We have a few special use-cases in Terraform where an object is
constructed from a mixture of different sources, such as a configuration
file, command line arguments, and environment variables.
To represent this within the HCL model, we introduce a new "synthetic"
HCL body type that just represents a map of values that are interpreted
as attributes.
We then export the previously-private MergeBodies function to allow the
synthetic body to be used as an override for a "real" body, which then
allows us to combine these various sources together while still retaining
the proper source location information for each individual attribute.
Since a synthetic body doesn't actually exist in configuration, it does
not produce source locations that can be turned into source snippets but
we can still use placeholder strings to help the user to understand
which of the many different sources a particular value came from.
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