TOML encoder: prefer readable table sections over inline tables (#2649)

* Initial plan

* Fix TOML encoder to prefer readable table sections over inline tables

When converting from YAML/JSON to TOML, the encoder now always uses
readable TOML table section syntax ([section]) instead of compact inline
hash table syntax (key = { ... }), which better matches TOML's goal as
a human-focused configuration format.

Changes:
- decoder_toml.go: Mark inline TOML tables with FlowStyle so round-trips
  can be distinguished from YAML flow mappings
- encoder_toml.go:
  - encodeTopLevelEntry: use FlowStyle check instead of EncodeSeparate to
    decide inline vs table section (all block mappings now become tables)
  - encodeSeparateMapping: count FlowStyle children as attributes; use
    recursive encodeSeparateMapping for nested non-flow mappings
  - encodeMappingBodyWithPath: emit non-flow child mappings as sub-table
    sections instead of inline tables
- toml_test.go: add encode (YAML→TOML) test scenarios, update roundtrip
  expectations for inline tables (now expanded to table sections)

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* Fix TOML roundtrip: use TomlInline flag instead of FlowStyle to preserve inline tables

FlowStyle affected YAML decode output (causing inline tables to appear as
YAML flow mappings). Replace it with a new TOML-specific TomlInline bool
on CandidateNode that:
- Is set by the TOML decoder for inline tables (not FlowStyle)
- Is copied by UpdateAttributesFrom so it survives DeeplyAssign merges
- Is checked by the TOML encoder alongside FlowStyle (for YAML flow maps)
- Has no effect on the YAML encoder, preserving existing TOML→YAML output

TOML roundtrip tests are restored to their original expected values (inline
tables stay inline, table sections stay as sections).

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* Refactor EncodeSeparate+TomlInline into a single EncodeHint enum

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* Fix stale comment in hasStructuralChildren

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* Remove unused hasStructuralChildren method from tomlEncoder

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---------

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Co-authored-by: mikefarah <1151925+mikefarah@users.noreply.github.com>

pkg/yqlib/candidate_node.go

@@ -27,6 +27,22 @@ const (
 	FlowStyle
 )
 
+// EncodeHint controls how a mapping node is serialised by format-specific encoders
+// that distinguish between inline and block/section representations (e.g. TOML, HCL).
+type EncodeHint int
+
+const (
+	// EncodeHintDefault lets the encoder choose the representation (e.g. TOML block
+	// mappings default to [section] headers).
+	EncodeHintDefault EncodeHint = iota
+	// EncodeHintSeparateBlock forces the node to be emitted as a separate block or
+	// table-section header (used by TOML [section] and HCL block decoders).
+	EncodeHintSeparateBlock
+	// EncodeHintInline forces the node to be emitted as an inline / flow table
+	// (used by TOML inline-table decoder and TOML encoder).
+	EncodeHintInline
+)
+
 func createStringScalarNode(stringValue string) *CandidateNode {
 	var node = &CandidateNode{Kind: ScalarNode}
 	node.Value = stringValue
@@ -97,9 +113,9 @@ type CandidateNode struct {
 	// (e.g. top level cross document merge). This property does not propagate to child nodes.
 	EvaluateTogether bool
 	IsMapKey         bool
-	// For formats like HCL and TOML: indicates that child entries should be emitted as separate blocks/tables
+	// EncodeHint controls how a mapping node is serialised by format-specific encoders
-	// rather than consolidated into nested mappings (default behaviour)
+	// (e.g. TOML, HCL) that support both inline and block/section representations.
-	EncodeSeparate bool
+	EncodeHint EncodeHint
 }
 
 func (n *CandidateNode) CreateChild() *CandidateNode {
@@ -411,7 +427,7 @@ func (n *CandidateNode) doCopy(cloneContent bool) *CandidateNode {
 		EvaluateTogether: n.EvaluateTogether,
 		IsMapKey:         n.IsMapKey,
 
-		EncodeSeparate: n.EncodeSeparate,
+		EncodeHint: n.EncodeHint,
 	}
 
 	if cloneContent {
@@ -465,8 +481,8 @@ func (n *CandidateNode) UpdateAttributesFrom(other *CandidateNode, prefs assignP
 		n.Anchor = other.Anchor
 	}
 
-	// Preserve EncodeSeparate flag for format-specific encoding hints
+	// Preserve EncodeHint for format-specific encoding hints
-	n.EncodeSeparate = other.EncodeSeparate
+	n.EncodeHint = other.EncodeHint
 
