal.t27 exits 0
• t27b specs/tri/t27b/slice_lit_plan.t27 exits 0
• t27b specs/automation/wrapup-auto.t27 exits 0
• 3 tests in the conformance suite for string slice literals pass

Boundary

• specs/tri/t27b/conformance/string_slice_literal.t27
• specs/tri/t27b/slice_lit_plan.t27
• specs/automation/wrapup-auto.t27
• scripts/tri_loop/t27b.py