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codegen: iterate over a slice whose elements are not base.u8 fails with "cannot convert Wuffs type" #259
Description
Hi,
General context of this work is in #188, including the AI-disclaimers.
No in-tree codec is affected by this issue; std never declares a variable or argument of a slice type other than slice base.u8, and never iterates over one. Claude judges the prospective risk of it as "low": generation fails, so the accepted program cannot be built.
The following is Sol 6.1's writeup of the issue. Includes a repro, the generation error, the same error for seven more element types, parameter-only and read-only-slice variants, and a base.u8 control that generates. The cause is reasoned from source.
The checker accepts the non-base.u8 slice types below and iterate over them, but cgen has a C type only for slices and tables of unrefined base.u8. A parameter or local of an unsupported slice type is enough to make generation fail, even without iterate.
Minimal repro:
pub struct repro?(
visits : base.u32,
)
pub func repro.run!(s: slice base.u16) {
var p : slice base.u16
iterate (p = args.s)(length: 1, advance: 1, unroll: 1) {
p[0] = p[0] ^ 0x1234
this.visits ~mod+= 1
}
}
Actual: at b36a81d3, wuffs-c gen -package_name repro repro.wuffs exits 1:
cannot convert Wuffs type "slice base.u16" to C
The same iterate fails for slices with element types base.u32, base.u64, base.i8, base.i16, base.i32, base.i64 and array[3] base.u8, with the matching slice type in the message. The parameter alone is enough: pub func repro.run!(s: slice base.u16) {} fails the same way, as does a roslice base.u16 parameter. Control: the same iterate over a slice base.u8 generates.
Expected: support slices of other element types, at least as iterate subjects, or reject them in the checker where cgen cannot lower them.
Cause: writeCTypeName has a C type only for slices and tables of unrefined base.u8, with a TODO:
Lines 576 to 606 in b36a81d
#198 reports that slice expressions of wider elements are lowered with byte offsets where they do generate.
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