 	// merge will pickup the style of the new thing
 	// when autocreating nodes

pkg/yqlib/decoder_hcl.go

@@ -226,7 +226,7 @@ func addBlockToMapping(parent *CandidateNode, block *hclsyntax.Block, src []byte
 		// Mark the type node if there are multiple blocks of this type at this level
 		// This tells the encoder to emit them as separate blocks rather than consolidating them
 		if isMultipleBlocksOfType {
-			typeNode.EncodeSeparate = true
+			typeNode.EncodeHint = EncodeHintSeparateBlock
 		}
 	}
 	current = typeNode

pkg/yqlib/decoder_toml.go

@@ -150,9 +150,10 @@ func (dec *tomlDecoder) createInlineTableMap(tomlNode *toml.Node) (*CandidateNod
 	}
 
 	return &CandidateNode{
-		Kind:    MappingNode,
+		Kind:       MappingNode,
-		Tag:     "!!map",
+		Tag:        "!!map",
-		Content: content,
+		EncodeHint: EncodeHintInline,
+		Content:    content,
 	}, nil
 }
 
@@ -345,10 +346,10 @@ func (dec *tomlDecoder) processTable(currentNode *toml.Node) (bool, error) {
 	}
 
 	tableNodeValue := &CandidateNode{
-		Kind:           MappingNode,
+		Kind:       MappingNode,
-		Tag:            "!!map",
+		Tag:        "!!map",
-		Content:        make([]*CandidateNode, 0),
+		Content:    make([]*CandidateNode, 0),
-		EncodeSeparate: true,
+		EncodeHint: EncodeHintSeparateBlock,
 	}
 
 	// Attach pending head comments to the table
@@ -442,9 +443,9 @@ func (dec *tomlDecoder) processArrayTable(currentNode *toml.Node) (bool, error)
 	hasValue := dec.parser.NextExpression()
 
 	tableNodeValue := &CandidateNode{
-		Kind:           MappingNode,
+		Kind:       MappingNode,
-		Tag:            "!!map",
+		Tag:        "!!map",
-		EncodeSeparate: true,
+		EncodeHint: EncodeHintSeparateBlock,
 	}
 
