feat(wasm): byte-level load/store IR + env_at/arg_at (ADR-015 S5) (#339)

## Summary

Unblocks INT-03 S5 (#180) by extending the wasm IR with the full
WebAssembly 1.0 §5.4.6 byte-level load/store family (opcodes
`0x2C..0x35`, `0x3A..0x3E`) and shipping the `env_at(i)` / `arg_at(i)`
string accessors that were waiting on them.

- **IR + encoder** (`lib/wasm.ml` + `lib/wasm_encode.ml`) — 14 new
`instr` constructors (`I32Load8{S,U}`, `I32Load16{S,U}`,
`I64Load8{S,U}`, `I64Load16{S,U}`, `I64Load32{S,U}`, `I32Store8`,
`I32Store16`, `I64Store{8,16,32}`). Encoder pattern mirrors the existing
full-width family one-for-one.
- **`env_at` / `arg_at` lowering** (`lib/typecheck.ml` +
`lib/codegen.ml` + `lib/wasi_runtime.ml`) — bind `Int -> String / { Time
}`; lower via on-demand `wasi_snapshot_preview1.{environ,args}_get`
imports paired with the existing `*_sizes_get` import. The codegen
helper emits a 7-phase sequence: `sizes_get` → alloc ptrvec+bytebuf →
`*_get` → resolve `ptrvec[n]` → null-terminator scan (`I32Load8U`) →
alloc length-prefixed AS string → byte-copy (`I32Load8U` / `I32Store8`).
- **Import-table dedup** (`lib/codegen.ml`) — `env_count` and `env_at`
both require `environ_sizes_get` (ditto `arg_count`/`arg_at`). The
`optional_wasi` table now lists every `(builtin, wasi_import)` pair and
the assembly pass dedupes by WASI import name (first occurrence wins),
preserving the canonical-order indexing the existing combo regression
hardened.
- Docs updated in `docs/ECOSYSTEM.adoc` and `docs/TECH-DEBT.adoc`; stale
"tracked follow-up" comments in codegen/typecheck/wasi_runtime cleared.

## Test plan

- [ ] CI `dune build` + `dune runtest` (the only build environment
available — see note below)
- [ ] `./tools/run_codegen_wasm_tests.sh` picks up the three new
fixtures automatically:
- `tests/codegen/env_at.affine` + `test_env_at.mjs` — smoke for
`string_length(env_at(0))`; verifies `environ_sizes_get` + `environ_get`
both fire and the byte-scan terminates at the right offset.
- `tests/codegen/arg_at.affine` + `test_arg_at.mjs` — same shape over
`args_get`, indexed at **1** to exercise the `n*4` ptrvec offset (not
just `index = 0` short-circuit).
- `tests/codegen/env_count_and_at.affine` + `test_env_count_and_at.mjs`
— dedup regression: instantiation fails fast if `environ_sizes_get` is
double-imported.

## Caveat — local verification

The session container's network policy blocks `opam.ocaml.org`
(`host_not_allowed`), so `opam install . --deps-only` fails and `dune
build` cannot run locally. The five modified `.ml` files were
syntax-checked with `ocamlc -stop-after parsing` (all OK) and the code
mirrors existing patterns exactly, but the type-checker hasn't been run
end-to-end against this branch. CI is the first real validation —
autofix-on-failure is wired up to follow.

https://claude.ai/code/session_01FoWm87AnVtLS5euDEAGu2A

---
_Generated by [Claude
Code](https://claude.ai/code/session_01FoWm87AnVtLS5euDEAGu2A)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

docs/ECOSYSTEM.adoc

@@ -219,11 +219,14 @@ builtins lower to on-demand `wasi_snapshot_preview1.*` imports
 `ctx.wasi_func_indices`; zero impact on units that don't use them;
 verified with a multi-import combo regression). Component path
 bridges to `wasi:clocks`/`wasi:cli`. Real-host main-invoke deferred
-to S6 (WIT export-lifting / wasi:cli/run command shape). String
-accessors (env_at/arg_at) gated on a byte-level wasm-IR extension
-(I32Load8U/I32Store8 absent today) — tracked as the next slice
-before/with S5 filesystem.** WIT world of
-record: `wit/affinescript.wit`
+to S6 (WIT export-lifting / wasi:cli/run command shape).
+**S5 string accessors (env_at/arg_at) DONE: the wasm IR gained
+the byte-level load/store family (I32Load8U/I32Store8 + the full
+WebAssembly 1.0 §5.4.6 row, opcodes 0x2C..0x35 / 0x3A..0x3E);
+accessors lower to on-demand `environ_get`/`args_get` paired with
+the existing `*_sizes_get` import (dedup keeps each WASI import
+once even when both `*_count` and `*_at` are used).** WIT world
+of record: `wit/affinescript.wit`
 |INT-04 |Publish compiler + runtime to JSR (then npm) |#181 |runtime
 packaging READY (affine-js + affinescript-tea JSR dry-run green;
 manual-only `publish-jsr.yml`; docs/PACKAGING.adoc). INT-01 dep

