Context sensitive tab expansion for Vim

With this plugin, hitting <Tab> in insert mode will insert a tab or spaces depending on the characters around the cursor:

1. If the character before the cursor is a space, then spaces will be inserted up to the next 'tabstop'.
2. If the character before the cursor is a tab, then a literal <Tab> will be inserted.
3. If the character before the cursor is neither a tab nor a space, and the character after the cursor is a space, then spaces will be inserted up to the next 'tabstop'.
4. If the character before the cursor is neither a tab nor a space, and the character after the cursor is a tab, then a literal <Tab> will be inserted.
5. If none of these cases apply, then this behaves like a regular i_<Tab> that respects 'expandtab'.

Let _ be a space and ---> be a tab.

Suppose you hit <Tab> here:

--->
    ^<Tab>

It doesn't make sense to insert spaces here, but if 'expandtab' is on, then spaces would get inserted anyway.

Suppose you hit <Tab> here:

____
    ^<Tab>

It doesn't make sense to insert a tab here, but if 'expandtab' is off, then a tab would get inserted anyway.

Using 'smarttab' or 'softtabstop' forces your 'expandtab' behavior onto the file, while using this plugin will respect whatever choice of indentation or alignment that was already there.

Customization

Use g:expandtab_nomap to disable the default <Tab> mapping.
E.g.,

let g:expandtab_nomap = 1

Use <Plug>ExpandTab to define your own mapping.
E.g.,

imap <Leader><Tab> <Plug>ExpandTab

Caveats

Vim mixes tabs and spaces in these cases when 'expandtab' is off:

1. at the start of a line, 'smarttab' is on, and 'shiftwidth' is not a multiple of 'tabstop'.
2. 'softtabstop' is negative, and 'shiftwidth' is not a multiple of 'tabstop'.
3. 'softtabstop' is set, and not a multiple of 'tabstop'.

It doesn't make sense for the plugin to function in these cases, so it will just behave like a regular i_<Tab>.