How can I insert a character repeatedly until even with next line?

Question

Say I have a comment like this:

//This is a comment

And I want to make it like this:

//*****************
//This is a comment
//*****************

Is there an easy way to insert a character repeatedly until even with the next/previous line?

Answer 1

Short answer

v $ r *

:help v_r

Longer

y y p / P     Duplicate line to below/above

2 l                    Move to position from where to overwrite (in this case 3rd column ← 2 to the right)

v $                  Select till end of line

r                          Replace all selected characters by char…

*

Answer 2

At the occasion°:

Insert mode mappings to replicate the remainder of adjacent line

" Insert the rest of the line below the cursor.
" Mnemonic: Elevate characters from below line
inoremap <A-e> 
    \<Esc>
    \jl
        \y$
    \hk
        \p
        \a
" Insert the rest of the line above the cursor.
" Mnemonic:  Y depicts a funnel, through which the above line's characters pour onto the current line.
inoremap <A-y> 
    \<Esc>
    \kl
        \y$
    \hj
        \p
        \a

---

Copying single characters from neighbouring lines in Insert mode

…is built-in: :help i_CTRL-E:

CTRL-E   Insert the character which is below the cursor.
CTRL-Y   Insert the character which is above the cursor.

---

_{° Not what the question text was about, but what other users might be looking for here, going from the current question title "insert until even with next line".}