zclsyntax: Implement the ContextString interface on our Nav

This allows tooling to get a string describing the context of a particular
offset into the file. This is used, for example, to provide context
above the source code snippets in console-printed diagnostic messages.

zcl/zclsyntax/navigation.go

@@ -0,0 +1,41 @@
+package zclsyntax
+
+import (
+	"bytes"
+	"fmt"
+)
+
+type navigation struct {
+	root *Body
+}
+
+// Implementation of zcled.ContextString
+func (n navigation) ContextString(offset int) string {
+	// We will walk our top-level blocks until we find one that contains
+	// the given offset, and then construct a representation of the header
+	// of the block.
+
+	var block *Block
+	for _, candidate := range n.root.Blocks {
+		if candidate.Range().ContainsOffset(offset) {
+			block = candidate
+			break
+		}
+	}
+
+	if block == nil {
+		return ""
+	}
+
+	if len(block.Labels) == 0 {
+		// Easy case!
+		return block.Type
+	}
+
+	buf := &bytes.Buffer{}
+	buf.WriteString(block.Type)
+	for _, label := range block.Labels {
+		fmt.Fprintf(buf, " %q", label)
+	}
+	return buf.String()
+}

zcl/zclsyntax/public.go

@@ -23,6 +23,10 @@ func ParseConfig(src []byte, filename string, start zcl.Pos) (*zcl.File, zcl.Dia
 	return &zcl.File{
 		Body:  body,
 		Bytes: src,
+
+		Nav: navigation{
+			root: body,
+		},
 	}, diags
 }