feat(yaml): prefer implicit key for empty value (#4972)

* feat(yaml): prefer implicit key for empty value

* fix: prefer `?` for `!!set`

* refactor

src/language-yaml/printer-yaml.js

@@ -323,7 +323,21 @@ function _print(node, parentNode, path, options, print) {
         return node.type === "flowMappingItem" &&
           path.getParentNode().type !== "flowSequence"
           ? key
-          : concat(["? ", align(2, key)]);
+          : node.type === "mappingItem" &&
+            node.key.type !== "null" &&
+            isAbsolutelyPrintedAsSingleLineNode(node.key.node, options) &&
+            !hasTrailingComments(node.key.node) &&
+            !(
+              parentNode.tag.type === "shorthandTag" &&
+              parentNode.tag.handle === "!!" &&
+              parentNode.tag.suffix === "set"
+            )
+            ? concat([
+                key,
+                needsSpaceInFrontOfMappingValue(node) ? " " : "",
+                ":"
+              ])
+            : concat(["? ", align(2, key)]);
       }
 
       if (node.key.type === "null") {

tests/yaml_comment/__snapshots__/jsfmt.spec.js.snap

@@ -88,7 +88,7 @@ a:
   #a
 
 A:
-  ? B
+  B:
   #A
   #A
 

tests/yaml_root/__snapshots__/jsfmt.spec.js.snap

@@ -382,7 +382,7 @@ merge:
 null:
   # This mapping has four keys,
   # one has a value.
-  ? empty
+  empty:
   canonical: ~
   english: null
   ~: null key

tests/yaml_spec/__snapshots__/jsfmt.spec.js.snap

@@ -349,9 +349,9 @@ exports[`block-mapping-with-missing-values.yml - yaml-verify 1`] = `
 ? b
 c:
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-? a
-? b
-? c
+a:
+b:
+c:
 
 `;
 
@@ -360,9 +360,9 @@ exports[`block-mapping-with-missing-values.yml - yaml-verify 2`] = `
 ? b
 c:
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-? a
-? b
-? c
+a:
+b:
+c:
 
 `;
 
@@ -954,7 +954,7 @@ a: 1
 &anchor c: 3
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 a: 1
-? b
+b:
 &anchor c: 3
 
 `;
@@ -966,7 +966,7 @@ a: 1
 &anchor c: 3
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 a: 1
-? b
+b:
 &anchor c: 3
 
 `;