 	// Attach pending head comments to the array table

pkg/yqlib/doc/usage/toml.md

@@ -384,3 +384,20 @@ ip = "10.0.0.2"
 role = "backend"
 ```
 
+## Encode: Simple mapping produces table section
+Given a sample.yml file of:
+```yaml
+arg:
+  hello: foo
+
+```
+then
+```bash
+yq -o toml '.' sample.yml
+```
+will output
+```toml
+[arg]
+hello = "foo"
+```
+

pkg/yqlib/encoder_hcl.go

@@ -449,8 +449,8 @@ func (he *hclEncoder) encodeBlockIfMapping(body *hclwrite.Body, key string, valu
 		return false
 	}
 
-	// If EncodeSeparate is set, emit children as separate blocks regardless of label extraction
+	// If EncodeHintSeparateBlock is set, emit children as separate blocks regardless of label extraction
-	if valueNode.EncodeSeparate {
+	if valueNode.EncodeHint == EncodeHintSeparateBlock {
 		if handled, _ := he.encodeMappingChildrenAsBlocks(body, key, valueNode); handled {
 			return true
 		}
@@ -537,9 +537,9 @@ func (he *hclEncoder) encodeMappingChildrenAsBlocks(body *hclwrite.Body, blockTy
 		return false, nil
 	}
 
-	// Only emit as separate blocks if EncodeSeparate is true
+	// Only emit as separate blocks if EncodeHintSeparateBlock is set
 	// This allows the encoder to respect the original block structure preserved by the decoder
-	if !valueNode.EncodeSeparate {
+	if valueNode.EncodeHint != EncodeHintSeparateBlock {
 		return false, nil
 	}
 

pkg/yqlib/encoder_toml.go

@@ -184,12 +184,9 @@ func (te *tomlEncoder) encodeTopLevelEntry(w io.Writer, path []string, node *Can
 		// Regular array attribute
 		return te.writeArrayAttribute(w, path[len(path)-1], node)
 	case MappingNode:
-		// Inline table if not EncodeSeparate, else emit separate tables/arrays of tables for children under this path
+		// Use inline table syntax for nodes explicitly marked as TOML inline tables
-		if !node.EncodeSeparate {
+		// or YAML flow mappings. All other mappings become readable TOML table sections.
-			// If children contain mappings or arrays of mappings, prefer separate sections
+		if node.EncodeHint == EncodeHintInline || node.Style&FlowStyle != 0 {
-			if te.hasEncodeSeparateChild(node) || te.hasStructuralChildren(node) {
-				return te.encodeSeparateMapping(w, path, node)
-			}
 			return te.writeInlineTableAttribute(w, path[len(path)-1], node)
 		}
 		return te.encodeSeparateMapping(w, path, node)
@@ -451,7 +448,8 @@ func (te *tomlEncoder) writeTableHeader(w io.Writer, path []string, m *Candidate
 // encodeSeparateMapping handles a mapping that should be encoded as table sections.
 // It emits the table header for this mapping if it has any content, then processes children.
 func (te *tomlEncoder) encodeSeparateMapping(w io.Writer, path []string, m *CandidateNode) error {
-	// Check if this mapping has any non-mapping, non-array-of-tables children (i.e., attributes)
+	// Check if this mapping has any non-mapping, non-array-of-tables children (i.e., attributes).
+	// Inline mapping children also count as attributes since they render as key = { ... }.
 	hasAttrs := false
 	for i := 0; i < len(m.Content); i += 2 {
 		v := m.Content[i+1]
@@ -459,6 +457,10 @@ func (te *tomlEncoder) encodeSeparateMapping(w io.Writer, path []string, m *Cand
 			hasAttrs = true
 			break
 		}
+		if v.Kind == MappingNode && (v.EncodeHint == EncodeHintInline || v.Style&FlowStyle != 0) {
+			hasAttrs = true
+			break
+		}
 		if v.Kind == SequenceNode {
 			// Check if it's NOT an array of tables
 			allMaps := true
@@ -486,18 +488,14 @@ func (te *tomlEncoder) encodeSeparateMapping(w io.Writer, path []string, m *Cand
 		return nil
 	}
 
-	// No attributes, just nested structures - process children
+	// No attributes, just nested table structures - process children recursively
 	for i := 0; i < len(m.Content); i += 2 {
 		k := m.Content[i].Value
 		v := m.Content[i+1]
 		switch v.Kind {
 		case MappingNode:
-			// Emit [path.k]
 			newPath := append(append([]string{}, path...), k)
-			if err := te.writeTableHeader(w, newPath, v); err != nil {
+			if err := te.encodeSeparateMapping(w, newPath, v); err != nil {
-				return err
-			}
-			if err := te.encodeMappingBodyWithPath(w, newPath, v); err != nil {
 				return err
 			}
 		case SequenceNode:
@@ -535,39 +533,6 @@ func (te *tomlEncoder) encodeSeparateMapping(w io.Writer, path []string, m *Cand
 	return nil
 }
 
-func (te *tomlEncoder) hasEncodeSeparateChild(m *CandidateNode) bool {
-	for i := 0; i < len(m.Content); i += 2 {
-		v := m.Content[i+1]
-		if v.Kind == MappingNode && v.EncodeSeparate {
-			return true
-		}
-	}
-	return false
-}
-
-func (te *tomlEncoder) hasStructuralChildren(m *CandidateNode) bool {
-	for i := 0; i < len(m.Content); i += 2 {
