Handle missing comma i func params

Sources/SwiftParser/Parameters.swift

@@ -156,7 +156,22 @@ extension Parser {
       defaultValue = nil
     }
 
-    let trailingComma = self.consume(if: .comma)
+    var trailingComma: Token?
+    if self.at(.comma) {
+      trailingComma = self.consume(if: .comma)
+    } else if !self.at(.rightParen) {
+      let canParseIdentifier: Bool = withLookahead {
+        $0.canParseTypeIdentifier(allowKeyword: false)
+      }
+
+      let canParseAttribute: Bool = withLookahead {
+        $0.consume(if: .atSign) != nil && $0.canParseCustomAttribute()
+      }
+
+      if canParseIdentifier || canParseAttribute {
+        trailingComma = Token(missing: .comma, arena: self.arena)
+      }
+    }
 
     return RawFunctionParameterSyntax(
       attributes: attrs,

Tests/SwiftParserTest/DeclarationTests.swift

@@ -1531,7 +1531,8 @@ final class DeclarationTests: ParserTestCase {
           leftSquare: .leftSquareToken(),
           element: IdentifierTypeSyntax(name: .identifier("third")),
           rightSquare: .rightSquareToken(presence: .missing)
-        )
+        ),
+        trailingComma: .commaToken(presence: .missing)
       ),
       diagnostics: [
         DiagnosticSpec(
@@ -1547,11 +1548,12 @@ final class DeclarationTests: ParserTestCase {
         ),
         DiagnosticSpec(
           locationMarker: "4️⃣",
-          message: "unexpected code 'fourth: Int' in parameter clause"
+          message: "expected ',' in parameter",
+          fixIts: ["insert ','"]
         ),
       ],
       fixedSource: """
-        func foo(first second: [third]fourth: Int) {}
+        func foo(first second: [third], fourth: Int) {}
         """
     )
   }
@@ -3417,4 +3419,85 @@ final class DeclarationTests: ParserTestCase {
       ]
     )
   }
+
+  func testMissingCommaInParameters() {
+    assertParse(
+      "func a(foo: Bar1️⃣ foo2: Bar2) {}",
+      diagnostics: [
+        DiagnosticSpec(
+          message: "expected ',' in parameter",
+          fixIts: ["insert ','"]
+        )
+      ],
+      fixedSource: "func a(foo: Bar, foo2: Bar2) {}"
+    )
+  }
+
+  func testMissingMultipleCommasInParameters() {
+    assertParse(
+      "func a(foo: Bar1️⃣ foo2: Bar2, foo3: Bar32️⃣ foo4: Bar4) {}",
+      diagnostics: [
+        DiagnosticSpec(
+          locationMarker: "1️⃣",
+          message: "expected ',' in parameter",
+          fixIts: ["insert ','"]
+        ),
+        DiagnosticSpec(
+          locationMarker: "2️⃣",
+          message: "expected ',' in parameter",
+          fixIts: ["insert ','"]
+        ),
+      ],
+      fixedSource: "func a(foo: Bar, foo2: Bar2, foo3: Bar3, foo4: Bar4) {}"
+    )
+  }
+
+  func testMissingCommaInParametersAndAttributes() {
+    assertParse(
+      "func a(foo: Bar1️⃣ @escaping foo1: () -> Void) {}",
+      diagnostics: [
+        DiagnosticSpec(
+          locationMarker: "1️⃣",
+          message: "expected ',' in parameter",
+          fixIts: ["insert ','"]
+        )
+      ],
+      fixedSource: "func a(foo: Bar, @escaping foo1: () -> Void) {}"
+    )
+  }
+
+  func testMissingCommaInParametersWithNewline() {
+    assertParse(
+      """
+      func foo(a: Int1️⃣
+      x: Int = 2
+      ) {}
+      """,
+      diagnostics: [
+        DiagnosticSpec(
+          locationMarker: "1️⃣",
+          message: "expected ',' in parameter",
+          fixIts: ["insert ','"]
+        )
+      ],
+      fixedSource:
+        """
+        func foo(a: Int,
+        x: Int = 2
+        ) {}
+        """
+    )
+  }
+
+  func testMissingCommaWithgarbageCode() {
+    assertParse(
+      "func foo(x: Int 1️⃣Int<@abc)",
+      diagnostics: [
+        DiagnosticSpec(
+          locationMarker: "1️⃣",
+          message: "unexpected code 'Int<@abc' in parameter clause"
+        )
+      ]
+    )
+  }
 }