Macro u8_from_env_or 

macro_rules! u8_from_env_or {
    ($env_var:literal, $default:expr, $doc:literal) => { ... };
}

Reads a u8 at compile time from the given environment variable, with a default.

• The $default parameter allows to provide a fallback value for when the environment variable is not found.
• The $doc parameter allows to provide a documentation string for this tunable (see str_from_env!).

Errors

• Produces a compile-time error when option_env! does.
• Produces a compile-time error when the environment variable cannot be parsed into a u8.