Rust: core::ops::Fn(char) -> bool

Here’s a Rust error:

error: the trait core::ops::Fn(char) -> bool is not implemented for the type &str

I was trying to split a string of numbers with spaces a delimiter. Here’s the line for which this error occured:

Apparently, split was expecting a single character, not a substring. This obtuse error was easily solved by changing the double-quotes to single-quotes.