e78320637f
Stream B1 B1.2 (Io capability + context.io + Future + cancel) is blocked on two newly-discovered, independent compiler bugs, both with standalone repros: - 0150: a `void` struct field crashes the compiler with an unsized-type SIGTRAP in LLVM getTypeSizeInBits. Blocks `Future(void)` -> `timeout`. - 0151: a type-var inferred from a fn-pointer parameter's RETURN type is not bound as a usable type in the function body (`unknown type 'R'`). Blocks the central `async(io, worker: ($A)->$R, arg)` free-fn's `Future(R)`. The B1.2 design itself is validated end-to-end (the Io protocol threaded on Context like Allocator, the stateless blocking CBlockingIo default, both __sx_default_context materializers, and `context.io.now_ms()` all work live). Only the async/await/timeout ergonomic layer hits the two bugs. Per the IMPASSABLE STOP rule, all B1.2 working changes were reverted (master green, 726/0) and the work paused pending fixes; WIP is saved at .sx-tmp/b12-wip/. Checkpoint + plan updated to mark B1.2 BLOCKED with full resume notes.
73 lines
3.4 KiB
Markdown
73 lines
3.4 KiB
Markdown
# 0150 — a `void` struct field crashes the compiler (unsized-type SIGTRAP in LLVM)
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## Status
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OPEN — surfaced by Stream B1 (fibers) B1.2: `Future(void)` (needed by
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`timeout(io, ms) -> Future(void)`) instantiates a struct with a `result: void`
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field, which hits this bug. Independent of the fibers work (a plain
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`struct { v: void; }` reproduces it standalone).
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## Symptom
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Declaring or instantiating any struct that has a field of type `void` aborts the
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compiler with `SIGTRAP` (exit 133/134) — no sx diagnostic. The trap is LLVM's
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`llvm_unreachable("Cannot getTypeInfo() on a type that is unsized!")`:
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```
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libLLVM`llvm::DataLayout::getTypeSizeInBits + 912 brk #0x1 (EXC_BREAKPOINT)
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```
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Reached via `declareFunction` → `toLLVMType(func.ret)` when a function returns
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such a struct, or directly when laying out the struct.
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Observed: SIGTRAP, no output, no diagnostic.
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Expected: either zero-size the `void` field (a `void`/zero-sized field is a
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legitimate construct — cf. Zig) OR emit a clean type diagnostic
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("a struct field may not have type `void`") — never a raw backend crash.
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## Reproduction
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```sx
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#import "modules/std.sx";
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Holder :: struct { v: void; ok: bool; }
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main :: () -> i32 {
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h : Holder = .{ ok = true };
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if h.ok { print("ok\n"); }
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return 0;
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}
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```
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`./zig-out/bin/sx run repro.sx` → SIGTRAP (exit 133), no output.
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Also reproduces through a generic: `Box :: struct($T: Type) { v: T; }` then
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`Box(void)` — i.e. any monomorphization that binds a struct field to `void`.
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## Suspected area
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- `src/backend/llvm/types.zig` `toLLVMTypeInfo` (struct field loop ~line 111):
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a `void` field's LLVM type is the unsized `void` type, then `getTypeSizeInBits`
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on the enclosing struct traps.
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- The type layout / size code (`src/ir/types.zig` `typeSizeBytes` and the LLVM
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struct builder) should treat a `void` field as zero-sized (skip it in the LLVM
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struct, size 0, align 1) — the same way a zero-field struct is handled.
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## Investigation prompt (paste into a fresh session)
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> A `void` struct field crashes the sx compiler with an unsized-type SIGTRAP in
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> LLVM `getTypeSizeInBits` (no diagnostic). Repro: `issues/0150-...` (run it →
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> exit 133). Decide the semantics: a `void` field should be ZERO-SIZED (preferred
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> — it is a legitimate construct, e.g. `Future(void).result`), laid out as
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> nothing (size 0, align 1) and OMITTED from the LLVM struct body; OR, if
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> zero-sized fields are out of scope, a clean front-end diagnostic ("a struct
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> field may not have type `void`, found in field `<name>` of `<Struct>`") before
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> emission — NEVER a backend trap. Likely sites: `src/backend/llvm/types.zig`
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> `toLLVMTypeInfo` (skip `void` fields when building the LLVM struct element
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> list) + `src/ir/types.zig` size/align (`typeSizeBytes`/align: a `void` field
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> contributes 0). If choosing the diagnostic route, add it where struct fields
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> are validated at type-resolution time. Verify: the repro prints `ok` (zero-size
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> route) or emits the diagnostic + clean exit 1 (diagnostic route); then move the
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> repro into `examples/` as a regression test.
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## Why this matters for B1 (fibers)
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`Future($R)` with `$R = void` is the natural shape for `timeout(io, ms) ->
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Future(void)` (B1.2 spec) and for any future-of-no-value. B1.2 deferred
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`timeout` pending this fix rather than route around it with a substitute
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non-void shape (which would hide the bug). Once 0150 lands, re-add `timeout`
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with `Future(void)` (see the saved WIP at `.sx-tmp/b12-wip/io.sx`).
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