libbpf-cargo: Generate C enums as custom types with const fields

[javierhonduco]

Rather than generating Rust enums from C enums, which might not be correct, as for example, C allows enum variants to share a value while Rust does not.

This fixes #982.

Test Plan

Verified that tests pass in CI in my fork and that my project compiles fine and works will with this change.

[danielocfb]

Thanks for working on this fix! Overall this looks great, but I think I spotted an issue with the handling of default values. Please take another look. Also, can you please add a test for the generation of a Rust enum for a C correspondent with two variants of the same value?

[javierhonduco]

Apologies for the delay and thanks for the feedback. Applied all the changes and added an integration tests to ensure compilation succeeds. I've also re-tested this branch with my project and it compiles fine. Just for completeness, the type that was causing issues before:

        #[derive(Debug, Copy, Clone)]
        #[repr(transparent)]
        pub struct bpf_map_type(u32);
        #[allow(non_upper_case_globals)]
        impl bpf_map_type {
            pub const BPF_MAP_TYPE_UNSPEC: bpf_map_type = bpf_map_type(0);
            pub const BPF_MAP_TYPE_HASH: bpf_map_type = bpf_map_type(1);
            pub const BPF_MAP_TYPE_ARRAY: bpf_map_type = bpf_map_type(2);
            pub const BPF_MAP_TYPE_PROG_ARRAY: bpf_map_type = bpf_map_type(3);
            pub const BPF_MAP_TYPE_PERF_EVENT_ARRAY: bpf_map_type = bpf_map_type(4);
            pub const BPF_MAP_TYPE_PERCPU_HASH: bpf_map_type = bpf_map_type(5);
            pub const BPF_MAP_TYPE_PERCPU_ARRAY: bpf_map_type = bpf_map_type(6);
            pub const BPF_MAP_TYPE_STACK_TRACE: bpf_map_type = bpf_map_type(7);
            pub const BPF_MAP_TYPE_CGROUP_ARRAY: bpf_map_type = bpf_map_type(8);
            pub const BPF_MAP_TYPE_LRU_HASH: bpf_map_type = bpf_map_type(9);
            pub const BPF_MAP_TYPE_LRU_PERCPU_HASH: bpf_map_type = bpf_map_type(10);
            pub const BPF_MAP_TYPE_LPM_TRIE: bpf_map_type = bpf_map_type(11);
            pub const BPF_MAP_TYPE_ARRAY_OF_MAPS: bpf_map_type = bpf_map_type(12);
            pub const BPF_MAP_TYPE_HASH_OF_MAPS: bpf_map_type = bpf_map_type(13);
            pub const BPF_MAP_TYPE_DEVMAP: bpf_map_type = bpf_map_type(14);
            pub const BPF_MAP_TYPE_SOCKMAP: bpf_map_type = bpf_map_type(15);
            pub const BPF_MAP_TYPE_CPUMAP: bpf_map_type = bpf_map_type(16);
            pub const BPF_MAP_TYPE_XSKMAP: bpf_map_type = bpf_map_type(17);
            pub const BPF_MAP_TYPE_SOCKHASH: bpf_map_type = bpf_map_type(18);
            pub const BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED: bpf_map_type = bpf_map_type(19);
            pub const BPF_MAP_TYPE_CGROUP_STORAGE: bpf_map_type = bpf_map_type(19);
            pub const BPF_MAP_TYPE_REUSEPORT_SOCKARRAY: bpf_map_type = bpf_map_type(20);
            pub const BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE: bpf_map_type = bpf_map_type(21);
            pub const BPF_MAP_TYPE_QUEUE: bpf_map_type = bpf_map_type(22);
            pub const BPF_MAP_TYPE_STACK: bpf_map_type = bpf_map_type(23);
            pub const BPF_MAP_TYPE_SK_STORAGE: bpf_map_type = bpf_map_type(24);
            pub const BPF_MAP_TYPE_DEVMAP_HASH: bpf_map_type = bpf_map_type(25);
            pub const BPF_MAP_TYPE_STRUCT_OPS: bpf_map_type = bpf_map_type(26);
            pub const BPF_MAP_TYPE_RINGBUF: bpf_map_type = bpf_map_type(27);
            pub const BPF_MAP_TYPE_INODE_STORAGE: bpf_map_type = bpf_map_type(28);
            pub const BPF_MAP_TYPE_TASK_STORAGE: bpf_map_type = bpf_map_type(29);
            pub const BPF_MAP_TYPE_BLOOM_FILTER: bpf_map_type = bpf_map_type(30);
            pub const BPF_MAP_TYPE_USER_RINGBUF: bpf_map_type = bpf_map_type(31);
            pub const BPF_MAP_TYPE_CGRP_STORAGE: bpf_map_type = bpf_map_type(32);
        }
        impl Default for bpf_map_type {
            fn default() -> Self {
                bpf_map_type::BPF_MAP_TYPE_UNSPEC
            }
        }

[javierhonduco]

(Pushed another update to write Rust compiler generated code to a temp folder and to fix a lint that I didn't see with my local version of Clippy)

[danielocfb]

Looks good to me, thanks. I think I'll cut another patch release before merging this change, because I believe we have to treat it as breaking.

[javierhonduco]

Thanks! :)