Pathcomplete for path stored in variable

[jeanlucthumm]

Related problem

Inspired from fish behavior, I want path auto complete when hitting <Tab> after a variable storing a path.

Describe the solution you'd like

Example:

$ tree lib
lib
└── random
    └── test.txt
$ let mypath = "./lib"
$ cd $mypath/ # pressing TAB here should autocomplete as if we're doing `cd ./lib`

This is also useful for storing common paths in env vars:

$ $env.I3CONF
/home/<....>/.config/i3
$ vim $env.I3CONF/ # pressing TAB here should autocomplete as if we're doing `vim ~/.config/i3/`

Describe alternatives you've considered

Can't think of anything to replicate this behavior.

Additional context and details

Test the behavior in Fish which has this feature