Write char in line as many times as the length of previous line

Question

I would like to write a char as many times as the length of the previous line

Starting point:

This is a line and I want to write a char under it.

I want to write char = as many times as the length of this line. This is the result I would like to obtain:

This is a line and I want to write a char under it.
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How I can achieve this result with the most reduced key/commands combination?

Answer 1

I suspect there is something shorter but the following works with the cursor starting over the initial line:

Yp:s/./=/g

It duplicates the line (Yp), then replaces each character on the new line with an = (:s/./=/g)

Update

An even shorter version from Doktor OSwaldo

YpVr=

Duplicates the line, selects it and replaces all characters with =

And, if you are using this a lot, it'll be even shorter as a macro.