Terraform has a _lot_ of functions written against HIL's function API, and
we're not ready to rewrite them all yet, so instead we shim the HIL
function API to conform to the HCL2 (really: cty) function API and thus
allow most of our existing functions to work as expected when called from
HCL2-based config files.
Not all of the functions can be fully shimmed in this way due to depending
on HIL implementation details that we can't mimic through the HCL2 API.
We don't attempt to address that yet, and instead just let them fail when
called. We will eventually address this by using first-class HCL2
functions for these few cases, thus avoiding the HIL API altogether where
we need to. (The methodology for that is already illustrated here in the
provision of jsonencode and jsondecode functions that are HCL2-native.)
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