docs/TECH-DEBT.adoc

@@ -230,11 +230,14 @@ Component-Model re-target, S1..S6); S2 toolchain #251 closed;
 S3 componentize done; **S4a (clock) + S4b (env_count, arg_count)
 DONE — on-demand preview1 imports via Effect_sites pre-scan,
 canonical-order indexing through `ctx.wasi_func_indices`; combo
-regression proves no collision. String accessors (env_at/arg_at)
-gated on byte-level wasm IR (I32Load8U/I32Store8 absent today) —
-tracked next slice. Real-host main-invoke = S6 (WIT export
-lifting). Next S5
-(native clocks/env/argv)**
+regression proves no collision. **S5 (env_at/arg_at) DONE — wasm
+IR extended with the byte-level load/store family
+(I32Load8U/I32Store8 and siblings); accessors lower to on-demand
+`environ_get`/`args_get` imports paired with the existing
+`*_sizes_get` (dedup keeps each WASI import exactly once even when
+both `*_count` and `*_at` are used in the same unit); guest
+allocates a length-prefixed AS string and byte-copies from the
+WASI buffer.** Real-host main-invoke = S6 (WIT export lifting)
 |INT-04 |Publish to JSR/npm |S2 |#181 packaging READY (dry-run green,
 manual workflow); compiler-binary distribution decided = **ADR-019**
 (#260, Releases + thin Deno/JSR shim, staged S1..S4) — S1/S2/S3

lib/codegen.ml

@@ -873,8 +873,9 @@ let rec gen_expr (ctx : context) (expr : expr) : (context * instr list) result =
            (added on-demand at module assembly; idx looked up in
            [ctx.wasi_func_indices]). The Unit arg satisfies the
            zero-param-fn collapse wart; it is evaluated but its value
-           is unused. String accessors (env_at/arg_at) need byte-level
-           wasm IR ops (currently absent) and are a tracked follow-up. *)
+           is unused. The companion string accessors `env_at`/`arg_at`
+           landed alongside the byte-level wasm IR extension — see
+           the case below. *)
         let wasi_name =
           if id.name = "env_count" then "environ_sizes_get"
           else "args_sizes_get"
@@ -894,6 +895,47 @@ let rec gen_expr (ctx : context) (expr : expr) : (context * instr list) result =
         in
         Ok (ctx_with_heap, code)
 
