[derive] Rename is_c_like -> is_fieldless

The implementation of this method checks for the presence of fields.
While this is /similar/ to the concept of a C-like enum, it's not the
same. Fieldless enums are exactly as described: none of their variants
has fields. C-like enums, on the other hand, are more recently referred
to as "unit-only" because their variants are not permitted to contain
parentheses or curly braces. E.g., the following is a fieldless enum,
but is not a C-like/unit-only enum:

  #[repr(u8)]
  enum Foo {
      A,   // This is fine
      B(), // This is not fine
      C{}  // This is not fine
  }

This distinction is important specifically when it comes to
discriminants. The syntax `Foo::A as u8` computes the discriminant value
of the `A` variant. However, `Foo::B` is a type constructor, and thus is
a function. As a result, `Foo::B as u8` computes the address of the
function `Foo::B`.

zerocopy-derive/src/ext.rs

@@ -69,11 +69,11 @@ fn map_fields<'a>(
 }
 
 pub trait EnumExt {
- fn is_c_like(&self) -> bool;
+ fn is_fieldless(&self) -> bool;
 }
 
 impl EnumExt for DataEnum {
- fn is_c_like(&self) -> bool {
+ fn is_fieldless(&self) -> bool {
  self.fields().is_empty()
  }
 }

zerocopy-derive/src/lib.rs

@@ -413,7 +413,7 @@ fn derive_from_zeros_struct(ast: &DeriveInput, strct: &DataStruct) -> proc_macro
 // - one of the variants has a discriminant of `0`
 
 fn derive_from_zeros_enum(ast: &DeriveInput, enm: &DataEnum) -> proc_macro2::TokenStream {
- if !enm.is_c_like() {
+ if !enm.is_fieldless() {
  return Error::new_spanned(ast, "only C-like enums can implement FromZeros")
  .to_compile_error();
  }
@@ -475,7 +475,7 @@ fn derive_from_bytes_struct(ast: &DeriveInput, strct: &DataStruct) -> proc_macro
 // this would require ~4 billion enum variants, which obviously isn't a thing.
 
 fn derive_from_bytes_enum(ast: &DeriveInput, enm: &DataEnum) -> proc_macro2::TokenStream {
- if !enm.is_c_like() {
+ if !enm.is_fieldless() {
  return Error::new_spanned(ast, "only C-like enums can implement FromBytes")
  .to_compile_error();
  }
@@ -580,7 +580,7 @@ const STRUCT_UNION_AS_BYTES_CFG: Config<StructRepr> = Config {
 // An enum is `IntoBytes` if it is C-like and has a defined repr.
 
 fn derive_as_bytes_enum(ast: &DeriveInput, enm: &DataEnum) -> proc_macro2::TokenStream {
- if !enm.is_c_like() {
+ if !enm.is_fieldless() {
  return Error::new_spanned(ast, "only C-like enums can implement IntoBytes")
  .to_compile_error();
  }
@@ -668,7 +668,7 @@ const STRUCT_UNION_UNALIGNED_CFG: Config<StructRepr> = Config {
 // - `repr(u8)` or `repr(i8)`
 
 fn derive_unaligned_enum(ast: &DeriveInput, enm: &DataEnum) -> proc_macro2::TokenStream {
- if !enm.is_c_like() {
+ if !enm.is_fieldless() {
  return Error::new_spanned(ast, "only C-like enums can implement Unaligned")
  .to_compile_error();
  }