Function filter_from_env 

pub fn filter_from_env(default: Targets) -> Targets

Builds a log filter from the RUST_LOG environment variable, falling back to default.

RUST_LOG is parsed with parse_filter_directives. If it’s unset or empty, default is returned. If it can’t be parsed, a warning is written to standard error – logging isn’t initialized yet at this point – and default is returned.