The array INV contains the multiplicative inverses of the indices modulo 26 or 0 if the index is not invertible modulo 26.

The triplets in the return value should be ordered a < b < c, right? I kind of get mixed messages:

With n = 849
Expected (134, 1998, [629, 540, 829]) but got (134, 1998, [540, 629, 829]) at src/lib.rs:46:9
With n = 541
Expected (87, 1302, [341, 420, 541]) but got (87, 1302, [420, 341, 541]) at src/lib.rs:46:9

Is this an issue with the rust translation/the random tests for rust or am I missing something?

It should panic, according to the description that's an invalid input.