artiq/symbol_resolver: Handle type of zero-length lists

nac3artiq/src/symbol_resolver.rs

@@ -991,8 +991,15 @@ impl InnerResolver {
                 }
                 _ => unreachable!("must be list"),
             };
-            let ty = ctx.get_llvm_type(generator, elem_ty);
             let size_t = generator.get_size_type(ctx.ctx);
+            let ty = if len == 0
+                && matches!(&*ctx.unifier.get_ty_immutable(elem_ty), TypeEnum::TVar { .. })
+            {
+                // The default type for zero-length lists of unknown element type is size_t
+                size_t.into()
+            } else {
+                ctx.get_llvm_type(generator, elem_ty)
+            };
             let arr_ty = ctx
                 .ctx
                 .struct_type(&[ty.ptr_type(AddressSpace::default()).into(), size_t.into()], false);

nac3core/src/codegen/expr.rs

@@ -951,9 +951,9 @@ pub fn destructure_range<'ctx>(
 /// Allocates a List structure with the given [type][ty] and [length]. The name of the resulting
 /// LLVM value is `{name}.addr`, or `list.addr` if [name] is not specified.
 ///
-/// Setting `ty` to [`None`] implies that the list does not have a known element type, which is only
+/// Setting `ty` to [`None`] implies that the list is empty **and** does not have a known element
-/// valid for empty lists. It is undefined behavior to generate a sized list with an unknown element
+/// type, and will therefore set the `list.data` type as `size_t*`. It is undefined behavior to
-/// type.
+/// generate a sized list with an unknown element type.
 pub fn allocate_list<'ctx, G: CodeGenerator + ?Sized>(
     generator: &mut G,
     ctx: &mut CodeGenContext<'ctx, '_>,