hclsyntax: Fix another template loop panic

If individual template expressions in a loop have marks, merge those
marks into the final result when joining.

hclsyntax/expression_template.go

@@ -153,6 +153,7 @@ func (e *TemplateJoinExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 	}
 
 	tuple, marks := tuple.Unmark()
+	allMarks := []cty.ValueMarks{marks}
 	buf := &bytes.Buffer{}
 	it := tuple.ElementIterator()
 	for it.Next() {
@@ -193,10 +194,14 @@ func (e *TemplateJoinExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 			return cty.UnknownVal(cty.String).WithMarks(marks), diags
 		}
 
+		strVal, strValMarks := strVal.Unmark()
+		if len(strValMarks) > 0 {
+			allMarks = append(allMarks, strValMarks)
+		}
 		buf.WriteString(strVal.AsString())
 	}
 
-	return cty.StringVal(buf.String()).WithMarks(marks), diags
+	return cty.StringVal(buf.String()).WithMarks(allMarks...), diags
 }
 
 func (e *TemplateJoinExpr) Range() hcl.Range {

hclsyntax/expression_template_test.go

@@ -330,6 +330,34 @@ trim`,
 			cty.StringVal("foobarbaz").Mark("sensitive"),
 			0,
 		},
+		{ // marks on individual elements propagate to the result
+			`%{ for s in secrets }${s}%{ endfor }`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"secrets": cty.ListVal([]cty.Value{
+						cty.StringVal("foo"),
+						cty.StringVal("bar").Mark("sensitive"),
+						cty.StringVal("baz"),
+					}),
+				},
+			},
+			cty.StringVal("foobarbaz").Mark("sensitive"),
+			0,
+		},
+		{ // lots of marks!
+			`%{ for s in secrets }${s}%{ endfor }`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"secrets": cty.ListVal([]cty.Value{
+						cty.StringVal("foo").Mark("x"),
+						cty.StringVal("bar").Mark("y"),
+						cty.StringVal("baz").Mark("z"),
+					}).Mark("x"), // second instance of x
+				},
+			},
+			cty.StringVal("foobarbaz").WithMarks(cty.NewValueMarks("x", "y", "z")),
+			0,
+		},
 	}
 
 	for _, test := range tests {