encoding/yaml: add support for YAML tags via @yaml(tag) attribute

internal/encoding/yaml/encode.go

@@ -234,6 +234,27 @@ func encodeExprs(exprs []ast.Expr) (n *yaml.Node, err error) {
 	return n, nil
 }
 
+// extractYAMLTag looks for @yaml(tag="...") attribute and returns the tag value.
+// Returns an empty string if no @yaml attribute or no tag argument is found.
+// Returns an error if the attribute is malformed.
+func extractYAMLTag(attrs []*ast.Attribute) (string, error) {
+	for _, attr := range attrs {
+		key, body := attr.Split()
+		if key == "yaml" {
+			parsed := internal.ParseAttrBody(attr.Pos(), body)
+			if parsed.Err != nil {
+				return "", parsed.Err
+			}
+			if val, found, err := parsed.Lookup(0, "tag"); err != nil {
+				return "", err
+			} else if found {
+				return val, nil
+			}
+		}
+	}
+	return "", nil
+}
+
 // encodeDecls converts a sequence of declarations to a value. If it encounters
 // an embedded value, it will return this expression. This is more relaxed for
 // structs than is currently allowed for CUE, but the expectation is that this
@@ -289,6 +310,15 @@ func encodeDecls(decls []ast.Decl) (n *yaml.Node, err error) {
 			if err != nil {
 				return nil, err
 			}
+
+			yamlTag, err := extractYAMLTag(x.Attrs)
+			if err != nil {
+				return nil, err
+			}
+			if yamlTag != "" {
+				value.Tag = yamlTag
+			}
+
 			lastHead = label
 			lastFoot = value
 			addDocs(x, label, value)

internal/encoding/yaml/encode_test.go

@@ -243,6 +243,70 @@ route:
   receiver: pager
   group_by: [alertname, cluster]
 		`,
+	}, {
+		name: "yaml_tag_scalar",
+		in: `
+		key: "value" @yaml(tag="!Custom")
+		env: "VAR_NAME" @yaml(tag="!Env")
+		`,
+		out: `
+key: !Custom value
+env: !Env VAR_NAME
+		`,
+	}, {
+		name: "yaml_tag_sequence",
+		in: `
+		lookup: ["table", ["key", "value"]] @yaml(tag="!Find")
+		items: [1, 2, 3] @yaml(tag="!Seq")
+		`,
+		out: `
+lookup: !Find [table, [key, value]]
+items: !Seq [1, 2, 3]
+		`,
+	}, {
+		name: "yaml_tag_mapping",
+		in: `
+		config: {
+			key: "value"
+			count: 42
+		} @yaml(tag="!Map")
+		`,
+		out: `
+config: !Map
+  key: value
+  count: 42
+		`,
+	}, {
+		name: "yaml_tag_mixed",
+		in: `
+		plain: "no-tag"
+		tagged: "has-tag" @yaml(tag="!Custom")
+		nested: {
+			field: "value" @yaml(tag="!Nested")
+		}
+		`,
+		out: `
+plain: no-tag
+tagged: !Custom has-tag
+nested:
+  field: !Nested value
+		`,
+	}, {
+		name: "yaml_tag_verbatim",
+		in: `
+		custom: "value" @yaml(tag="!<tag:example.com,2000:app/foo>")
+		`,
+		out: `
+custom: !%3Ctag:example.com,2000:app/foo%3E value
+		`,
+	}, {
+		name: "yaml_attribute_without_tag",
+		in: `
+		field: "value" @yaml(other="ignored")
+		`,
+		out: `
+field: value
+		`,
 	}}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {