Function resolve_static_header_value 

pub fn resolve_static_header_value(value: HeaderValue) -> HeaderValue

Resolves a header value that cannot change for the life of the process - $hostname and $<ENV_VAR> - so it can be stored as a plain value when the configuration is loaded instead of being looked up on every request (std::env::var takes a process-wide lock and allocates each time). Values that depend on the request ($host, $http_*, :key, …) and plain values come back unchanged, as does a $NAME whose variable is not set, so the request path treats it exactly as before.