[derive] Rename is_c_like -> is_fieldless #804
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jswrenn
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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`.
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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:
This distinction is important specifically when it comes to discriminants. The syntax
Foo::A as u8computes the discriminant value of theAvariant. However,Foo::Bis a type constructor, and thus is a function. As a result,Foo::B as u8computes the address of the functionFoo::B.