Objects containing `importc` fields without `completeStruct` fail to
compile when used as const/static. The C codegen generates "aggregate
initialization" which is invalid for opaque types.
Fixes#25405.
Nim code:
```nim
type
OpaqueInt {.importc: "_Atomic int", nodecl.} = object
ContainsImportc = object
normal: int
opaque: OpaqueInt
const c = default(ContainsImportc)
```
Resulting C code:
```c
// Invalid C - cannot aggregate-init opaque type
NIM_CONST ContainsImportc c = {((NI) 0), {}};
^^ error: illegal initializer type
```
## Solution
Fix in `ccgexprs.nim`:
1. Skip opaque importc fields when building aggregate initializers
2. Use "designated initializers" (`siNamedStruct`) when opaque fields
are present to avoid positional misalignment
```c
// Valid C:
// - opaque field is omitted and implicitly zero-initialized by C
// - other fields are explitly named and initialized
NIM_CONST ContainsImportc c = {.normal = ((NI) 0)};
```
This correctly handles the case where the opaque fields might be in any
order.
A field is considered "opaque importc" if:
- Has `sfImportc` flag
- Does NOT have `tfCompleteStruct` flag
- Either has `tfIncompleteStruct` OR is an object with no visible fields
The `containsOpaqueImportcField` proc recursively checks all object
fields, including nested objects and variant branches.
Anonymous unions (from variant objects) are handled by passing an empty
field name, which skips the `.fieldname = ` prefix since C anonymous
unions have no field name.
Note that initialization for structs without opaque importc fields
remains the same as before this changeset.
## Test Coverage
`tests/ccgbugs/timportc_field_init.nim` covers:
- Simple struct with one importc field
- Nested struct containing struct with importc field
- Variant object (case object) with importc field in a branch
- Array of structs with importc fields
- Tuple containing struct with importc field
- `completeStruct` importc types (still use aggregate init)
- Sandwich case (opaque field between two non-opaque fields)
- Fields with different C names (`{.importc: "c_name".}`, `{.exportc.}`)
- `{.packed.}` structs with opaque fields
- `{.union.}` types with opaque fields
- Deep nesting (3+ levels)
- Multiple opaque fields with renamed fields between them
780c9eeef0