Typesafe, `0x` prefixed, hex strings up to 32 bytes long

typesafe-hex-strings.ts

type Cast<T, U> = T extends U ? T : U;
type Prop<T, K> = K extends keyof T ? T[K] : never;
type HexChars = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "A" | "B" | "C" | "D" | "E" | "F" | "a" | "b" | "c" | "d" | "e" | "f";
type HexPairs = `${HexChars}${HexChars}`;

/**
 * returns a Tuple of hex pairs (bytes), or `never` if the input doesn't contain an even number of hex characters
 */
type Hex<S extends string> =
  S extends "" ? [] :
  S extends `${HexPairs}${infer R}` ? S extends `${infer C}${R}`
    ? [C, ...(Hex<R> extends infer X ? Cast<X, string[]> : never)]
    : never : never;

/**
 * Gets up to the first 8 bytes following the given prefix. `S` _must not_ start with `0x`.
 */
type Raw<S extends string, P extends string = ""> =
      S extends `${P}${infer I0}${infer I1}${infer I2}${infer I3}${infer I4}${infer I5}${infer I6}${infer I7}${infer I8}${infer I9}${infer I10}${infer I11}${infer I12}${infer I13}${infer I14}${infer I15}${infer _}` ? `${I0}${I1}${I2}${I3}${I4}${I5}${I6}${I7}${I8}${I9}${I10}${I11}${I12}${I13}${I14}${I15}`
        : S extends `${P}${infer I0}${infer I1}${infer I2}${infer I3}${infer I4}${infer I5}${infer I6}${infer I7}${infer I8}${infer I9}${infer I10}${infer I11}${infer I12}${infer I13}` ? `${I0}${I1}${I2}${I3}${I4}${I5}${I6}${I7}${I8}${I9}${I10}${I11}${I12}${I13}`
          : S extends `${P}${infer I0}${infer I1}${infer I2}${infer I3}${infer I4}${infer I5}${infer I6}${infer I7}${infer I8}${infer I9}${infer I10}${infer I11}` ? `${I0}${I1}${I2}${I3}${I4}${I5}${I6}${I7}${I8}${I9}${I10}${I11}`
            : S extends `${P}${infer I0}${infer I1}${infer I2}${infer I3}${infer I4}${infer I5}${infer I6}${infer I7}${infer I8}${infer I9}` ? `${I0}${I1}${I2}${I3}${I4}${I5}${I6}${I7}${I8}${I9}`
              : S extends `${P}${infer I0}${infer I1}${infer I2}${infer I3}${infer I4}${infer I5}${infer I6}${infer I7}` ? `${I0}${I1}${I2}${I3}${I4}${I5}${I6}${I7}`
                : S extends `${P}${infer I0}${infer I1}${infer I2}${infer I3}${infer I4}${infer I5}` ? `${I0}${I1}${I2}${I3}${I4}${I5}`
                  : S extends `${P}${infer I0}${infer I1}${infer I2}${infer I3}` ? `${I0}${I1}${I2}${I3}`
                    : S extends `${P}${infer I0}${infer I1}` ? `${I0}${I1}`
                    : "";


/**
 * Add up all our lengths. If any length is `never`, this returns `never`.
 */
type ValidLengthTogether<L1 extends number, L2 extends number, L3 extends number, L4 extends number> = (
    L1 extends 8
    ? L2 extends 8
        ? L3 extends 8
        ? Add<`${L4}`, "24">
        : Add<`${L3}`, "16">
        : Add<`${L2}`, "8">
    : L1 extends never
        ? never
        : `${L1}`
);

type Trim<S extends string, Prefix extends string> = S extends `${Prefix}${infer I}` ? I : never;

type U1<S extends string> = Raw<S>;
type U2<S extends string> = Raw<S, `${U1<S>}`>;
type U3<S extends string> = Raw<S, `${U1<S>}${U2<S>}`>;
type U4<S extends string> = Raw<S, `${U1<S>}${U2<S>}${U3<S>}`>;

type L1<S extends string> = Prop<Hex<U1<S>>, "length">;
type L2<S extends string> = Prop<Hex<U2<S>>, "length">;
type L3<S extends string> = Prop<Hex<U3<S>>, "length">;
type L4<S extends string> = Prop<Hex<U4<S>>, "length">;