-		v := m.Content[i+1]
-		// Only consider it structural if mapping has EncodeSeparate or is non-empty
-		if v.Kind == MappingNode && v.EncodeSeparate {
-			return true
-		}
-		if v.Kind == SequenceNode {
-			allMaps := true
-			for _, it := range v.Content {
-				if it.Kind != MappingNode {
-					allMaps = false
-					break
-				}
-			}
-			if allMaps {
-				return true
-			}
-		}
-	}
-	return false
-}
-
 // encodeMappingBodyWithPath encodes attributes and nested arrays of tables using full dotted path context
 func (te *tomlEncoder) encodeMappingBodyWithPath(w io.Writer, path []string, m *CandidateNode) error {
 	// First, attributes (scalars and non-map arrays)
@@ -621,13 +586,21 @@ func (te *tomlEncoder) encodeMappingBodyWithPath(w io.Writer, path []string, m *
 		}
 	}
 
-	// Finally, child mappings that are not marked EncodeSeparate get inlined as attributes
+	// Finally, child mappings: inline-hint or flow-style ones become inline table attributes,
+	// while all others are emitted as separate sub-table sections.
 	for i := 0; i < len(m.Content); i += 2 {
 		k := m.Content[i].Value
 		v := m.Content[i+1]
-		if v.Kind == MappingNode && !v.EncodeSeparate {
+		if v.Kind == MappingNode {
-			if err := te.writeInlineTableAttribute(w, k, v); err != nil {
+			if v.EncodeHint == EncodeHintInline || v.Style&FlowStyle != 0 {
-				return err
+				if err := te.writeInlineTableAttribute(w, k, v); err != nil {
+					return err
+				}
+			} else {
+				subPath := append(append([]string{}, path...), k)
+				if err := te.encodeSeparateMapping(w, subPath, v); err != nil {
+					return err
+				}
 			}
 		}
 	}

pkg/yqlib/toml_test.go

@@ -626,6 +626,34 @@ var tomlScenarios = []formatScenario{
 		expected:     tomlTableWithComments,
 		scenarioType: "roundtrip",
 	},
+	// Encode (YAML → TOML) scenarios - verify readable table sections are produced
+	{
+		description:  "Encode: Simple mapping produces table section",
+		input:        "arg:\n  hello: foo\n",
+		expected:     "[arg]\nhello = \"foo\"\n",
+		scenarioType: "encode",
+	},
+	{
+		skipDoc:      true,
+		description:  "Encode: Nested mappings produce nested table sections",
+		input:        "a:\n  b:\n    c: val\n",
+		expected:     "[a.b]\nc = \"val\"\n",
+		scenarioType: "encode",
+	},
+	{
+		skipDoc:      true,
+		description:  "Encode: Mixed scalars and nested mapping",
+		input:        "a:\n  hello: foo\n  nested:\n    key: val\n",
+		expected:     "[a]\nhello = \"foo\"\n\n[a.nested]\nkey = \"val\"\n",
+		scenarioType: "encode",
+	},
+	{
+		skipDoc:      true,
+		description:  "Encode: YAML flow mapping stays inline",
+		input:        "arg: {hello: foo}\n",
+		expected:     "arg = { hello = \"foo\" }\n",
+		scenarioType: "encode",
+	},
 	{
 		skipDoc:      true,
 		description:  "Roundtrip: key with special characters in inline table",
@@ -665,6 +693,8 @@ func testTomlScenario(t *testing.T, s formatScenario) {
 		}
 	case "roundtrip":
 		test.AssertResultWithContext(t, s.expected, mustProcessFormatScenario(s, NewTomlDecoder(), NewTomlEncoder()), s.description)
+	case "encode":
+		test.AssertResultWithContext(t, s.expected, mustProcessFormatScenario(s, NewYamlDecoder(ConfiguredYamlPreferences), NewTomlEncoder()), s.description)
 	}
 }
 
@@ -690,6 +720,28 @@ func documentTomlDecodeScenario(w *bufio.Writer, s formatScenario) {
 	writeOrPanic(w, fmt.Sprintf("```yaml\n%v```\n\n", mustProcessFormatScenario(s, NewTomlDecoder(), NewYamlEncoder(ConfiguredYamlPreferences))))
 }
 
+func documentTomlEncodeScenario(w *bufio.Writer, s formatScenario) {
+	writeOrPanic(w, fmt.Sprintf("## %v\n", s.description))
+
+	if s.subdescription != "" {
+		writeOrPanic(w, s.subdescription)
+		writeOrPanic(w, "\n\n")
+	}
+
+	writeOrPanic(w, "Given a sample.yml file of:\n")
+	writeOrPanic(w, fmt.Sprintf("```yaml\n%v\n```\n", s.input))
+
+	writeOrPanic(w, "then\n")
+	expression := s.expression
+	if expression == "" {
+		expression = "."
+	}
+	writeOrPanic(w, fmt.Sprintf("```bash\nyq -o toml '%v' sample.yml\n```\n", expression))
+	writeOrPanic(w, "will output\n")
+
+	writeOrPanic(w, fmt.Sprintf("```toml\n%v```\n\n", mustProcessFormatScenario(s, NewYamlDecoder(ConfiguredYamlPreferences), NewTomlEncoder())))
+}
+
 func documentTomlRoundtripScenario(w *bufio.Writer, s formatScenario) {
 	writeOrPanic(w, fmt.Sprintf("## %v\n", s.description))
 
@@ -723,6 +775,8 @@ func documentTomlScenario(_ *testing.T, w *bufio.Writer, i interface{}) {
 		documentTomlDecodeScenario(w, s)
 	case "roundtrip":
 		documentTomlRoundtripScenario(w, s)
+	case "encode":
+		documentTomlEncodeScenario(w, s)
 
 	default:
 		panic(fmt.Sprintf("unhandled scenario type %q", s.scenarioType))