End the tabs versus space war

Vim, 37, 34, 33, 32 bytes

:%s/\t/    /g|%norm ^hr/hv0r*r/

Try it online!

Note that this requires a trailing carriage return (enter) in vim, although not in the online interpreter.

This uses the V interpreter because it's backwards compatible. A very straightforward solution.

Here's a gif that lets you see the solution happen in real time. This uses a slightly older version, and I added some extra keystrokes to make it run slower so you can see what happens:

And here is the explanation of how it works:

:%s/\t/    /g           "Replace every tab with 4 spaces
|                       "AND
%norm                   "On every line:
      ^                 "  Move to the first non-whitespace char
       h                "  Move one character to the left. If there is none, the command will end here.
         r/             "  Replace it with a slash
           h            "  Move to the left
            v0          "  Visually select everything until the first column
              r*        "  Replace this selection with asterisks
                r/      "  Replace the first character with a slash

Perl, 41 bytes

s,␉,    ,g;s,^  ( +),/@{[$1=~y| |*|r]}/,

Run with the -p flag, like so:

perl -pe 's,␉,    ,g;s,^  ( +),/@{[$1=~y| |*|r]}/,'
#     ↑   └───────────────────┬───────────────────┘
#     1 byte               40 bytes

Replace ␉ by a tab (in Bash, try typing Control-V Tab.)

Cheddar, 60 57 56 bytes

Saved 3 bytes thanks to @Conor O'Brien

@.sub(/\t/g," "*4).sub(/^ +/gm,i->"/"+"*"*(i.len-2)+"/")

I wish Cheddar had better string formatting.

Try it online!

Explanation

This is a function. @ is a represents functionized property (e.g. ruby's &:) letting you do things like: `ar.map(@.head(-1))

@                      // Input
 .sub( /\t/g, " "*4)   // Replace tabs with four spaces
 .sub(
   /^ +/gm,            // Regex matches leading spaces
   i ->                // i is the matched leading spaces
     "/"+              // The / at the beginning
     "*"*(i.len-2)+    // Repeat *s i-2 times
     "/"                // The / at the end
 )

If you aren't familiar with regex the:

/^ +/gm

this basically matched one or more (+) spaces () at the beginning (^) of every (g) line (m).