Make a list continuous! Take an input list L containing positive integers, and insert the least amount of integers into the sequence, so that you make it a continuous one.

Example:

L = 4 2 6 1
Out: 4 3 2 3 4 5 6 5 4 3 2 1


The input and output formats are optional.

Test cases:

L = 1
Out: 1

L = 10 4
Out: 10 9 8 7 6 5 4

L = 13 13
Out: 13 13

L = 3 6
Out: 3 4 5 6

L = 4 4 1 1 4 4
Out: 4 4 3 2 1 1 2 3 4 4


This is code-golf... You know the rules.
Builtin
Husk:
…

Try it online!
I bet this builtin exists in more than one language, so I made this a community wiki, just add in more languages where this is a builtin if you know them!