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

The HCL lexer reserves `IF` and `IN` as dedicated tokens for use in
for-expressions, but the spec doesn't reserve these as keywords at the
body level. Terraform itself accepts `if` and `in` as attribute names
and object keys, so the parser should too.

Extend the `attribute` and `qualifiedIdentifier` rules to accept `IF`
and `IN` alongside `Identifier`. The visitor walks `qualifiedIdentifier`
children in source order to keep dotted-name positions correct when the
alternatives are interleaved.

Fixes #6446

Author: scuba10steve

rewrite-hcl/src/main/antlr/HCLParser.g4

@@ -19,7 +19,7 @@ bodyContent
     ;
 
 attribute
-    : (Identifier | NULL) ASSIGN expression
+    : (Identifier | NULL | IF | IN) ASSIGN expression
     ;
 
 block
@@ -91,7 +91,7 @@ objectelem
     ;
 
 qualifiedIdentifier
-    : Identifier (DOT Identifier)*
+    : (Identifier | IF | IN) (DOT (Identifier | IF | IN))*
     ;
 
 // For Expressions

rewrite-hcl/src/main/java/org/openrewrite/hcl/internal/HclParserVisitor.java

@@ -71,7 +71,7 @@ public Hcl visitAttribute(HCLParser.AttributeContext ctx) {
                 randomId(),
                 Space.format(prefix),
                 Markers.EMPTY,
-                c.Identifier() != null ? visitIdentifier(c.Identifier()) : visitIdentifier(c.NULL()),
+                visitIdentifier(attributeNameToken(c)),
                 new HclLeftPadded<>(
                         sourceBefore("="),
                         Hcl.Attribute.Type.Assignment,
@@ -82,6 +82,33 @@ public Hcl visitAttribute(HCLParser.AttributeContext ctx) {
         ));
     }
 
+    private static TerminalNode attributeNameToken(HCLParser.AttributeContext ctx) {
+        if (ctx.Identifier() != null) {
+            return ctx.Identifier();
+        } else if (ctx.NULL() != null) {
+            return ctx.NULL();
+        } else if (ctx.IF() != null) {
+            return ctx.IF();
+        } else if (ctx.IN() != null) {
+            return ctx.IN();
+        }
+        throw new IllegalStateException("Unexpected attribute name token in: " + ctx.getText());
+    }
+
+    private static List<TerminalNode> qualifiedIdentifierNames(HCLParser.QualifiedIdentifierContext ctx) {
+        List<TerminalNode> names = new ArrayList<>();
+        for (int i = 0; i < ctx.getChildCount(); i++) {
+            ParseTree child = ctx.getChild(i);
+            if (child instanceof TerminalNode) {
+                int type = ((TerminalNode) child).getSymbol().getType();
+                if (type != HCLParser.DOT) {
+                    names.add((TerminalNode) child);
+                }
+            }
+        }
+        return names;
+    }
+
     @Override
     public Hcl visitAttributeAccessExpression(HCLParser.AttributeAccessExpressionContext ctx) {
         return convert(ctx, (c, prefix) -> new Hcl.AttributeAccess(
@@ -744,7 +771,7 @@ public Hcl visitVariableExpr(HCLParser.VariableExprContext ctx) {
         if (ctx == null) {
             return null;
         }
-        List<TerminalNode> identifiers = ctx.Identifier();
+        List<TerminalNode> identifiers = qualifiedIdentifierNames(ctx);
         if (identifiers.size() == 1) {
             return visitIdentifier(identifiers.get(0));
         }