Constant STD_PACKAGE_IDS 

pub const STD_PACKAGE_IDS: &[&str];

The complete standard-package universe (the 13 std packages of the standard-package campaign). use std.<name> outside this list is a check error: std resolution is a built-in registry, so an unknown name can never resolve later — a typo’d use std.coercon would otherwise silently import nothing (and downstream missing-import bite is advisory only).