A heredoc inside a function call splices raw newlines into expression source

[livingstaccato]

Summary

SerializationContext.inside_dollar_string tells a rule it is being
serialized into expression source rather than into a value. StringRule
checks it and keeps its quotes. HeredocTemplateRule and
HeredocTrimTemplateRule never read it, so a heredoc argument is written into
the expression as its bare body, newlines and all.

Reproduction

hcl2.loads("a = upper(<<E\nx\nE\n)\n",
           serialization_options=SerializationOptions(preserve_heredocs=False,
                                                      strip_string_quotes=True))
# {'a': '${upper(x)}'}

x is now a bare reference rather than a string, and with a multi-line body
the result contains a raw newline inside ${...}, which is not parseable HCL.
The quoted-string case one line away is correct: upper("x") gives
${upper("x")}.

Both << and <<- are affected: HeredocTemplateRule.serialize
(hcl2/rules/strings.py:164 at v8.1.3) and
HeredocTrimTemplateRule.serialize (:197) both take the context and pass it
to their child, but branch only on options.strip_string_quotes.
StringRule.serialize (:115) checks context.inside_dollar_string for
exactly this case.

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This issue, and the investigation behind it, were produced by an AI assistant (Claude) working on behalf of the author. Please review with that provenance in mind.