I think the docs about default Source is pretty clear if you read it in full. ``` Top-level functions, such as Float64 and Int, use a default shared Source that produces a deterministic sequence of values each time a program is run. Use the Seed function to initialize the default Source if different behavior is required for each run. The default Source is safe for concurrent use by multiple goroutines. ``` It explicitly mentions the default Source is the one used by the top-level functions. Following Go documentation convention, anything not explicitly documented for concurrency safety is not safe for concurrent use. The NewSource doesn't say Source is safe for concurrent use, so it's not.