hclsyntax: Pass marks through template expressions

If a template expression interpolates values which have marks, we should
apply all of those marks to the output value. This allows template
expressions to function like native cty functions with respect to marks.

hclsyntax/expression_template.go

@@ -26,6 +26,9 @@ func (e *TemplateExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics)
 	var diags hcl.Diagnostics
 	isKnown := true
 
+	// Maintain a set of marks for values used in the template
+	marks := make(cty.ValueMarks)
+
 	for _, part := range e.Parts {
 		partVal, partDiags := part.Value(ctx)
 		diags = append(diags, partDiags...)
@@ -71,14 +74,24 @@ func (e *TemplateExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics)
 			continue
 		}
 
-		buf.WriteString(strVal.AsString())
+		// Unmark the part and merge its marks into the set
+		unmarked, partMarks := strVal.Unmark()
+		for k, v := range partMarks {
+			marks[k] = v
+		}
+
+		buf.WriteString(unmarked.AsString())
 	}
 
+	var ret cty.Value
 	if !isKnown {
-		return cty.UnknownVal(cty.String), diags
+		ret = cty.UnknownVal(cty.String)
+	} else {
+		ret = cty.StringVal(buf.String())
 	}
 
-	return cty.StringVal(buf.String()), diags
+	// Apply the full set of marks to the returned value
+	return ret.WithMarks(marks), diags
 }
 
 func (e *TemplateExpr) Range() hcl.Range {

hclsyntax/expression_template_test.go

@@ -283,6 +283,27 @@ trim`,
 			cty.UnknownVal(cty.String),
 			1, // Unexpected endfor directive
 		},
+		{ // marks from uninterpolated values are ignored
+			`hello%{ if false } ${target}%{ endif }`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"target": cty.StringVal("world").WithMarks(cty.NewValueMarks("sensitive")),
+				},
+			},
+			cty.StringVal("hello"),
+			0,
+		},
+		{ // marks from interpolated values are passed through
+			`${greeting} ${target}`,
+			&hcl.EvalContext{
+				Variables: map[string]cty.Value{
+					"greeting": cty.StringVal("hello").WithMarks(cty.NewValueMarks("english")),
+					"target":   cty.StringVal("world").WithMarks(cty.NewValueMarks("sensitive")),
+				},
+			},
+			cty.StringVal("hello world").WithMarks(cty.NewValueMarks("english", "sensitive")),
+			0,
+		},
 	}
 
 	for _, test := range tests {