45 lines
1.7 KiB
Markdown
45 lines
1.7 KiB
Markdown
# 0116 — writes through module consts are not rejected (struct-const write bus-errors at runtime)
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> **RESOLVED** (2026-06-11, PLAN-CONST-AGG step 2). The assignment
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> lowering rejects any target chain ROOTED at a constant — a
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> const-flagged global (array consts, #run consts) or a module value
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> const (struct consts incl.) — with `cannot assign through constant
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> 'X'`. A deref along the chain (`p.*`) breaks the root (pointer writes
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> are permanently unchecked — the documented pointer contract, specs.md
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> §Pointer Types); a local
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> shadowing the const name stays writable. Regression test:
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> examples/1162-diagnostics-const-write-rejected.sx (struct field — the
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> crash repro, array element, compound, bare scalar).
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**Symptom.** Assigning through a module-level constant compiles silently.
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For a struct-literal const the store lands in read-only memory and the
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program crashes at runtime.
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- **Observed**: compiles; `Bus error` at runtime on the store.
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- **Expected**: compile-time diagnostic — `cannot assign through constant
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'WHITE'`.
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## Reproduction
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```sx
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#import "modules/std.sx";
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Color :: struct { r, g, b: s64; }
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WHITE :: Color.{ r = 255, g = 255, b = 255 };
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main :: () {
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WHITE.r = 0; // compiles; bus error at runtime
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print("{}\n", WHITE.r);
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}
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```
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(Copies are fine and stay fine: `w := WHITE; w.r = 0;` mutates the copy.)
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## Fix
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Scheduled as **step 2 of `current/PLAN-CONST-AGG.md`** (aggregate consts):
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one rejection rule for assignment/compound-assignment targets whose ROOT
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identifier resolves to a module const (module_const_map author) or a
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const-flagged global (the array consts that plan introduces). Diagnose at
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the assignment site; pin the repro above as a diagnostics example.
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