# [R], 58 bytes

<!-- language-all: lang-r -->

    function(v,x){d=diff(c(0,which(v==x)))
    sequence(d,d-1,-1)}

**output**

    f(int, 0)
    # [1] 2 1 0 2 1 0 0
    f(int, 1)
    # [1] 0 0
    f(int, 2)
    # [1] 4 3 2 1 0 2 1 0
    f(int, 3)
    # integer(0)
    
    f(char, "A")
    # [1] 0 2 1 0 0
    f(char, "B")
    # [1] 1 0
    f(char, "C")
    # [1] 2 1 0
    f(char, "D")
    # integer(0)

Works for R >= 4.0.0.

[R]: https://www.r-project.org/