It's to remove a syntax ambiguity with c-style function declarations https://en.wikipedia.org/wiki/Most_vexing_parse https://en.wikipedia.org/wiki/Most_vexing_p

It's to remove a syntax ambiguity with c-style function declarations https://en.wikipedia.org/wiki/Most_vexing_parse https://en.wikipedia.org/wiki/Most_vexing_parse The syntax ambiguity adds a lot of complexity to the grammar that makes parsing a lot more complicated than it needs to be. Sticking `fn` in front fixes a lot of problems.