Improve codegen for C style enums

[jrmuizel]

Because C style enums are loosely typed and aren't typed they tend to be expressed in C more loosely than they are actually used.

For example, one might have an enum like: https://github.com/zyantific/zydis/blob/4dd632463c122ee85ba9c5f0c1e56ac748bf246e/include/Zydis/DecoderTypes.h#L478

but that enum is stored in a ZydisCPUFlagAction/uint8_t (https://github.com/zyantific/zydis/blob/4dd632463c122ee85ba9c5f0c1e56ac748bf246e/include/Zydis/DecoderTypes.h#L473, https://github.com/zyantific/zydis/blob/4dd632463c122ee85ba9c5f0c1e56ac748bf246e/include/Zydis/DecoderTypes.h#L816).

It would be nice to be able map the ZydisCPUFlagAction type to the ZydisCPUFlagActions enum somehow.

[fitzgen]

Can you be more specific, and give an example definittion, its current translation, and what you would prefer instead? Which style of enum translation are you using? Default (into constants)? --constified-enum-modules? --rustified-enum-modules?

[jrmuizel]

From above: https://github.com/zyantific/zydis/blob/4dd632463c122ee85ba9c5f0c1e56ac748bf246e/include/Zydis/DecoderTypes.h#L478

So currently constified enums are being used so we get something like: (notice the difference between the field type ZydisCPUFlagAction and the enum type ZydisCPUFlagActions).

/// @brief   Defines the @c ZydisCPUFlagAction datatype.
pub type ZydisCPUFlagAction = u8;

pub const ZYDIS_CPUFLAG_ACTION_NONE: ZydisCPUFlagActions = 0;
pub const ZYDIS_CPUFLAG_ACTION_TESTED: ZydisCPUFlagActions = 1;
pub const ZYDIS_CPUFLAG_ACTION_MODIFIED: ZydisCPUFlagActions = 2;
pub const ZYDIS_CPUFLAG_ACTION_SET_0: ZydisCPUFlagActions = 3;
pub const ZYDIS_CPUFLAG_ACTION_SET_1: ZydisCPUFlagActions = 4;
pub const ZYDIS_CPUFLAG_ACTION_UNDEFINED: ZydisCPUFlagActions = 5;
pub const ZYDIS_CPUFLAG_ACTION_MAX_VALUE: ZydisCPUFlagActions = 5;
/// @brief   Values that represent CPU-flag actions.
pub type ZydisCPUFlagActions = ::std::os::raw::c_uint;

I'd like to be able to turn this into something like:

#[repr(u8)]
enum ZydisCPUFlagAction {
  NONE = 0,
  TESTED = 1,
  MODIFIED = 2,
  SET_0 = 3,
  SET_1 = 4,
  UNDEFINED = 5,
  MAX_VALUE = 5,
}

/// @brief   Values that represent CPU-flag actions.
pub type ZydisCPUFlagActions = ::std::os::raw::c_uint;

[fitzgen]

Using --rustified-enum '.*', this is outputted:

/* automatically generated by rust-bindgen */

pub const ZydisCPUFlagActions_ZYDIS_CPUFLAG_ACTION_MAX_VALUE : ZydisCPUFlagActions = ZydisCPUFlagActions :: ZYDIS_CPUFLAG_ACTION_UNDEFINED ;
#[repr(u32)]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum ZydisCPUFlagActions {
    ZYDIS_CPUFLAG_ACTION_NONE = 0,
    ZYDIS_CPUFLAG_ACTION_TESTED = 1,
    ZYDIS_CPUFLAG_ACTION_MODIFIED = 2,
    ZYDIS_CPUFLAG_ACTION_SET_0 = 3,
    ZYDIS_CPUFLAG_ACTION_SET_1 = 4,
    ZYDIS_CPUFLAG_ACTION_UNDEFINED = 5,
}

You have to be careful, though -- and this is why rustified-enum isn't the default -- because rustc relies on the assumption that there will never be a ZydisCPUFlagsActions whose value is 6, and breaking that assumption is UB.

[jrmuizel]

I need ZydisCPUFlagAction to be an enum not ZydisCPUFlagActions and I need it to be repr(u8)

[lilianmoraru]

Shouldn't the enums be "i32" by default?(These are "int" by default in C/C++), unless the code specifically wants another type? Seems weird that bindgen decides to use u32 when the value is not negative, while the C code uses i32(in C you could overflow, maybe intentionally and in Rust it will just fit).

[emilio]

Bindgen uses whatever enum type comes from clang, so if it's a u32, clang is picking a u32 type for it.

[emilio]

Also, signed integer overflow is auto-UB even in C, so it's not supposed to overflow :)