+      | ExprVar id when (id.name = "env_at" || id.name = "arg_at")
+                        && List.length args = 1 ->
+        (* ADR-015 S5 (#180): env_at(i: Int) / arg_at(i: Int) -> String.
+           Allocates a length-prefixed AS string and byte-copies the
+           i-th null-terminated entry from the WASI environ/argv
+           buffer. Uses [I32Load8U]/[I32Store8] (the byte-level wasm
+           IR extension landed alongside this slice). Pairs the
+           existing on-demand `*_sizes_get` import with the matching
+           `environ_get`/`args_get` import (registered above in the
+           `optional_wasi` table; deduped by wasi name). *)
+        let sizes_name, get_name =
+          if id.name = "env_at" then "environ_sizes_get", "environ_get"
+          else "args_sizes_get", "args_get"
+        in
+        let sizes_func_idx =
+          try List.assoc sizes_name ctx.wasi_func_indices
+          with Not_found -> 1
+        in
+        let get_func_idx =
+          try List.assoc get_name ctx.wasi_func_indices
+          with Not_found -> 2
+        in
+        let* (ctx0, arg_code) = gen_expr ctx (List.hd args) in
+        let (c1, n_local)       = alloc_local ctx0 ("__" ^ id.name ^ "_n") in
+        let (c2, scratch_local) = alloc_local c1  ("__" ^ id.name ^ "_scratch") in
+        let (c3, count_local)   = alloc_local c2  ("__" ^ id.name ^ "_count") in
+        let (c4, bufsize_local) = alloc_local c3  ("__" ^ id.name ^ "_bufsize") in
+        let (c5, ptrvec_local)  = alloc_local c4  ("__" ^ id.name ^ "_ptrvec") in
+        let (c6, src_local)     = alloc_local c5  ("__" ^ id.name ^ "_src") in
+        let (c7, dst_local)     = alloc_local c6  ("__" ^ id.name ^ "_dst") in
+        let (c8, result_local)  = alloc_local c7  ("__" ^ id.name ^ "_result") in
+        let (ctx_with_heap, heap_idx) = ensure_heap_ptr c8 in
+        let code =
+          arg_code @
+          Wasi_runtime.gen_str_at_via_get
+            heap_idx n_local scratch_local count_local bufsize_local
+            ptrvec_local src_local dst_local result_local
+            sizes_func_idx get_func_idx
+        in
+        Ok (ctx_with_heap, code)
+
       | ExprVar id when List.mem_assoc id.name ctx.variant_tags ->
         (* Enum constructor called as a function: Circle(5), Rect({x:1,y:2}), etc.
            Layout: [tag: i32][field1: i32][field2: i32]...
@@ -2549,13 +2591,25 @@ let generate_module ?loader (prog : program) : wasm_module result =
       false prog
   in
   let optional_wasi =
-    (* (guest_builtin_name, wasi_import_name, factory) — canonical order. *)
+    (* (guest_builtin_name, wasi_import_name, factory) — canonical order.
+       Multiple builtins MAY require the same WASI import (e.g. both
+       `env_count` and `env_at` need `environ_sizes_get`); the dedup
+       pass below keeps the first occurrence so each wasm import shows
+       up exactly once with a stable index. *)
     [ ("clock_now_ms", "clock_time_get",     Wasi_runtime.create_clock_time_get_import);
       ("env_count",    "environ_sizes_get",  Wasi_runtime.create_environ_sizes_get_import);
       ("arg_count",    "args_sizes_get",     Wasi_runtime.create_args_sizes_get_import);
+      ("env_at",       "environ_sizes_get",  Wasi_runtime.create_environ_sizes_get_import);
+      ("env_at",       "environ_get",        Wasi_runtime.create_environ_get_import);
+      ("arg_at",       "args_sizes_get",     Wasi_runtime.create_args_sizes_get_import);
+      ("arg_at",       "args_get",           Wasi_runtime.create_args_get_import);
     ]
-    |> List.filter_map
-         (fun (b, w, f) -> if uses b then Some (w, f ()) else None)
+    |> List.filter (fun (b, _, _) -> uses b)
+    |> List.fold_left
+         (fun acc (_, w, f) ->
+           if List.exists (fun (w', _) -> w' = w) acc then acc
+           else acc @ [(w, f ())])
+         []
     |> List.mapi (fun i (w, (imp, ty)) -> (i + 1, w, imp, ty))
   in
   let opt_types = List.map (fun (_, _, _, ty) -> ty) optional_wasi in

lib/typecheck.ml

@@ -1324,11 +1324,18 @@ let register_builtins (ctx : context) : unit =
     (TArrow (ty_int, QOmega, ty_int, ESingleton "Time"));
   (* ADR-015 S4b (#180): WASI environment / argv COUNTS. The Unit arg
      satisfies the zero-param-fn collapse wart (`fn()->T` lowers to
-     bare `T`; callable zero-arg builtins take `Unit -> R`). String
-     accessors (env_at/arg_at) need byte-level wasm IR ops — tracked
-     follow-up. Effect row `Time` (reserved). *)
+     bare `T`; callable zero-arg builtins take `Unit -> R`).
+     Effect row `Time` (reserved). *)
   bind_var ctx "env_count" (TArrow (ty_unit, QOmega, ty_int, ESingleton "Time"));
   bind_var ctx "arg_count" (TArrow (ty_unit, QOmega, ty_int, ESingleton "Time"));
+  (* ADR-015 S5 (#180): WASI environment / argv STRING ACCESSORS. Returns
+     the i-th entry as a length-prefixed AS string. Lowered via
+     `environ_get`/`args_get` + a byte-level scan + byte-copy, which
+     became expressible once `I32Load8U`/`I32Store8` joined the wasm IR.
+     Index out-of-bounds is UB at this layer — the guest is expected to
+     bound-check against `env_count(())`/`arg_count(())`. *)
+  bind_var ctx "env_at" (TArrow (ty_int, QOmega, ty_string, ESingleton "Time"));
+  bind_var ctx "arg_at" (TArrow (ty_int, QOmega, ty_string, ESingleton "Time"));
   bind_var ctx "eprint" (TArrow (ty_string, QOmega, ty_unit, ESingleton "IO"));
   bind_var ctx "eprintln" (TArrow (ty_string, QOmega, ty_unit, ESingleton "IO"));
   bind_var ctx "read_line"

