Investigating parsing error for struct/union

[Aditya <yashsri421@xxxxxxxxx>]

Hi Jonathan, Lukas

While investigating "error: Cannot parse struct or union!", I
discovered few more patterns causing this error:
1) A large part of this error(~80%) occurs due to the presence of one
or more lines(such as '#define' lines) between commented code and
struct declaration.

For e.g., in include/linux/platform_data/apds990x.h:

/**
 * struct apds990x_chip_factors - defines effect of the cover window
 * @ga: Total glass attenuation
 * @cf1: clear channel factor 1 for raw to lux conversion
 * @irf1: IR channel factor 1 for raw to lux conversion
 * @cf2: clear channel factor 2 for raw to lux conversion
 * @irf2: IR channel factor 2 for raw to lux conversion
 * @df: device factor for conversion formulas
 *
 * Structure for tuning ALS calculation to match with environment.
 * Values depend on the material above the sensor and the sensor
 * itself. If the GA is zero, driver will use uncovered sensor default
values
 * format: decimal value * APDS_PARAM_SCALE except df which is plain
integer.
 */
#define APDS_PARAM_SCALE 4096
struct apds990x_chip_factors {
	int ga;
	int cf1;
	int irf1;
	int cf2;
	int irf2;
	int df;
};


2) If struct does not contain any members, for e.g., in
include/linux/xz.h:

/**
 * struct xz_dec - Opaque type to hold the XZ decoder state
 */
struct xz_dec;

Here, it causes error as the curly braces and members expected by the
regex, are absent.
This kind of use has also been made in include/linux/zstd.h:

/**
 * struct ZSTD_DDict - a digested dictionary to be used for decompression
 */
typedef struct ZSTD_DDict_s ZSTD_DDict;


3) Different Syntax than expected. For e.g.:
    a) struct xyz struct_name {} syntax. This syntax has been used in
arch/arm/mach-omap2/omap_hwmod_common_data.c file
    b) "static __maybe_unused const struct st_sensors_platform_data
default_press_pdata = {" in drivers/iio/pressure/st_pressure.h.
This kind of syntax has also been used in
drivers/iio/accel/st_accel.h, and drivers/iio/gyro/st_gyro.h

I wanted to take your opinion if we should extend support for any of
these syntax, causing the error. If not, perhaps we can make the
documentation a bit clear, atleast for (1), which causes most of these
errors; so as to not include any lines between comment and struct
declaration.

What do you think?

Thanks
Aditya