Since Howl is a multi-language editor, it's obvious that properly parsing all languages to figure out where functions begin or end is... difficult. Instead, I offer an alternative suggestion: A way to minimise content enclosed within a given set of braces/brackets/parentheses. As an example,

304 | 
305 | fn foo(a: i32, b: i32) -> i32 {
306 |     // Some useless code
307 |     let c = 5;
308 |     let d = c + a
309 |     a + d
310 | }
311 | 

Would minimise to

304 | 
305 | fn foo(a: i32, b: i32) -> i32 { ... }
311 | 

The character to perform the minimisation upon could be tweaked on a per-language basis.

I've not fully thought through how this might work with nesting, but I thought that starting a discussion about this would be the best thing to do.