lib/wasi_runtime.ml

@@ -359,8 +359,8 @@ let gen_print_str (heap_ptr_global : int) (str_ptr_local : int) (fd_write_idx :
     Signature: `(envc_out: i32, envbuf_size_out: i32) -> errno: i32`.
     Writes the env-var count and the total byte size of the
     null-terminated `KEY=VAL\0…` buffer the next call would need.
-    String accessor (`env_at`) is gated on byte-level wasm IR ops,
-    deferred to a follow-up slice. *)
+    Paired with `environ_get` (created by
+    {!create_environ_get_import}) for the `env_at` string accessor. *)
 let create_environ_sizes_get_import () : import * func_type =
   let func_type = {
     ft_params = [I32; I32];  (* envc_out_ptr, envbuf_size_out_ptr *)
@@ -413,3 +413,159 @@ let gen_count_via_sizes_get
     LocalGet scratch_local;
     I32Load (2, 0);
   ]
+
+(** Create the WASI `environ_get` import (ADR-015 S5, #180).
+    Signature: `(environ_ptr_ptr: i32, environ_buf_ptr: i32) -> errno: i32`.
+    Fills two regions: a vector of pointers (one per env-var, written
+    at `environ_ptr_ptr`) and a contiguous buffer of null-terminated
+    `KEY=VAL` strings (written at `environ_buf_ptr`). The sizes that
+    must be allocated are reported by `environ_sizes_get`. *)
+let create_environ_get_import () : import * func_type =
+  let func_type = {
+    ft_params = [I32; I32];  (* environ_ptr_ptr, environ_buf_ptr *)
+    ft_results = [I32];      (* errno *)
+  } in
+  let import = {
+    i_module = "wasi_snapshot_preview1";
+    i_name = "environ_get";
+    i_desc = ImportFunc 0;
+  } in
+  (import, func_type)
+
+(** Create the WASI `args_get` import (ADR-015 S5, #180).
+    Signature: `(argv_ptr_ptr: i32, argv_buf_ptr: i32) -> errno: i32`.
+    Same shape as `environ_get`. *)
+let create_args_get_import () : import * func_type =
+  let func_type = {
+    ft_params = [I32; I32];
+    ft_results = [I32];
+  } in
+  let import = {
+    i_module = "wasi_snapshot_preview1";
+    i_name = "args_get";
+    i_desc = ImportFunc 0;
+  } in
+  (import, func_type)
+
+(** Emit `env_at(i)` / `arg_at(i)`: fetch the i-th entry from the WASI
+    environ/argv vector and return it as a length-prefixed AffineScript
+    string. Sequence:
+      1. `*_sizes_get(&count, &bufsize)`
+      2. allocate `count*4` bytes for the pointer vector + `bufsize`
+         bytes for the string buffer
+      3. `*_get(ptrvec, ptrvec + count*4)`
+      4. resolve `src = ptrvec[i]`
+      5. scan `src` for the null terminator to compute length
+      6. allocate `(4 + length)` bytes for the result string,
+         store length at +0, byte-copy `src..src+length` to `result+4`
+      7. leave the result pointer on the stack
+
+    The byte loops use `I32Load8U`/`I32Store8` (added with the
+    byte-level wasm IR extension). The caller has placed the index `i`
+    on the stack; this helper consumes it via [LocalSet n_local].
+
+    All locals must be pre-allocated by the caller (8 in total). The
+    helper itself does not modify the type or scope context — it only
+    emits instructions. *)
+let gen_str_at_via_get
+    (heap_ptr_global : int)
+    (n_local : int)
+    (scratch_local : int)
+    (count_local : int)
+    (bufsize_local : int)
+    (ptrvec_local : int)
+    (src_local : int)
+    (dst_local : int)
