Thanks, the tan case works for me now.

Rust 1.44

Contains the following assertions in the tests:

assert_eq!(diff("(tan x)"), "(+ 1 (^ (tan x) 2))");
// ...
assert_eq!(diff("(tan (* 2 x))"), "(* 2 (+ 1 (^ (tan (* 2 x)) 2)))");

Who do I contact to fix the translation?

I believe the two tan-related test cases are broken and/or not very robust.

The first one

f(x) = (tan x)
f'(x) = (/ 1 (^ (cos x) 2))

 actual:   `"(/ 1 (^ (cos x) 2))"`,
 expected: `"(+ 1 (^ (tan x) 2))"`

The second one

====
f(x) = (tan (* 2 x))
f'(x) = (* 2 (/ 1 (^ (cos (* 2 x)) 2)))

  actual:   `"(* 2 (/ 1 (^ (cos (* 2 x)) 2)))"`,
  expected: `"(* 2 (+ 1 (^ (tan (* 2 x)) 2)))"`

Also the second one begs for another simplification: (* 2 (/ 1 $Expr)) => (/ 2 $Expr).

In my opinion, the simplification rules are to be codified more explicitly.