Tracking: implicit-conversion / flat-path coercion arc → Rust numeric model

Master task for the type-conversion + flat-path coercion arc: make the flat codegen path handle Vx's implicit numeric coercions correctly, then adopt Rust's stricter model (no implicit conversions; literals infer to context) and delete the coercion machinery. Groups the work done this arc and what remains.

Done ✅

Phase 1 — flat path correct under Vx's old (permissive) semantics (unblocked the flat corpus, flat-used 61 → 87):

• hiraditya/Vx.1#231 — string values (let s = "…", string args): LoweredTy::Ptr + StringConst opcode. 95cfbdc
• Extern symbols are module-independent (dedup vx_stdout_write across a program + std::io). 5898f30
• hiraditya/Vx.1#235 — FFI pointer ABI (*const/*mut/& as params/returns/args/memory-mode locals, pointer-returning externs). 95cfbdc
• hiraditya/Vx.1#236 — coerce call arguments to parameter types (type checker). 2ec70d6 (later removed by hiraditya/Vx.1#240 Stage B)
• hiraditya/Vx.1#237 — coerce binary-op / comparison / compound-assign operands (flat lowerer). adae4c5 (later removed by hiraditya/Vx.1#240 Stage B)
• hiraditya/Vx.1#238 — materialize implicit coercions in the flat lowerer (let slot, assignment, TensorStore, struct field, value-if, return). adae4c5, 5062c95 (later removed by hiraditya/Vx.1#240 Stage B)

Phase 2 — adopt Rust's numeric model (#240), which superseded Phase 1 by removing the reason it existed:

• hiraditya/Vx.1#240 Stage A — unsuffixed literals are untyped (ty: None) and infer to context; one shared default_number_elem fallback for the checker and the flat lowerer. Non-breaking behind still-permissive is_assignable. b71fdf0
• hiraditya/Vx.1#240 Stage B — is_assignable scalar rule tightened to identical-only (no implicit numeric conversion). Inference wired at every checking position (check_operand_pair for binary/relational/logical operands + range bounds; assignment/compound RHS; array-literal elements; call args via refine_literal_arg). Deleted coerce_val (#238) and coerce_call_arg (#236) — a probe confirmed zero residual casts. Nine-program explicit-as migration; stdlib needed none. 14719b4
• Design docs + journal (Entries 55–61): flat_call_argument_coercion.md, rust_numeric_model.md (§8 Stage A, §9 Stage B).

Key finding (drove the design): coercion is codegen-type-dependent (a loop var is MLIR index in the AST codegen, i64 then i32 in the flat lowerer), so materialization belonged per-backend — until hiraditya/Vx.1#240 removed implicit conversion entirely, making both backends agree by construction.

Latent bugs the Stage B tightening exposed and fixed (coercion had hidden them): for-loop induction variables were hardcoded i64 in the checker (now take the range's element type, matching the flat lowerer); array literals and Vec<i32> indexing defaulted their element type to f32 (now resolved from the first element / the container's data : *mut T field).

Planned 🔜

• hiraditya/Vx#239 — lower the expression constructs the flat path still declines (method / closure / fn-pointer calls, value-match, array literals, logical ops). Now pure lowering: with hiraditya/Vx.1#240 done there is no implicit coercion at these value positions.
• hiraditya/Vx.1#235 — remainder: scalar/aggregate &x / *p and void externs (no corpus driver yet).
• hiraditya/Vx.1#233 — data-carrying (tagged-union) enums + value-producing match. Blocked: the AST oracle itself fails MLIR verification on a user-defined tagged union; also needs generic Option<T> + Vec for the one corpus driver.
• (optional UX) point a scalar-mismatch error explicitly at as (the message is clear but doesn't yet name the fix).

Sequencing note

hiraditya/Vx.1#240 is complete, so hiraditya/Vx#239 is now pure lowering (no coercion follow-ups). hiraditya/Vx.1#235's remaining pointer work is independent. hiraditya/Vx.1#233 waits on an AST-path fix or the generics/Vec surface.

Part of the flat-codegen convergence (#200/#201).

Start by reading the rust_numeric_model.md and flat_call_argument_coercion.md design documents, then choose one of the remaining linked tasks: #239, #235, or the blocked #233. The issue names no source file or test; done means completing the selected lowering, pointer, or enum/match work while preserving agreement between the AST and flat paths.