Format property wrappers correctly.

Sources/SwiftFormatPrettyPrint/TokenStreamCreator.swift

@@ -1391,6 +1391,22 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
     return .visitChildren
   }
 
+  override func visit(_ node: CustomAttributeSyntax) -> SyntaxVisitorContinueKind {
+    // "Custom attributes" are better known to users as "property wrappers".
+    before(node.firstToken, tokens: .open)
+    if let argumentList = node.argumentList,
+      let leftParen = node.leftParen, let rightParen = node.rightParen
+    {
+      arrangeFunctionCallArgumentList(
+        argumentList,
+        leftDelimiter: leftParen,
+        rightDelimiter: rightParen,
+        forcesBreakBeforeRightDelimiter: false)
+    }
+    after(node.lastToken, tokens: .close)
+    return .visitChildren
+  }
+
   override func visit(_ node: AvailabilitySpecListSyntax) -> SyntaxVisitorContinueKind {
     insertTokens(.break(.same, size: 1), betweenElementsOf: node)
     return .visitChildren

Tests/SwiftFormatPrettyPrintTests/AttributeTests.swift

@@ -314,4 +314,49 @@ final class AttributeTests: PrettyPrintTestCase {
 
     assertPrettyPrintEqual(input: input, expected: expected, linelength: 30)
   }
+
+  func testPropertyWrappers() {
+    // Property wrappers are `CustomAttributeSyntax` nodes (not `AttributeSyntax`) and their
+    // arguments are `TupleExprElementListSyntax` (like regular function call argument lists), so
+    // make sure that those are formatted properly.
+    let input =
+      """
+      struct X {
+        @Wrapper var value: String
+
+        @Wrapper ( ) var value: String
+
+        @Wrapper (arg1:"value")var value: String
+
+        @Wrapper (arg1:"value")
+        var value: String
+
+        @Wrapper (arg1:"value",arg2:otherValue)
+        var value: String
+      }
+      """
+
+    let expected =
+      """
+      struct X {
+        @Wrapper var value: String
+
+        @Wrapper() var value: String
+
+        @Wrapper(arg1: "value")
+        var value: String
+
+        @Wrapper(arg1: "value")
+        var value: String
+
+        @Wrapper(
+          arg1: "value",
+          arg2: otherValue)
+        var value: String
+      }
+
+      """
+
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 32)
+  }
 }

Tests/SwiftFormatPrettyPrintTests/XCTestManifests.swift

@@ -69,6 +69,7 @@ extension AttributeTests {
         ("testObjCAttributesDiscretionaryLineBreaking", testObjCAttributesDiscretionaryLineBreaking),
         ("testObjCAttributesPerLineBreaking", testObjCAttributesPerLineBreaking),
         ("testObjCBinPackedAttributes", testObjCBinPackedAttributes),
+        ("testPropertyWrappers", testPropertyWrappers),
     ]
 }