HCL: allow if and in as attribute and object key names

[scuba10steve]

Summary

• The HCL lexer reserves IF and IN as dedicated tokens for use in for-expressions, but the HCL spec does not reserve these as keywords at the body level. Terraform itself accepts if and in as attribute names and object keys.

• Extend attribute and qualifiedIdentifier rules in HCLParser.g4 to accept IF and IN alongside Identifier. The visitor walks qualifiedIdentifier children in source order so dotted-name positions stay correct when alternatives are interleaved.

• For-expression parsing (for x in xs : x if cond) is unaffected — those tokens are still consumed at the for-rule level. Verified by the existing HclForTest suite.

• Fixes HclParser might fail if you use if as property name #6446

Test plan

• rewrite-hcl:test — 9 new + existing tests pass:
  • usingIfAsAttributeName / usingInAsAttributeName — issue's reproducer
  • keywordsAsTopLevelAttributes — body-level form, no enclosing block
  • keywordAttributeAlongsideForExpression — coexistence with for ... in ... if ...
  • keywordAttributeWithCommentAndUnusualSpacing — formatting variations
  • keywordsAsObjectKeys / keywordObjectKeyMixedWithRegularKeys — object literal keys
• rewrite-hcl:check — license, lint, all green
• Full repo ./gradlew build — BUILD SUCCESSFUL in 10m 13s, 442 tasks, no regressions in any downstream consumer
• HclForTest — for-expression regression coverage still passes

[timtebeek]

Thanks! We'd had a parallel effort open as well with a slightly different approach:

• Allow 'if' as HCL attribute name #6473

For both I'm wondering whether we should have these keywords called out in the grammar and generated parser, or whether we can better scope when to interpret and if as a conditional. No immediate suggestions here, but thought to call it out. Greg or Knut might have some ideas as well.

[scuba10steve]

Thanks! We'd had a parallel effort open as well with a slightly different approach:

• Allow 'if' as HCL attribute name #6473

For both I'm wondering whether we should have these keywords called out in the grammar and generated parser, or whether we can better scope when to interpret and if as a conditional. No immediate suggestions here, but thought to call it out. Greg or Knut might have some ideas as well.

ha!

If you already had something going let's just take yours. I haven't really tested this at all so it was just something I threw together.