C Enum equivalent in Rust

In C code we often use enums to represent constants like so:

enum TokenType {
	TOK_OFS = 256,
	TOK_and,
	TOK_break,
	TOK_STRING,

	FIRST_RESERVED = TOK_OFS + 1,
	LAST_RESERVED = TOK_while - TOK_OFS
};

Rust also has an enum type but it is not at all like the C enum. The Rust enum is more like a discriminated union in C.

Superficially though you can almost write something like above in Rust.

enum TokenType {
    TOK_OFS = 256,
    TOK_and,
    TOK_break,
    TOK_STRING,

    FIRST_RESERVED = TOK_OFS + 1,
    LAST_RESERVED = TOK_while - TOK_OFS
}

But this will not compile. Firstly Rust expects explicit conversion from enum to int, so you can try:

enum TokenType {
    TOK_OFS = 256,
    TOK_and,
    TOK_break,
    TOK_STRING,

    FIRST_RESERVED = TOK_OFS as isize + 1,
    LAST_RESERVED = TOK_while as isize - TOK_OFS as isize
}

However this will not work either because an enum in Rust is not really a constant. The enum discriminant value needs to be unique so we cannot have two instances TOK_and and FIRST_RESERVED with the same value.

Perhaps we could try this:

enum TokenType {
    TOK_OFS = 256,
    TOK_and,
    TOK_break,
    TOK_STRING,

}

const FIRST_RESERVED :isize =  TOK_OFS as isize + 1;
const LAST_RESERVED: isize  = TOK_while as isize - TOK_OFS as isize;

This compiles, but the enums are a pain to use as constants because of the need to explicitly convert to integer values.

In the end I ended up doing:

const TOK_OFS: i32 = 256;

const TOK_and: i32 = 257;
const TOK_break: i32 = 258;
const TOK_STRING: i32 = 301;

const FIRST_RESERVED: i32 = TOK_OFS + 1;
const LAST_RESERVED: i32 = TOK_while - TOK_OFS;

Not great but it gives me what I need.