Preserve one-item inline spacing on insert

docs/reference/support-matrix.md

@@ -93,7 +93,7 @@ The default API behavior is TOML 1.1-compatible. Strict TOML 1.0 parsing/decodin
 | Trivia preservation on document items and table entries | Partial | Available through `Toml::parse($input, true)` for leading/trailing trivia on parsed items |
 | Trivia preservation inside parsed arrays and inline tables | Partial | Collection-local item spacing and comments are preserved where represented in the AST |
 | Comment preservation on re-encode | Partial | Preserved for parsed document items, table entries, and collection items when trivia is available |
-| Formatting preservation on re-encode | Partial | Available in `DocumentFormattingMode::SourceAware` for trivia-preserving ASTs |
+| Formatting preservation on re-encode | Partial | Available in `DocumentFormattingMode::SourceAware` for trivia-preserving ASTs, including local spacing preservation for common single-line collection edits |
 | `encodeDocument()` round-trip fidelity | Partial | Normalized by default; source-aware mode is lossless for unchanged parsed regions, while edited regions preserve local style when inferable and otherwise canonicalize locally |
 
 ## Tooling and Errors

src/Encoder/Encoder.php

@@ -1121,8 +1121,18 @@ private function inferSingleLineInlineTableStyle(InlineTable $value): ?array
         $afterComma = null;
         $observedPair = false;
         $itemCount = count($value->items);
+        $reusableItemCount = 0;
+        $lastReusableItem = null;
 
         for ($index = 1; $index < $itemCount; $index++) {
+            if (!$this->isSyntheticNode($value->items[$index - 1])) {
+                $reusableItemCount++;
+                $lastReusableItem = $value->items[$index - 1];
+            }
+            if ($index === $itemCount - 1 && !$this->isSyntheticNode($value->items[$index])) {
+                $reusableItemCount++;
+                $lastReusableItem = $value->items[$index];
+            }
             if ($this->isSyntheticNode($value->items[$index - 1]) || $this->isSyntheticNode($value->items[$index])) {
                 continue;
             }
@@ -1140,6 +1150,19 @@ private function inferSingleLineInlineTableStyle(InlineTable $value): ?array
         }
 
         if (!$observedPair) {
+            if ($reusableItemCount === 1) {
+                $closingSpacing = $closing;
+                if ($closingSpacing === '' && $lastReusableItem !== null) {
+                    $closingSpacing = $this->singleLineWhitespaceTrivia($lastReusableItem->getTrailingTrivia()) ?? '';
+                }
+
+                return [
+                    'opening' => $opening !== '' ? $opening : ' ',
+                    'afterComma' => $closingSpacing !== '' ? $closingSpacing : ($opening !== '' ? $opening : ' '),
+                    'closing' => $closingSpacing !== '' ? $closingSpacing : ' ',
+                ];
+            }
+
             return null;
         }
 

tests/Integration/EditingFixtureTest.php

@@ -38,6 +38,8 @@ public function testEditedFixturesReencodeAsExpected(string $caseDir): void
             'single-line-array-remove-to-one' => $this->applySingleLineArrayRemovalToOne($document),
             'single-line-array-remove-consistent' => $this->applySingleLineArrayRemoval($document),
             'single-line-inline-insert' => $this->applySingleLineInlineInsert($document),
+            'single-line-inline-insert-from-one' => $this->applySingleLineInlineInsert($document),
+            'nested-single-line-inline-insert-from-one' => $this->applyNestedSingleLineInlineInsert($document),
             'single-line-inline-remove-to-one' => $this->applySingleLineInlineRemovalToOne($document),
             'single-line-inline-remove-consistent' => $this->applySingleLineInlineRemoval($document),
             'single-line-value-replace' => $this->applySingleLineValueReplace($document),
@@ -274,6 +276,20 @@ private function applyNestedInlineReplacement(Document $document): void
         );
     }
 
+    private function applyNestedSingleLineInlineInsert(Document $document): void
+    {
+        $item = $document->items[0];
+        self::assertInstanceOf(KeyValue::class, $item);
+        self::assertInstanceOf(InlineTable::class, $item->value);
+        self::assertInstanceOf(InlineTable::class, $item->value->items[0]->value);
+
+        $item->value->items[0]->value->items[] = new KeyValue(
+            new Key(['y'], [KeyStyle::Bare], $this->span()),
+            new IntegerValue(2, IntegerBase::Decimal, $this->span()),
+            $this->span(),
+        );
+    }
+
     private function readFixture(string $path): string
     {
         $contents = file_get_contents($path);

tests/fixtures/Editing/nested-single-line-inline-insert-from-one/expected.toml

@@ -0,0 +1 @@
+outer = { inner = { x = 1, y = 2 } }

tests/fixtures/Editing/nested-single-line-inline-insert-from-one/input.toml

@@ -0,0 +1 @@
+outer = { inner = { x = 1 } }

tests/fixtures/Editing/single-line-inline-insert-from-one/expected.toml

@@ -0,0 +1 @@
+point = { x = 1, z = 3 }

tests/fixtures/Editing/single-line-inline-insert-from-one/input.toml

@@ -0,0 +1 @@
+point = { x = 1 }