+    (result_local : int)
+    (sizes_func_idx : int)
+    (get_func_idx : int)
+    : instr list =
+  [
+    (* Index `i` is on the stack from the caller's arg_code. *)
+    LocalSet n_local;
+
+    (* --- Phase 1: sizes_get -> count, bufsize --- *)
+    GlobalGet heap_ptr_global;
+    I32Const 8l; I32Add;
+    GlobalSet heap_ptr_global;
+    GlobalGet heap_ptr_global;
+    I32Const 8l; I32Sub;
+    LocalSet scratch_local;
+    LocalGet scratch_local;                       (* count_ptr *)
+    LocalGet scratch_local; I32Const 4l; I32Add;  (* bufsize_ptr *)
+    Call sizes_func_idx;
+    Drop;
+    LocalGet scratch_local; I32Load (2, 0); LocalSet count_local;
+    LocalGet scratch_local; I32Load (2, 4); LocalSet bufsize_local;
+
+    (* --- Phase 2: allocate ptrvec (count*4) + bytebuf (bufsize) --- *)
+    GlobalGet heap_ptr_global;
+    LocalSet ptrvec_local;
+    GlobalGet heap_ptr_global;
+    LocalGet count_local; I32Const 4l; I32Mul;
+    LocalGet bufsize_local; I32Add;
+    I32Add;
+    GlobalSet heap_ptr_global;
+
+    (* --- Phase 3: get(ptrvec, ptrvec + count*4) --- *)
+    LocalGet ptrvec_local;
+    LocalGet ptrvec_local; LocalGet count_local; I32Const 4l; I32Mul; I32Add;
+    Call get_func_idx;
+    Drop;
+
+    (* --- Phase 4: src = *(ptrvec + i*4) --- *)
+    LocalGet ptrvec_local;
+    LocalGet n_local; I32Const 4l; I32Mul; I32Add;
+    I32Load (2, 0);
+    LocalSet src_local;
+
+    (* --- Phase 5: scan for null terminator. Use scratch as cursor. --- *)
+    LocalGet src_local; LocalSet scratch_local;
+    Block (BtEmpty, [
+      Loop (BtEmpty, [
+        LocalGet scratch_local;
+        I32Load8U (0, 0);
+        I32Eqz; BrIf 1;                           (* exit on 0 byte *)
+        LocalGet scratch_local; I32Const 1l; I32Add;
+        LocalSet scratch_local;
+        Br 0
+      ])
+    ]);
+    (* length = cursor - src (excludes the null terminator).
+       Stash it back into count_local, which we are done with. *)
+    LocalGet scratch_local; LocalGet src_local; I32Sub;
+    LocalSet count_local;
+
+    (* --- Phase 6: allocate (4 + length) for the AS string --- *)
+    GlobalGet heap_ptr_global;
+    LocalSet result_local;
+    GlobalGet heap_ptr_global;
+    I32Const 4l; LocalGet count_local; I32Add;
+    I32Add;
+    GlobalSet heap_ptr_global;
+
+    (* Store length at result+0. *)
+    LocalGet result_local;
+    LocalGet count_local;
+    I32Store (2, 0);
+
+    (* --- Phase 7: byte-copy src..src+length -> result+4 ---
+       Reuses scratch as src cursor and count_local as the loop count. *)
+    LocalGet src_local; LocalSet scratch_local;
+    LocalGet result_local; I32Const 4l; I32Add; LocalSet dst_local;
+    Block (BtEmpty, [
+      Loop (BtEmpty, [
+        LocalGet count_local; I32Eqz; BrIf 1;
+        LocalGet dst_local;
+        LocalGet scratch_local; I32Load8U (0, 0);
+        I32Store8 (0, 0);
+        LocalGet scratch_local; I32Const 1l; I32Add; LocalSet scratch_local;
+        LocalGet dst_local; I32Const 1l; I32Add; LocalSet dst_local;
+        LocalGet count_local; I32Const 1l; I32Sub; LocalSet count_local;
+        Br 0
+      ])
+    ]);
+
+    (* --- Result: leave the string pointer on the stack. --- *)
+    LocalGet result_local;
+  ]

