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I'd count the number of space characters before the cursor at the moment <cr> is hit and produce as many <BS>. It can be done with substitute:

:inoremap <expr> <cr> substitute(matchstr(getline('.')[: col('.')-2], '\S\zs\s*$'), '.', "<bs>", 'g')."<cr>"

or with repeat+len:

:inoremap <expr> <cr> repeat("<bs>", len(matchstr(getline('.')[: col('.')-2], '\S\zs\s*$')))."<cr>"

Notes:

• As there is no movement/break (with <esc>) in the flow of this solution, anything that is typed can be undone, redone , etc. However, if you start a new insert sequence to cut after a series of spaces, redoing the same insertion sequence with . elsewhere is likely to remove too many characters.
• The \S\zs is a workaround when we're on en empty line and when each <BS> deletes &sw characters. In that case I should check getline() result and do something smarter.

I'd count the number of space characters before the cursor at the moment <cr> is hit and produce as many <BS>. It can be done with substitute:

:inoremap <expr> <cr> substitute(matchstr(getline('.')[: col('.')-2], '\S\zs\s*$'), '.', "<bs>", 'g')."<cr>"

or with repeat+len:

:inoremap <expr> <cr> repeat("<bs>", len(matchstr(getline('.')[: col('.')-2], '\S\zs\s*$')))."<cr>"

Notes:

• As there is no movement/break (with <esc>) in the flow of this solution, anything that is typed can be undone, redone , etc. However, if you start a new insert sequence to cut after a series of spaces, redoing the same insertion sequence with . elsewhere is likely to remove too many characters.
• The \S\zs is a workaround when we're on en empty line and when each <BS> deletes &sw characters. In that case I should check getline() result and do something smarter.

A version that behaves a little bit better regarding indented empty lines is the following:

function! s:trim_ws_on_the_fly()
  let line = getline('.')
  " TODO: set and reset &bs
  if line =~ '^\s*$'
    let nb = (virtcol('.') + 2*&sw-2) / &sw - 1
    " echomsg "remove "virtcol('.').'/'.&sw.' -> '.nb
  else
    let nb = len(matchstr(line[: col('.')-2], '\s*$'))
    let line = ''
  endif
  return repeat("\<bs>", nb)."\<cr>".line
endfunction
inoremap <expr> <cr> <sid>trim_ws_on_the_fly()

I'd count the number of space characters before the cursor at the moment <cr> is hit and produce as many <BS>. It can be done with substitute:

:inoremap <expr> <cr> substitute(matchstr(getline('.')[: col('.')-2], '\s*$'), '.', "<bs>", 'g')."<cr>"

or with repeat+len:

:inoremap <expr> <cr> repeat("<bs>", len(matchstr(getline('.')[: col('.')-2], '\s*$')))."<cr>"

Note: As there is no movement/break (with <esc>) in the flow of this solution, anything that is typed can be undone, redone , etc. However, if you start a new insert sequence to cut after a series of spaces, redoing the same insertion sequence with . elsewhere is likely to remove too many characters.

I'd count the number of space characters before the cursor at the moment <cr> is hit and produce as many <BS>. It can be done with substitute:

:inoremap <expr> <cr> substitute(matchstr(getline('.')[: col('.')-2], '\S\zs\s*$'), '.', "<bs>", 'g')."<cr>"

or with repeat+len:

:inoremap <expr> <cr> repeat("<bs>", len(matchstr(getline('.')[: col('.')-2], '\S\zs\s*$')))."<cr>"

Notes:

• As there is no movement/break (with <esc>) in the flow of this solution, anything that is typed can be undone, redone , etc. However, if you start a new insert sequence to cut after a series of spaces, redoing the same insertion sequence with . elsewhere is likely to remove too many characters.
• The \S\zs is a workaround when we're on en empty line and when each <BS> deletes &sw characters. In that case I should check getline() result and do something smarter.

I'd count the number of space characters before the cursor at the moment <cr> is hit and produce as many <BS>. It can be done with substitute also:

:inoremap <expr> <cr> substitute(matchstr(getline('.')[: col('.')-2], '\s*$'), '.', "<bs>", 'g')."<cr>"

or

:inoremap <expr> <cr> repeat("<bs>", len(matchstr(getline('.')[: col('.')-2], '\s*$')))."<cr>"

Note: As there is no movement/break (with <esc>) in the flow of this solution, anything that is typed can be undone, redone , etc. However, if you start a new insert sequence to cut after a series of spaces, redoing the same insertion sequence with . elsewhere is likely to remove too many characters.

I'd count the number of space characters before the cursor at the moment <cr> is hit and produce as many <BS>. It can be done with substitute:

:inoremap <expr> <cr> substitute(matchstr(getline('.')[: col('.')-2], '\s*$'), '.', "<bs>", 'g')."<cr>"

or with repeat+len:

:inoremap <expr> <cr> repeat("<bs>", len(matchstr(getline('.')[: col('.')-2], '\s*$')))."<cr>"

Note: As there is no movement/break (with <esc>) in the flow of this solution, anything that is typed can be undone, redone , etc. However, if you start a new insert sequence to cut after a series of spaces, redoing the same insertion sequence with . elsewhere is likely to remove too many characters.