The zarith-ppx package provides literals
for arbitrary-precision integers and rationals
with the Zarith library.
Here are some examples:

# 1z ;;
- : Z.t = 1
# 1.234e40z ;;
- : Z.t = 12340000000000000000000000000000000000000
# 0x18z ;; (* Hexadecimal *)
- : Z.t = 24
# 1.2q ;;
- : Q.t = 6/5
# 2.45e10q ;;
- : Q.t = 24500000000
# 2.45e-5q ;;
- : Q.t = 49/2000000
# -0O123q ;; (* Octal *)
- : Q.t = -83

It also raises errors for incorrect literals:

# 1.234e2z ;;
Characters 0-8:
Error: This literal does not fit in an integer. You should use a rational number.

This package depends on vanilla Zarith, but the generated code is compatible with any cross-compiled version of Zarith such as zarith-freestanding and zarith-xen.