How to use rewrite rule to replace tabs with spaces in gofmt tool?

gofmt command supports -r flag to use rewrite rule during formatting the code. How to use it to replace tabs indentation with the spaces indentation?

Go's format forced by gofmt dictates to use tab indentation.
Also see the decision lead to it:
https://code.google.com/p/go/issues/detail?id=7101

> Command gofmt
>
> The rewrite rule specified with the -r flag must be a string of the
> form:
>
> pattern -> replacement
>
> Both pattern and replacement must be valid Go expressions.

The tab and space characters are not valid Go expressions. It won't work.

> The Go Programming Language
>
> Alan A. A. Donovan & Brian W. Kernighan
>
> ISBN: 978-0134190440
>
> gopl.io
>
> Go takes a strong stance on code formatting. The gofmt tool rewrites
> code into the standard format, and the go tool’s fmt subcommand
> applies gofmt to all the files in the specified package, or the ones
> in the current directory by default. All Go source files in the book
> have been run through gofmt, and you should get into the habit of
> doing the same for your own code. Declaring a standard format by fiat
> eliminates a lot of pointless debate about trivia and, more
> importantly, enables a variety of automated source code
> transformations that would be infeasible if arbitrary formatting were
> allowed.

Always use gofmt code formatting.