# 0125 — any_to_string's array arms materialize every interned array type by value

## Symptom

A program that (a) interns any large (~64KB+) array type and (b) uses
`{}` formatting anywhere — `print("{}\n", 5)` of a plain int is enough —
crashes `sx build` inside libLLVM (`DAGCombiner::visitMERGE_VALUES` →
`SelectionDAG::ReplaceAllUsesWith`), and makes `sx run` (-O0) take ~18s
to compile a trivial file. The two triggers are independent: the array
need never be printed, sliced, or passed anywhere near the format call.

- **Observed**: segfault under `sx build`; multi-second compiles under
  `sx run`.
- **Expected**: formatting an int is unaffected by an unrelated large
  array type; printing the array itself formats in place.

Root cause shape: `any_to_string`'s comptime type-switch
(library/modules/std/fmt.sx, `case array:` arm) expands one arm per
interned array type, and each arm is
`array_to_string(cast(type) val)`:

1. the `cast(type) val` unbox loads the WHOLE array from the Any
   payload pointer (`coerceFromI64`, src/ir/emit_llvm.zig ~2240,
   `ua.load`),
2. the call passes the array BY VALUE to the `array_to_string` mono,
3. the mono spills its by-value param to an alloca and (since the
   param is an SSA value, not addressable storage) reads elements via
   `index_get` on the value — copy-whole-array per element.

LLVM's legalizer scalarizes each whole-aggregate op into one
SelectionDAG node per element; at ~64K elements the DAG combiner
recurses to death (the sibling of issue 0124, which fixed the
local-variable shapes: `---` undef store and index reads on
addressable storage).

## Reproduction

```sx
#import "modules/std.sx";

f :: () {
    buf : [65536]u8 = ---;
    buf[0] = 1;
    out(string.{ ptr = @buf[0], len = 1 });
}

main :: () -> i32 {
    f();
    print("{}\n", 5);
    return 0;
}
```

Observed (with 0124's fix in place): `sx build` segfaults in libLLVM;
`sx ir` shows the giant arm inside `@any_to_string`:

```llvm
%ua.load = load [65536 x i8], ptr %ua.ptr, align 1
%call = call { ptr, i64 } @array_to_string__AR_65536_u8(ptr %0, [65536 x i8] %ua.load)
```

## Investigation prompt

The fix needs the array formatting chain to never materialize the
array as a first-class value. The Any payload for an array IS a
pointer to its storage (that is what `coerceFromI64` intToPtr+loads),
so the arm has everything it needs to format in place. Plausible
routes, most contained first:

1. Lower the `case array:` arm to a slice view: box the payload
   pointer + the array's element count as a `[]elem` and call
   `slice_to_string` (slices unbox as a 16-byte {ptr,len} — no giant
   ops). Needs the element type at arm-expansion time — the comptime
   type-switch already has the concrete array TypeId in hand; an
   `element_type(T)`-style comptime accessor may need to be added for
   the sx-level spelling, or the arm can be synthesized in the
   compiler where both pieces are known.
2. Teach `array_to_string :: (a: $T)` monos (and the unbox `cast`) an
   indirect ABI for array-typed params — bigger blast radius: touches
   call emission, param spills, and many `.ir` snapshots.

Suspected files: src/ir/lower/comptime.zig / lower/call.zig (the
type-switch arm expansion and `cast(type)` lowering),
src/ir/emit_llvm.zig `coerceFromI64`,
library/modules/std/fmt.sx (`any_to_string`, `array_to_string`).

Verification: the repro builds and runs printing `5`; printing the
array itself (`print("{}\n", buf)` on a small array) still renders
element lists (pinned by 0101/0904 et al.); `zig build test` and
`bash tests/run_examples.sh` green; the repro pinned as an example.
