Rework generic Boxy descriptor and nominal backing resolution

[lukewilliamboswell]

Split out of #10717. Stacked on #10776 — the first commit here is that PR's digest canonicalization; review the second commit. Without it, planning trips one worker body representation had conflicting specialization terminals on digest-distinct but equivalent solved types.

Problem

Validating real applications (RocRay) against the generic (--specialize=no) Boxy backend surfaced a family of descriptor identity bugs: runtime type descriptors for erased callables, stored callables, and aggregate fields were resolved from the wrong context when nominal backing, transparent aliases, or specialization branches separated a value's construction site from its use.

Fix

• Resolve nominal backing substitutions through the declaring module and scope them to their lexical traversal paths, avoiding self-cycles.
• Preserve stored callable representations and descriptor-wrapper substitutions through Boxy nominal backings.
• Reserve descriptors for pending aggregate and tag payload fields from source provenance before construction, and track runtime descriptor writes so materialized slots are marked bound.
• Read erased callable argument and result descriptors at runtime via two new low-level ops (erased_callable_arg_desc / erased_callable_result_desc), wired through the interpreter, dev, LLVM, and wasm backends and the Boxy runtime.
• Plan representations for field-access segment success types; segment types are interned per access and appear nowhere else in the checked body, so lowerFieldAccessInto's prefix locals could not resolve them.
• Prune static body specialization branches with identical contexts and align worker body alias descriptor contexts.
• Keep fixed-lifetime LLVM allocations in procedure entry blocks when an erased call is emitted from a branch.

Tests

Regression fixtures cover stored function fields (plain and generic), generic platform required-init consts (red on main: panics with checked body referenced a type missing from the boxy representation plan), and boxed erased callables across the host boundary inside control flow.

Full MiniCI is green on this stack (75/75 phases).

Known limitation (pre-existing, out of scope)

erased_callable_alias_digest/app.roc still panics under --specialize=no (conflicting specialization terminals); its registered test intentionally uses the specialized path. Worth a follow-up issue.

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

This PR reworks generic Boxy representation planning and lowering so descriptors retain the correct nominal, alias, aggregate, and callable provenance across construction and use sites.

• Adds runtime erased-callable argument and result descriptor operations across the interpreter, dev, LLVM, and wasm backends.
• Introduces lexical nominal-backing substitutions and explicit descriptor reservation and binding for aggregate fields.
• Plans field-access segment representations and aligns specialization and worker-alias descriptor contexts.
• Moves fixed-lifetime LLVM scratch allocations into procedure entry blocks and adds regression coverage for stored functions and host-boundary erased callables.

Confidence Score: 4/5

The PR appears safe to merge after replacing the non-blocking nested-callable fallback with an explicit context-keyed plan or invariant failure.

The runtime and backend descriptor wiring is internally consistent in the reviewed paths, while the accepted concern is a compiler-planning discipline violation that can hide missing contextual planning rather than a demonstrated current blocking failure.

Files Needing Attention: src/postcheck/boxy/lower.zig

Important Files Changed

Filename	Overview
src/postcheck/boxy/plan.zig	Extensively reworks representation identity, nominal-backing substitutions, aggregate descriptor planning, field-access planning, and specialization contexts.
src/postcheck/boxy/lower.zig	Consumes the expanded plans and emits runtime descriptor operations, but introduces a prohibited fallback for missing context-keyed nested-callable plans.
src/eval/boxy_abi.zig	Adds registered erased-callable metadata lookups with keyed argument-offset and bounds validation.
src/eval/interpreter.zig	Implements interpreter equivalents of the new erased-callable descriptor reads using interpreter-owned callable context.
src/backend/llvm/MonoLlvmCodeGen.zig	Wires the new low-level operations and relocates fixed-lifetime scratch allocations to entry-block slots.
src/backend/dev/LirCodeGen.zig	Registers and emits calls to the two new Boxy runtime descriptor wrappers.
src/backend/wasm/WasmCodeGen.zig	Registers wasm signatures for the new erased-callable descriptor runtime symbols.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    A[Checked types and callable uses] --> B[Boxy representation plan]
    B --> C[Nominal and alias descriptor substitutions]
    B --> D[Aggregate field descriptor reservations]
    B --> E[Erased callable descriptor keys]
    C --> F[Boxy lowering to LIR]
    D --> F
    E --> F
    F --> G[Interpreter]
    F --> H[Dev backend]
    F --> I[LLVM backend]
    F --> J[Wasm backend]
    G --> K[Boxy runtime descriptors]
    H --> K
    I --> K
    J --> K

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[greptile-apps]

Context-blind callable plan fallback

When the exact contextual lookup returns no plan, this scan drops contextual_type and reuses a plan selected only by expression, caller, and worker. This violates the compiler's explicit-planning requirement and can conceal a missing context-specific plan by silently coupling lowering to another specialization's compatible-looking arguments.

Context Used: AGENTS.md (source)