This ppx adds a construct similar to if let in Rust.
The following two snippets are equivalent:

let%if Some x = Sys.getenv_opt "HELLO" in
print_endline x

match Sys.getenv_opt "HELLO" with
| Some x -> print_endline x
| _ -> ()

New: if%true then do

Do you often want to perform side effects if an expression is true while still returning the boolean value?
Then if%true is for you!
Simply write:

if%true cond then something else otherwise

... and your code is automagically transformed into:

let c = cond in (if c then something else otherwise); c

The else branch is optional!