HEREDOC tokens are a little more fussy than normal string sequences
because we need to preserve the whitespace within them along with the
start and end markers while we upgrade any interpolated expressions inside.
We need to do some work locally here because the HCL heredoc processing
"does too much" and throws away information we need to do a faithful
upgrade.
We also need to contend with the fact that Terraform <=0.11 had an older
version of HCL that accidentally permitted a degenerate form of heredoc
where the marker was at the end of the final line, like this:
degenerate = <<EOT
this should never have workedEOT
When we migrate this, we'll introduce the additional newline that is now
required, which will unfortunately slightly change the result string to
include a newline when parsed by 0.12, and so we'll need to call this out
as a caveat in the upgrade guide.
Martin Atkins