// @ts-ignore: Ignore type instantiation depth errors as we have already limited our depth
type ValidLength<S extends string> = ValidLengthTogether<L1<S>, L2<S>, L3<S>, L4<S>>;

type Trimmed<S extends string> = Trim<S, "0x">;

/**
 * Supports hex-encoded strings up to 32 bytes (64 hex chars), `0x` prefixed
 */
type DATA<
  S extends string,
  Length extends number = -1,
> = Length extends -1
    ? ValidLength<Trimmed<S>> extends never
        ? never
        : S
    : `${Length}` extends ValidLength<Trimmed<S>>
        ? S
        : never;



/* ************************** <HELPER TYPES> ********************************** */

type TxHash<S extends string> = DATA<S, 32>;
type Address<S extends string> = DATA<S, 20>;
type Nonce<S extends string> = DATA<S, 4>;

/* ************************** </HELPER TYPES> ********************************* */



/* ************************** <DEFINE OPTIONS> ******************************** */

type BaseOptions = {
  nonce: string;
  coinbase: string;
};

type OptionsTypeMap<N> = {
    nonce: Nonce<Cast<N, string>>,
    coinbase: Address<Cast<N, string>>
}

type Cond<B, T extends B, U> = B extends T ? B : U;

type ValidOptions<O extends BaseOptions> = {
  [P in keyof O]?:
    P extends keyof OptionsTypeMap<O[P]>
      ? Cond<string, O[P], OptionsTypeMap<O[P]>[P]>
      : O[P]
};

/* ************************** </DEFINE OPTIONS> ******************************* */



/* ************************** <BASIC TESTS> *********************************** */

type a = DATA<"0x123"> // never
type b = DATA<"0x123", 3> // never (odd numbers aren't currently allowed)
type c = DATA<"0x1234"> // 0x1234
type d = DATA<"0xbda2ca0611c2a952a4a25bfc175803912842c839", 4> // never, too long
type e = DATA<"0xbda2ca0611c2a952a4a25bfc175803912842c839"> // 0xbda2ca0611c2a952a4a25bfc175803912842c839
type f = DATA<"0xbda2ca0611c2a952a4a25bfc175803912842c839", 20> // 0xbda2ca0611c2a952a4a25bfc175803912842c839
type g = DATA<"0xff6b938acb7d72e261c51fa255a8119d9e23d021bed440298f91342706b1538d"> // 0xff6b938acb7d72e261c51fa255a8119d9e23d021bed440298f91342706b1538d
type h = DATA<"0xff6b938acb7d72e261c51fa255a8119d9e23d021bed440298f91342706b1538d", 32> // 0xff6b938acb7d72e261c51fa255a8119d9e23d021bed440298f91342706b1538d
type i = DATA<"0123"> // never, invalid

/* ************************** </BASIC TESTS> *********************************** */



/* ************************** <COMPLEX TESTS> ********************************** */

declare function provider<
    O extends BaseOptions
>(options: ValidOptions<O>): void;
// VALID INPUT:
provider({
    coinbase: "0xbda2ca0611c2a952a4a25bfc175803912842c839",
    nonce: "0xa3079c9f"
});
provider({
    coinbase: "0xbda2ca0611c2a952a4a25bfc175803912842c839"
});
provider({
    nonce: "0xa3079c9f"
});
// INVALID INPUTS:
provider({
    coinbase: "0xZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ", // invalid chars
    nonce: "0xa3079c9f"
});
provider({
    coinbase: "0x1234", // too short
    nonce: "0xa3079c9f"
});
provider({
    coinbase: "0x", // too short
    nonce: "0xa3079c9f"
});
provider({
    coinbase: "0", // no 0x and too short
    nonce: "0xa3079c9f"
});
provider({
    coinbase: "bda2ca0611c2a952a4a25bfc175803912842c839",  // no 0x
    nonce: "0xa3079c9f"
});
provider({
    coinbase: "0xbda2ca0611c2a952a4a25bfc175803912842c839",
    nonce: "a3079c9f" // no 0x
});
provider({
    coinbase: "0xbda2ca0611c2a952a4a25bfc175803912842c839",
    nonce: "0xQ3079c9f" // invalid character (Q)
});
provider({
    coinbase: "0xbda2ca0611c2a952a4a25bfc175803912842c839",
    nonce: "0xA3079c9f123456789" // too long
});

