Fixes https://github.com/hashicorp/terraform/issues/8886
While parsing JSON, an empty list value would be assumed to be a
non-existent list of objects, and would be removed from the result. This
may have never been the correct behavior but always worked okay because
we previously didn't support lists as first class types.
With the support of lists, we need to actually return the empty list as
the type. If we return nothing, then projects like Terraform will think
that the value was never set, which is false.
At some point we ignored the " in interpolations. We do this for HCL and
it is correct but this is invalid JSON syntax and for JSON we've always
had the stance that we have to escape them.
Before the parser rewrite, HCL would silently convert `\"` within
interpolation braces to `"`.
After the conversion, this became a syntax error.
We've found several instances of Terraform configs in the wild using
this syntax. It results in a hard "syntax error" message during config
parsing. While avoiding the extra escape on double quotes within
interpolations is definitely preferred, the UX of the syntax error feels
harsh enough to be worth inserting this backwards compatibility for now,
leaving us the option of deprecating it with a warning down the line.
With the previous Walk function it's not easy to rewrite the node as we
don't have any kind of reference to the parent. If we want to rewrite a
given AST, we have to manually traverse it as Walk is not usable. To
allow us rewriting the AST we change the signature of the function
passed to Walk. It'll allow us to rewrite the AST and return back.
Internally Walk() overrides the returned AST.
This idea was also talked here:
https://groups.google.com/forum/#!topic/golang-nuts/cRZQV36IckM
extensively.
