# 0116 — writes through module consts are not rejected (struct-const write bus-errors at runtime)

> **RESOLVED** (2026-06-11, PLAN-CONST-AGG step 2). The assignment
> lowering rejects any target chain ROOTED at a constant — a
> const-flagged global (array consts, #run consts) or a module value
> const (struct consts incl.) — with `cannot assign through constant
> 'X'`. A deref along the chain (`p.*`) breaks the root (pointer writes
> are permanently unchecked — the documented pointer contract, specs.md
> §Pointer Types); a local
> shadowing the const name stays writable. Regression test:
> examples/1162-diagnostics-const-write-rejected.sx (struct field — the
> crash repro, array element, compound, bare scalar).

**Symptom.** Assigning through a module-level constant compiles silently.
For a struct-literal const the store lands in read-only memory and the
program crashes at runtime.

- **Observed**: compiles; `Bus error` at runtime on the store.
- **Expected**: compile-time diagnostic — `cannot assign through constant
  'WHITE'`.

## Reproduction

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

Color :: struct { r, g, b: i64; }
WHITE :: Color.{ r = 255, g = 255, b = 255 };

main :: () {
    WHITE.r = 0;     // compiles; bus error at runtime
    print("{}\n", WHITE.r);
}
```

(Copies are fine and stay fine: `w := WHITE; w.r = 0;` mutates the copy.)

## Fix

Scheduled as **step 2 of `current/PLAN-CONST-AGG.md`** (aggregate consts):
one rejection rule for assignment/compound-assignment targets whose ROOT
identifier resolves to a module const (module_const_map author) or a
const-flagged global (the array consts that plan introduces). Diagnose at
the assignment site; pin the repro above as a diagnostics example.