/* ************************** </COMPLEX TESTS> ********************************* */



/* ***************************************************************************** */
/* ***************************************************************************** */
/* ***************************************************************************** */
/* ***************************************************************************** */
/* ***************************** STRING ADDITION ******************************* */
/* ***************************************************************************** */
/* ***************************************************************************** */
/* ***************************************************************************** */
/* ***************************************************************************** */

type Digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9";
type OneDigitPlus = {
    "00": [false, "0"];
    "01": [false, "1"];
    "02": [false, "2"];
    "03": [false, "3"];
    "04": [false, "4"];
    "05": [false, "5"];
    "06": [false, "6"];
    "07": [false, "7"];
    "08": [false, "8"];
    "09": [false, "9"];
    "10": [false, "1"];
    "11": [false, "2"];
    "12": [false, "3"];
    "13": [false, "4"];
    "14": [false, "5"];
    "15": [false, "6"];
    "16": [false, "7"];
    "17": [false, "8"];
    "18": [false, "9"];
    "19": [true, "0"];
    "20": [false, "2"];
    "21": [false, "3"];
    "22": [false, "4"];
    "23": [false, "5"];
    "24": [false, "6"];
    "25": [false, "7"];
    "26": [false, "8"];
    "27": [false, "9"];
    "28": [true, "0"];
    "29": [true, "1"];
    "30": [false, "3"];
    "31": [false, "4"];
    "32": [false, "5"];
    "33": [false, "6"];
    "34": [false, "7"];
    "35": [false, "8"];
    "36": [false, "9"];
    "37": [true, "0"];
    "38": [true, "1"];
    "39": [true, "2"];
    "40": [false, "4"];
    "41": [false, "5"];
    "42": [false, "6"];
    "43": [false, "7"];
    "44": [false, "8"];
    "45": [false, "9"];
    "46": [true, "0"];
    "47": [true, "1"];
    "48": [true, "2"];
    "49": [true, "3"];
    "50": [false, "5"];
    "51": [false, "6"];
    "52": [false, "7"];
    "53": [false, "8"];
    "54": [false, "9"];
    "55": [true, "0"];
    "56": [true, "1"];
    "57": [true, "2"];
    "58": [true, "3"];
    "59": [true, "4"];
    "60": [false, "6"];
    "61": [false, "7"];
    "62": [false, "8"];
    "63": [false, "9"];
    "64": [true, "0"];
    "65": [true, "1"];
    "66": [true, "2"];
    "67": [true, "3"];
    "68": [true, "4"];
    "69": [true, "5"];
    "70": [false, "7"];
    "71": [false, "8"];
    "72": [false, "9"];
    "73": [true, "0"];
    "74": [true, "1"];
    "75": [true, "2"];
    "76": [true, "3"];
    "77": [true, "4"];
    "78": [true, "5"];
    "79": [true, "6"];
    "80": [false, "8"];
    "81": [false, "9"];
    "82": [true, "0"];
    "83": [true, "1"];
    "84": [true, "2"];
    "85": [true, "3"];
    "86": [true, "4"];
    "87": [true, "5"];
    "88": [true, "6"];
    "89": [true, "7"];
    "90": [false, "9"];
    "91": [true, "0"];
    "92": [true, "1"];
    "93": [true, "2"];
    "94": [true, "3"];
    "95": [true, "4"];
    "96": [true, "5"];
    "97": [true, "6"];
    "98": [true, "7"];
    "99": [true, "8"];
}
type LastDigit<Str, D = Digit> = D extends Digit ? Str extends `${infer Rest}${D}` ? [Rest, D] : never : never;
type ConcatStr<Left, Right> =
  Left extends string ?
  Right extends string ?
  `${Left}${Right}`
  : never : never;


