How can I a put a line like “==========” quickly in vim

Question

I am editing restructuredtext files. I often need to put some charactors like \"=-`~\" in one line, and I want the length of the line match the previous line. How should I

Answer 1

How about yyp:s/./=/g ?

You can map that to a key, e.g. :map yyp:s/./=/g