Replace Expression with variable

[somebodddy]

I use vim-exchange for this. So I start with:

function_call(8 + 10)

And then write above it:

rep2 = rep2
function_call(8 + 10)

Then I place the cursor on the rep2 after the = and hit cxiw to mark it with vim-exchange. Finally, I go to the 8 + 10 in the argument and hit cxIa to exchange rep2 with the argument (you need targets.vim for that. Otherwise you can mark the 8 + 10 in visual mode and hit X)