Juri Linkov <juri@linkov.net> writes:

>>> Thanks, it would be nice to support displaying tabs in alternating
>>> colors.  But there are endless possibilities for look and feel of
>>> tabs, so instead of hard-coding only one of them for alternating
>>> colors, I suggest to add a customizable function with a name like
>>> 'tab-line-tab-face' that with a tab provided as its arg (and a list of
>>> tabs to be able to find the ordinal number of the given tab) would
>>> return its face to use.
>>
>> Yes, that seems like a good idea.  Something like this?  I also added
>> another face for non-file-backed buffers, which helps distinguish them.
>
> What if some users might want to display tabs with rainbow colors?
> Then this too specific option tab-line-alternate-colors won't help.
>
> What I suggest is that defcustom tab-line-tab-face-function could
> provide a separate function choice, e.g. tab-line-tab-face-alternate,
> so when customized to tab-line-tab-face-alternate, this function
> could call the default function tab-line-tab-face-default,
> and if the default function returns 'tab-line-tab-inactive',
> then replace it with 'tab-line-tab-inactive-alternate'
> depending on whether the tab is odd/even.

Okay, here's my next proposal: The tab-line-tab-face-default function
returns a face, and then a new option, tab-line-tab-face-modifiers, is a
list of functions, each of which is called with that face and returns
it, possibly modified.  This seems like a flexible mechanism which makes
it possible to modify the faces orthogonally according to arbitrary
criteria, and it can be used to add the alternating colors and
special-buffer faces when applicable (when so-configured by the user).
It could also be used to do a "rainbow" of faces, as you mentioned, etc.

To address Eli's feedback, in this patch I also defined the alternate
face to have a certain background color by default rather than
inheriting from mode-line.

However, the issue regarding italic slant on terminals remains; I'll be
glad to do whatever you or Eli suggest.

Thanks to you and Eli for your help.