Constant MAX_NV_LENGTH 

pub const MAX_NV_LENGTH: u32 = 0x7FFF_FFFF; // 2_147_483_647u32

Available on crate feature fastcgi only.

Maximum length that fits in the FastCGI variable-length integer format.

The 4-byte form reserves the high bit as a marker, leaving 31 bits for the length value.

Reference: FastCGI Specification §3.4