Fix missing newline for opening quote for multilines strings

Author: kimdv

Sources/SwiftBasicFormat/BasicFormat.swift

@@ -180,6 +180,22 @@ open class BasicFormat: SyntaxRewriter {
     }
   }
 
+  /// Whether a trailing newline on `token` should be added.
+  open func requiresTrailingNewline(_ token: TokenSyntax) -> Bool {
+    // We don't want to add newlines inside string interpolation
+    if isInsideStringInterpolation(token) && token.tokenKind != .multilineStringQuote {
+      return false
+    }
+
+    let syntaxToken: Syntax = Syntax(token)
+    switch syntaxToken.keyPathInParent {
+    case \StringLiteralExprSyntax.openQuote:
+      return token.tokenKind == .multilineStringQuote
+    default:
+      return false
+    }
+  }
+
   open func requiresWhitespace(between first: TokenSyntax?, and second: TokenSyntax?) -> Bool {
     switch (first?.tokenKind, second?.tokenKind) {
     case (.atSign, _),
@@ -368,6 +384,12 @@ open class BasicFormat: SyntaxRewriter {
       anchorPoints[token] = currentIndentationLevel
     }
 
+    if requiresTrailingNewline(token) {
+      if !trailingTrivia.endsWithNewline && nextToken?.leadingTrivia.startsWithNewline == false {
+        trailingTrivia += .newline
+      }
+    }
+
     // Add a trailing space to the token unless
     //  - it already ends with a whitespace or
     //  - the next token will start starts with a newline after the rewrite

Tests/SwiftSyntaxBuilderTest/StringLiteralExprSyntaxTests.swift

@@ -334,4 +334,42 @@ final class StringLiteralExprSyntaxTests: XCTestCase {
       """#
     )
   }
+
+  func testMultiStringOpeningQuote() {
+    //    assertBuildResult(
+    //      StringLiteralExprSyntax(openQuote: .multilineStringQuoteToken(), content: "a", closeQuote: .multilineStringQuoteToken()),
+    //      #"""
+    //      """
+    //      a
+    //      """
+    //      """#
+    //    )
+
+    assertBuildResult(
+      StringLiteralExprSyntax(
+        openQuote: .multilineStringQuoteToken(),
+        segments: StringLiteralSegmentsSyntax {
+          .expressionSegment(
+            ExpressionSegmentSyntax(
+              expressions: TupleExprElementListSyntax {
+                TupleExprElementSyntax(
+                  expression: StringLiteralExprSyntax(
+                    openQuote: .multilineStringQuoteToken(),
+                    content: "a",
+                    closeQuote: .multilineStringQuoteToken()
+                  )
+                )
+              }
+            )
+          )
+        },
+        closeQuote: .multilineStringQuoteToken()
+      ),
+      #"""
+      """
+      a
+      """
+      """#
+    )
+  }
 }