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

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 well with this change.

libbpf-cargo/CHANGELOG.md

@@ -1,6 +1,7 @@
 Unreleased
 ----------
 - Fixed handling of empty unions in BPF types
+- Represent C enums with custom types and const fields
 
 
 0.24.6

libbpf-cargo/src/gen/btf.rs

@@ -797,6 +797,8 @@ impl<'s> GenBtf<'s> {
             _ => bail!("Invalid enum size: {}", t.size()),
         };
 
+        let enum_name = self.anon_types.type_name_or_anon(&t);
+
         let mut signed = "u";
         for value in t.iter() {
             if value.value < 0 {
@@ -805,30 +807,22 @@ impl<'s> GenBtf<'s> {
             }
         }
 
-        writeln!(
-            def,
-            r#"#[derive(Debug, Copy, Clone, Default, PartialEq, Eq)]"#
-        )?;
-        writeln!(def, r#"#[repr({signed}{repr_size})]"#)?;
-        writeln!(
-            def,
-            r#"pub enum {name} {{"#,
-            name = self.anon_types.type_name_or_anon(&t),
-        )?;
+        writeln!(def, r#"#[derive(Debug, Copy, Clone, Default)]"#)?;
+        writeln!(def, r#"pub struct {enum_name}({signed}{repr_size});"#,)?;
 
-        for (i, value) in t.iter().enumerate() {
-            if i == 0 {
-                writeln!(def, r#"    #[default]"#)?;
-            }
+        writeln!(def, r#"impl {enum_name} {{"#,)?;
+
+        for value in t.iter() {
+            writeln!(def, "    #[allow(non_upper_case_globals)]")?;
             writeln!(
                 def,
-                r#"    {name} = {value},"#,
+                r#"    pub const {name}: {enum_name} = {enum_name}({value});"#,
                 name = value.name.unwrap().to_string_lossy(),
                 value = value.value,
             )?;
         }
 
-        writeln!(def, "}}")?;
+        writeln!(def, r#"}}"#,)?;
         Ok(())
     }
 

libbpf-cargo/src/test.rs

@@ -1824,13 +1824,15 @@ enum Foo foo;
 "#;
 
     let expected_output = r#"
-#[derive(Debug, Copy, Clone, Default, PartialEq, Eq)]
-#[repr(u32)]
-pub enum Foo {
-    #[default]
-    Zero = 0,
-    One = 1,
-    seven = 7,
+#[derive(Debug, Copy, Clone, Default)]
+pub struct Foo(u32);
+impl Foo {
+    #[allow(non_upper_case_globals)]
+    pub const Zero: Foo = Foo(0);
+    #[allow(non_upper_case_globals)]
+    pub const One: Foo = Foo(1);
+    #[allow(non_upper_case_globals)]
+    pub const seven: Foo = Foo(7);
 }
 "#;
 
@@ -2565,13 +2567,12 @@ impl Default for Foo {
         }
     }
 }
-#[derive(Debug, Copy, Clone, Default, PartialEq, Eq)]
-#[repr(u32)]
-pub enum __anon_1 {
-    #[default]
-    FOO = 1,
-}
-"#;
+#[derive(Debug, Copy, Clone, Default)]
+pub struct __anon_1(u32);
+impl __anon_1 {
+    #[allow(non_upper_case_globals)]
+    pub const FOO: __anon_1 = __anon_1(1);
+}"#;
 
     let mmap = build_btf_mmap(prog_text);
     let btf = btf_from_mmap(&mmap);