Format parameter packs.

Author: allevato

Sources/SwiftFormat/PrettyPrint/TokenStreamCreator.swift

@@ -2293,6 +2293,7 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
 
   override func visit(_ node: GenericParameterSyntax) -> SyntaxVisitorContinueKind {
     before(node.firstToken(viewMode: .sourceAccurate), tokens: .open)
+    after(node.eachKeyword, tokens: .break)
     after(node.colon, tokens: .break)
     if let trailingComma = node.trailingComma {
       after(trailingComma, tokens: .close, .break(.same))
@@ -2312,6 +2313,28 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
     return .visitChildren
   }
 
+  override func visit(_ node: PackElementExprSyntax) -> SyntaxVisitorContinueKind {
+    // `each` cannot be separated from the following token, or it is parsed as an identifier itself.
+    after(node.eachKeyword, tokens: .space)
+    return .visitChildren
+  }
+
+  override func visit(_ node: PackElementTypeSyntax) -> SyntaxVisitorContinueKind {
+    // `each` cannot be separated from the following token, or it is parsed as an identifier itself.
+    after(node.eachKeyword, tokens: .space)
+    return .visitChildren
+  }
+
+  override func visit(_ node: PackExpansionExprSyntax) -> SyntaxVisitorContinueKind {
+    after(node.repeatKeyword, tokens: .break)
+    return .visitChildren
+  }
+
+  override func visit(_ node: PackExpansionTypeSyntax) -> SyntaxVisitorContinueKind {
+    after(node.repeatKeyword, tokens: .break)
+    return .visitChildren
+  }
+
   override func visit(_ node: ExpressionPatternSyntax) -> SyntaxVisitorContinueKind {
     return .visitChildren
   }

Tests/SwiftFormatTests/PrettyPrint/ParameterPackTests.swift

@@ -0,0 +1,46 @@
+final class ParameterPackTests: PrettyPrintTestCase {
+  func testGenericPackArgument() {
+    assertPrettyPrintEqual(
+      input: """
+        func someFunction<each P>() {}
+        struct SomeStruct<each P> {}
+        """,
+      expected: """
+        func someFunction<
+          each P
+        >() {}
+        struct SomeStruct<
+          each P
+        > {}
+
+        """,
+      linelength: 22)
+  }
+
+  func testPackExpansionsAndElements() {
+    assertPrettyPrintEqual(
+      input: """
+        repeat checkNilness(of: each value)
+        """,
+      expected: """
+        repeat checkNilness(
+          of: each value)
+
+        """,
+      linelength: 25)
+
+    assertPrettyPrintEqual(
+      input: """
+        repeat f(of: each v)
+        """,
+      expected: """
+        repeat
+          f(
+            of:
+              each v
+          )
+
+        """,
+      linelength: 7)
+  }
+}