I just found out, that there is a different behaviour between the C-coreutils echo and the Rust version.

This command leads to an error, while working with the C version:

echo '"\n'CXXFLAGS=-g -O2'\n"'

It leads to this output:

echo '"\n'CXXFLAGS=-g -O2'\n"'
echo: error: Unrecognized option: 'O'

The expected output is:

echo '"\n'CXXFLAGS=-g -O2'\n"'
"\nCXXFLAGS=-g -O2\n"

It seems like the argument is somehow parsed wrong.