This is a plugin I extracted for formatting TOML the way I like. You're right, there are no rules, but this is the way I like it. I have been working with Hugo a lot and copy/pasta-ing those config files leaves all the tomls in different states of indent and it was bothering me. Everything else I write indents on save.

I use it with vim-toml.

Original from toml-lang

title = "TOML Example"

[owner]
  name = "Tom Preston-Werner"
  dob = 1979-05-27T07:32:00-08:00 # First class dates

[database]
  server = "192.168.1.1"
  ports = [ 8001, 8001, 8002 ]
  connection_max = 5000
  enabled = true

[servers]

  # Indentation (tabs and/or spaces) is allowed but not required
[servers.alpha]
  ip = "10.0.0.1"
  dc = "eqdc10"

[servers.beta]
  ip = "10.0.0.2"
  dc = "eqdc10"

[clients]
[clients.thing]
  data = [ ["gamma", "delta"], [1, 2] ]

  # Line breaks are OK when inside arrays
  hosts = [
  "alpha",
  "omega"
  ]

It's basically just the toml.vim file for the indent folder.

Use Install with your favorite Plugin manager.

Plug 'guygrigsby/vim-opine', { 'for': 'toml' }

Plugin 'guygrigsby/vim-opine' etc...

In your .vimrc put:

au BufRead,BufWritePre *.toml :normal gg=G``

No vars. Uses shiftwidth for indentation