type Add<Left, Right> =
  Left extends "" ? Right :
  Right extends "" ? Left :
  LastDigit<Left> extends [infer LeftRest, infer LeftDigit] ?
  LastDigit<Right> extends [infer RightRest, infer RightDigit] ?
  Add2<LeftRest, RightRest, ConcatStr<LeftDigit, RightDigit>> : never : never;

type Add2<LeftRest, RightRest, LR> =
  LR extends keyof OneDigitPlus ?
  OneDigitPlus[LR] extends [infer Carry, infer LD] ?
  Add<LeftRest, RightRest> extends infer RestSum ?
  (Carry extends true ?
    Add<RestSum, "1"> :
    RestSum) extends infer LeftCarriedSum ?
  ConcatStr<LeftCarriedSum, LD>
  : never : never : never : never;

type AddD<Left, Right> =
  Left extends "" ? Right :
  Right extends "" ? Left :
  LastDigit<Left> extends [infer LeftRest, infer LeftDigit] ?
  LastDigit<Right> extends [infer RightRest, infer RightDigit] ?
  [LeftRest, RightRest, ConcatStr<LeftDigit, RightDigit>] : never : never;

This utilizes template literals types that will (hopefully) be introduced in TypeScript 4.1.0 beta.

To try it today you can go to https://www.typescriptlang.org/play?#code/Q, switch the version to Nightly, then copy+paste this gist in!

Note: this version only handles complete hex pairs, like 0x0123 instead of 0x123.

Got rid of 8 type params on DATA:

/**
 * Add up all our lengths. If any length is `never`, this returns `never`.
 */
type ValidLengthTogether<L1 extends number, L2 extends number, L3 extends number, L4 extends number> = (
    L1 extends 8
    ? L2 extends 8
        ? L3 extends 8
        ? Add<`${L4}`, "24">
        : Add<`${L3}`, "16">
        : Add<`${L2}`, "8">
    : L1 extends never
        ? never
        : `${L1}`
);
type Trim<S extends string, Prefix extends string> = S extends `${Prefix}${infer I}` ? I : never;

type U1<S extends string> = Raw<S>;
type U2<S extends string> = Raw<S, `${U1<S>}`>;
type U3<S extends string> = Raw<S, `${U1<S>}${U2<S>}`>;
type U4<S extends string> = Raw<S, `${U1<S>}${U2<S>}${U3<S>}`>;

type L1<S extends string> = Prop<Hex<U1<S>>, "length">;
type L2<S extends string> = Prop<Hex<U2<S>>, "length">;
type L3<S extends string> = Prop<Hex<U3<S>>, "length">;
type L4<S extends string> = Prop<Hex<U4<S>>, "length">;

// @ts-ignore: Ignore type instantiation depth errors as we have already limited our depth
type ValidLength<S extends string> = ValidLengthTogether<L1<S>, L2<S>, L3<S>, L4<S>>;

type Trimmed<S extends string> = Trim<S, "0x">;

/**
 * Supports hex-encoded strings up to 32 bytes (64 hex chars), `0x` prefixed
 */
type DATA<
  S extends string,
  Length extends number = -1,
> = Length extends -1
    ? ValidLength<Trimmed<S>> extends never
        ? never
        : S
    : `${Length}` extends ValidLength<Trimmed<S>>
        ? S
        : never;

Updated with @gnidan's improvements!

Reducing provider to a single type param:

type BaseOptions = {
  nonce: string;
  coinbase: string;
};

type OptionsTypeMap<N> = {
    nonce: Nonce<Cast<N, string>>,
    coinbase: Address<Cast<N, string>>
}

type Cond<B, T extends B, U> = B extends T ? B : U;

type ValidOptions<O extends BaseOptions> = {
  [P in keyof O]?:
    P extends keyof OptionsTypeMap<O[P]>
      ? Cond<string, O[P], OptionsTypeMap<O[P]>[P]>
      : O[P]
};

declare function provider<
    O extends BaseOptions
>(options: ValidOptions<O>): void;

Updated again! Thanks, @gnidan!!!

For more fun things you can do with this see: https://gist.github.com/davidmurdoch/de393cca445effbed4d7e22fe33c5b85