How to use different rules for highlighting trailing whitespace inside a gitcommit diff?

Question

In my ~/.vimrc, I have:

highlight TrailingWhitespace ctermbg=blue
autocmd BufReadPost * syn match TrailingWhitespace /\s\+\%#\@<!$/

This highlights trailing spaces, except when I am still typing at the end of the line. (I obtained the regex pattern from the Vim wiki).

What I want to do now is to change the trailing whitespace pattern only inside a gitcommit diff. Background:

• When doing git commit --verbose, a diff will be shown at the bottom of the git commit template opened in Vim. The syntax file responsible for git commits is syntax/gitcommit.vim.

• The first column of a diff is either + (i.e. line added), - (i.e. line deleted), or (i.e. no change). If a blank line is added, the diff for that line will show +. However, if a blank line is unchanged, the diff for that line will show . That is the problem. When the diff for an unchanged blank line shows in the first column, the syntax highlighting scheme above has a defect: it will highlight the as trailing whitespace.

  

That is why I want to change trailing whitespace detection pattern for gitcommit diffs. I want to ignore the first column. To that end, I created a file ~/.vim/after/syntax/gitcommit.vim:

" HACK: disable the usual highlighting for trailing whitespace by using a regex
" pattern that never matches anything.
syn match TrailingWhitespace /.^/ containedin=gitcommitDiff

" Ignore the first column when finding trailing whitespace.
syn match diffTrailingWhitespace /^..\{-}\zs\s\+$/ containedin=gitcommitDiff
highlight diffTrailingWhitespace ctermbg=cyan

At this point, the vim configuration seems to work as intended: trailing whitespace is highlighted, and special rules are used for highlighting trailing whitespace in gitcommit diff. The problem is that this all looks like a hack. I wonder if there is a more straightforward, or less hacky way of achieving the same result. Could you provide a better solution?

Answer 1

I have found an acceptable solution.

First, use the solution provided in the Vim wiki to highlight trailing whitespace. i.e. add to ~/.vimrc:

autocmd BufReadPost,InsertEnter * match TrailingWhitespace /\s\+\%#\@<!$/
autocmd InsertLeave * match TrailingWhitespace /\s\+$/
highlight TrailingWhitespace ctermbg=blue

Then, handle gitcommit using special rules. To do so, override and extend the syntax defined by syntax/gitcommit.vim. Add to ~/.vimrc:

function! s:set_gitcommit_trailing_whitespace()
    " Stop the usual highlighting of trailing whitespace.
    highlight TrailingWhitespace NONE

    " gitcommit.vim only contains syntax for the first line of the commit
    " message (i.e. the "commit message summary") and the blank line following
    " the commit message summary. It does not have a syntax for the rest of the
    " commit message (i.e. the "commit message body"), so we need to add a 
    " syntax for it.
    "
    " Step 1: Override gitcommitBlank from gitcommit.vim.
    "         This is necessary for adding a syntax for the commit message body.
    syn match gitcommitBlank "^[^#]*$" contained
        \ nextgroup=mygitcommitBody
        \ skipnl
    " Step 2: Add syntax for the commit message's body.
    syn match mygitcommitBody "^[^#]*$" contained
        \ contains=@Spell
        \ nextgroup=mygitcommitBody
        \ skipempty

    " Highlight trailing whitespace in the commit message (i.e. commit message
    " summary + commit message body).
    syn match commitTrailingWhitespace /\zs\s\+\%#\@<!$/
        \ containedin=gitcommitSummary,mygitcommitBody
    highlight commitTrailingWhitespace ctermbg=blue

    " Highlight trailing whitespace in the diff. Ignores the first column.
    syn match diffTrailingWhitespace /^..\{-}\zs\s\+$/
        \ containedin=gitcommitDiff
    highlight diffTrailingWhitespace ctermbg=cyan
endfunction
autocmd Syntax gitcommit call s:set_gitcommit_trailing_whitespace()

Note 1: this solution depends on the syntax defined in syntax/gitcommit.vim. I have tested this using the "2019 Aug 29" version of gitcommit.vim that is bundled with Vim 8.1.2269 (the version of Vim I am currently using). In the future, this solution may break if there are changes to the syntax defined in gitcommit.vim. The current upstream for gitcommit.vim is vim-git.

Note 2: this solution inherits some of the weaknesses of gitcommit.vim. In particular, gitcommit.vim fails to highlight correctly when the user writes comments (i.e. lines starting with #) in the commit message area. This solution will similarly fail to highlight trailing whitespace correctly if the user writes comments in the commit message area.