Feature request: Offer "generate function" on unknown identifiers passed to higher-order functions

[jplatte]

Given an input like

let foo = vec![1, 2, 3].iter().map(make_foo);

where make_foo doesn't resolve, it would be nice if RA offered to generate an fn make_foo(arg0: i32) -> _.

[ChayimFriedman2]

This will have the same difficulty as #3148.