lib/wasm.ml

@@ -51,10 +51,25 @@ type instr =
   | I64Load of int * int
   | F32Load of int * int
   | F64Load of int * int
+  | I32Load8S of int * int   (** load 1 byte, sign-extend to i32 *)
+  | I32Load8U of int * int   (** load 1 byte, zero-extend to i32 *)
+  | I32Load16S of int * int  (** load 2 bytes, sign-extend to i32 *)
+  | I32Load16U of int * int  (** load 2 bytes, zero-extend to i32 *)
+  | I64Load8S of int * int
+  | I64Load8U of int * int
+  | I64Load16S of int * int
+  | I64Load16U of int * int
+  | I64Load32S of int * int
+  | I64Load32U of int * int
   | I32Store of int * int
   | I64Store of int * int
   | F32Store of int * int
   | F64Store of int * int
+  | I32Store8 of int * int   (** store low 1 byte of i32 *)
+  | I32Store16 of int * int  (** store low 2 bytes of i32 *)
+  | I64Store8 of int * int
+  | I64Store16 of int * int
+  | I64Store32 of int * int
   | MemorySize
   | MemoryGrow
 

lib/wasm_encode.ml

@@ -123,10 +123,25 @@ let rec add_instr buf = function
   | I64Load (align, offset) -> add_u8 buf 0x29; add_memarg buf align offset
   | F32Load (align, offset) -> add_u8 buf 0x2A; add_memarg buf align offset
   | F64Load (align, offset) -> add_u8 buf 0x2B; add_memarg buf align offset
+  | I32Load8S (align, offset) -> add_u8 buf 0x2C; add_memarg buf align offset
+  | I32Load8U (align, offset) -> add_u8 buf 0x2D; add_memarg buf align offset
+  | I32Load16S (align, offset) -> add_u8 buf 0x2E; add_memarg buf align offset
+  | I32Load16U (align, offset) -> add_u8 buf 0x2F; add_memarg buf align offset
+  | I64Load8S (align, offset) -> add_u8 buf 0x30; add_memarg buf align offset
+  | I64Load8U (align, offset) -> add_u8 buf 0x31; add_memarg buf align offset
+  | I64Load16S (align, offset) -> add_u8 buf 0x32; add_memarg buf align offset
+  | I64Load16U (align, offset) -> add_u8 buf 0x33; add_memarg buf align offset
+  | I64Load32S (align, offset) -> add_u8 buf 0x34; add_memarg buf align offset
+  | I64Load32U (align, offset) -> add_u8 buf 0x35; add_memarg buf align offset
   | I32Store (align, offset) -> add_u8 buf 0x36; add_memarg buf align offset
   | I64Store (align, offset) -> add_u8 buf 0x37; add_memarg buf align offset
   | F32Store (align, offset) -> add_u8 buf 0x38; add_memarg buf align offset
   | F64Store (align, offset) -> add_u8 buf 0x39; add_memarg buf align offset
+  | I32Store8 (align, offset) -> add_u8 buf 0x3A; add_memarg buf align offset
+  | I32Store16 (align, offset) -> add_u8 buf 0x3B; add_memarg buf align offset
+  | I64Store8 (align, offset) -> add_u8 buf 0x3C; add_memarg buf align offset
+  | I64Store16 (align, offset) -> add_u8 buf 0x3D; add_memarg buf align offset
+  | I64Store32 (align, offset) -> add_u8 buf 0x3E; add_memarg buf align offset
   | MemorySize -> add_u8 buf 0x3F; add_u8 buf 0x00
   | MemoryGrow -> add_u8 buf 0x40; add_u8 buf 0x00
   | I32Const v -> add_u8 buf 0x41; add_sleb32 buf v

tests/codegen/arg_at.affine

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: MPL-2.0
+// ADR-015 S5 (#180): arg_at(i) smoke. Same shape as env_at; differs
+// only in the underlying WASI import (`args_sizes_get` + `args_get`).
+// Indexing 1 (not 0) exercises the `n_local * 4` pointer-vector
+// offset path that env_at(0) would short-circuit.
+pub fn main() -> Int / { Time } {
+  string_length(arg_at(1))
+}

tests/codegen/env_at.affine

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: MPL-2.0
+// ADR-015 S5 (#180): env_at(i) smoke. Lowers to a `*_sizes_get` +
+// `environ_get` pair, then byte-scans the i-th entry to compute
+// length and byte-copies it into a length-prefixed AS string.
+// Returning `string_length` proves the scan terminated at the
+// host-written null terminator at the expected offset.
+pub fn main() -> Int / { Time } {
+  string_length(env_at(0))
+}