pkg/encoding/toml: add new builtin package for encoding/toml

With just Marshal and Unmarshal for now; validation APIs will follow,
as their semantics don't seem consistent between json and yaml.
Leave a TODO to come back to the validation APIs as well.

We add some txtar tests as well, driven by pkg/internal/builtintest.

Fixes #3344.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I5cd4c01ab39daa4a208afcd8c3303f5f8d4ef512
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201474
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

pkg/encoding/toml/manual.go

@@ -0,0 +1,43 @@
+// Copyright 2024 The CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package toml
+
+import (
+	"bytes"
+	"strings"
+
+	"cuelang.org/go/cue"
+	"cuelang.org/go/cue/ast"
+	"cuelang.org/go/encoding/toml"
+)
+
+// Marshal returns the TOML encoding of v.
+func Marshal(v cue.Value) (string, error) {
+	if err := v.Validate(cue.Concrete(true)); err != nil {
+		return "", err
+	}
+	var b strings.Builder
+	if err := toml.NewEncoder(&b).Encode(v); err != nil {
+		return "", err
+	}
+	return b.String(), nil
+}
+
+// Unmarshal parses the TOML to a CUE expression.
+func Unmarshal(data []byte) (ast.Expr, error) {
+	return toml.NewDecoder("", bytes.NewReader(data)).Decode()
+}
+
+// TODO(mvdan): add Validate too, but which semantics? encoding/json and encoding/yaml do not seem to agree.

pkg/encoding/toml/testdata/marshal.txtar

@@ -0,0 +1,165 @@
+-- in.cue --
+import "encoding/toml"
+
+marshal: {
+	_input: {
+		rootKey: r1: "foo"
+		rootKeys: {
+			r1: "foo"
+			r2: "bar"
+			r3: "baz"
+		}
+		rootKeysDots: {
+			a1: "foo"
+			b1: b2: "bar"
+			c1: c2: c3: "baz"
+		}
+		subtables: {
+			tables: [
+				{table1: "foo"},
+				{table2: "bar"},
+				{subtable: {sub1: "baz"}}
+			]
+		}
+		complexKeys: "123-456": " foo bar ": "value"
+		defaults: key: string | *"default"
+
+		failIncomplete: key: string | "nondefault"
+		failRequired: key!: "foo"
+	}
+	for name, value in _input {
+		output: (name): toml.Marshal(value)
+	}
+}
+
+unmarshal: {
+	_input: {
+		rootKeysDots: """
+			a1       = "A"
+			b1.b2    = "B"
+			c1.c2.c3 = "C"
+			"""
+		subtables: """
+			[[tables]]
+			table1 = 'foo'
+			[[tables]]
+			table2 = 'bar'
+			[[tables]]
+			[tables.subtable]
+			sub1 = 'baz'
+			"""
+		complexKeys: """
+			[123-456]
+			' foo bar ' = 'value'
+			"""
+		defaultEmpty: string | *""
+
+		failIncomplete: string | ""
+		failBadSyntax: """
+			= "no key name"
+			"""
+		failDuplicate: """
+			foo = "same key"
+			foo = "same key"
+			"""
+	}
+	for name, value in _input {
+		output: (name): toml.Unmarshal(value)
+	}
+}
+
+-- out/toml --
+Errors:
+unmarshal.output.failBadSyntax: error in call to encoding/toml.Unmarshal: invalid character at start of key: =:
+    ./in.cue:66:19
+    1:1
+unmarshal.output.failDuplicate: error in call to encoding/toml.Unmarshal: duplicate key: foo:
+    ./in.cue:66:19
+    2:1
+
+Result:
+import "encoding/toml"
+
+marshal: {
+	output: {
+		rootKey: """
+			r1 = 'foo'
+
+			"""
+		rootKeys: """
+			r1 = 'foo'
+			r2 = 'bar'
+			r3 = 'baz'
+
+			"""
+		rootKeysDots: """
+			a1 = 'foo'
+
+			[b1]
+			b2 = 'bar'
+
+			[c1]
+			[c1.c2]
+			c3 = 'baz'
+
+			"""
+		subtables: """
+			[[tables]]
+			table1 = 'foo'
+
+			[[tables]]
+			table2 = 'bar'
+
+			[[tables]]
+			[tables.subtable]
+			sub1 = 'baz'
+
+			"""
+		complexKeys: """
+			[123-456]
+			' foo bar ' = 'value'
+
+			"""
+		defaults: """
+			key = 'default'
+
+			"""
+		failIncomplete: toml.Marshal(value)
+		failRequired:   toml.Marshal(value)
+	}
+}
+unmarshal: {
+	output: {
+		rootKeysDots: {
+			a1: "A"
+			b1: {
+				b2: "B"
+			}
+			c1: {
+				c2: {
+					c3: "C"
+				}
+			}
+		}
+		subtables: {
+			tables: [{
+				table1: "foo"
+			}, {
+				table2: "bar"
+			}, {
+				subtable: {
+					sub1: "baz"
+				}
+			}]
+		}
+		complexKeys: {
+			"123-456": {
+				" foo bar ": "value"
+			}
+		}
+		defaultEmpty: {}
+		failIncomplete: toml.Unmarshal(value)
+		failBadSyntax:  _|_ // unmarshal.output.failBadSyntax: error in call to encoding/toml.Unmarshal: invalid character at start of key: =
+		failDuplicate:  _|_ // unmarshal.output.failDuplicate: error in call to encoding/toml.Unmarshal: duplicate key: foo
+	}
+}

pkg/encoding/toml/toml_test.go

@@ -0,0 +1,25 @@
+// Copyright 2024 CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package toml_test
+
+import (
+	"testing"
+
+	"cuelang.org/go/pkg/internal/builtintest"
+)
+
+func TestBuiltin(t *testing.T) {
+	builtintest.Run("toml", t)
+}