hcldec: Allow body-derived values to be marked

Similar to the previously-added UnknownBody, the new optional interface
MarkedBody allows hcl.Body implementations to suggest a set of marks that
ought to be applied to any value that's generated to represent the content
of that body.

The dynblock extension then uses this to get hcldec to mark the whole
object representing any block that was generated by a dynamic block whose
for_each was marked, for a better representation of the fact that a
block's existence was decided based on a marked value.

ext/dynblock/expand_body.go

@@ -270,3 +270,8 @@ func (b *expandBody) JustAttributes() (hcl.Attributes, hcl.Diagnostics) {
 func (b *expandBody) MissingItemRange() hcl.Range {
 	return b.original.MissingItemRange()
 }
+
+// hcldec.MarkedBody impl
+func (b *expandBody) BodyValueMarks() cty.ValueMarks {
+	return b.valueMarks
+}

ext/dynblock/expand_body_test.go

@@ -762,7 +762,7 @@ func TestExpandMarkedForEach(t *testing.T) {
 			cty.ObjectVal(map[string]cty.Value{
 				"val0": cty.StringVal("static c 1").Mark("boop"),
 				"val1": cty.StringVal("hey").Mark("boop"),
-			}),
+			}).Mark("boop"),
 		})
 		got, diags := hcldec.Decode(dynBody, decSpec, nil)
 		if diags.HasErrors() {

hcldec/spec.go

@@ -73,6 +73,12 @@ type UnknownBody interface {
 	Unknown() bool
 }
 
+// MarkedBody can be optionally implemented by an hcl.Body instance to
+// indicate that a value created from it ought to be marked.
+type MarkedBody interface {
+	BodyValueMarks() cty.ValueMarks
+}
+
 func (s ObjectSpec) visitSameBodyChildren(cb visitFunc) {
 	for _, c := range s {
 		cb(c)
@@ -473,6 +479,7 @@ func (s *BlockListSpec) decode(content *hcl.BodyContent, blockLabels []blockLabe
 
 		val, _, childDiags := decode(childBlock.Body, labelsForBlock(childBlock), ctx, s.Nested, false)
 		diags = append(diags, childDiags...)
+		val = prepareBodyVal(val, childBlock.Body)
 
 		if u, ok := childBlock.Body.(UnknownBody); ok {
 			if u.Unknown() {
@@ -635,6 +642,7 @@ func (s *BlockTupleSpec) decode(content *hcl.BodyContent, blockLabels []blockLab
 
 		val, _, childDiags := decode(childBlock.Body, labelsForBlock(childBlock), ctx, s.Nested, false)
 		diags = append(diags, childDiags...)
+		val = prepareBodyVal(val, childBlock.Body)
 
 		if u, ok := childBlock.Body.(UnknownBody); ok {
 			if u.Unknown() {
@@ -758,6 +766,7 @@ func (s *BlockSetSpec) decode(content *hcl.BodyContent, blockLabels []blockLabel
 
 		val, _, childDiags := decode(childBlock.Body, labelsForBlock(childBlock), ctx, s.Nested, false)
 		diags = append(diags, childDiags...)
+		val = prepareBodyVal(val, childBlock.Body)
 
 		if u, ok := childBlock.Body.(UnknownBody); ok {
 			if u.Unknown() {
@@ -928,6 +937,7 @@ func (s *BlockMapSpec) decode(content *hcl.BodyContent, blockLabels []blockLabel
 
 		childLabels := labelsForBlock(childBlock)
 		val, _, childDiags := decode(childBlock.Body, childLabels[len(s.LabelNames):], ctx, s.Nested, false)
+		val = prepareBodyVal(val, childBlock.Body)
 		targetMap := elems
 		for _, key := range childBlock.Labels[:len(s.LabelNames)-1] {
 			if _, exists := targetMap[key]; !exists {
@@ -1082,6 +1092,7 @@ func (s *BlockObjectSpec) decode(content *hcl.BodyContent, blockLabels []blockLa
 
 		childLabels := labelsForBlock(childBlock)
 		val, _, childDiags := decode(childBlock.Body, childLabels[len(s.LabelNames):], ctx, s.Nested, false)
+		val = prepareBodyVal(val, childBlock.Body)
 		targetMap := elems
 		for _, key := range childBlock.Labels[:len(s.LabelNames)-1] {
 			if _, exists := targetMap[key]; !exists {
@@ -1720,3 +1731,11 @@ func (s noopSpec) sourceRange(content *hcl.BodyContent, blockLabels []blockLabel
 		Filename: "noopSpec",
 	}
 }
+
+func prepareBodyVal(decodeResult cty.Value, body hcl.Body) cty.Value {
+	if m, ok := body.(MarkedBody); ok {
+		marks := m.BodyValueMarks()
+		return decodeResult.WithMarks(marks)
+	}
+	return decodeResult
+}