Parse Much Harder for a Right Bracket in Capture Specifiers

All the lookahead we did before this promised us a right bracket here, but the precedence of right brackets is too weak to overcome keywords. The attached test case shows that we fall all the way through closure signature parsing and nothing picks up the slack and eats the remaining parts of the signature.

Instead, do `expect`'s job for it and try to munge through to the right bracket ourselves.

Fixes swiftlang#721

rdar://99657721

Author: CodaFi

Sources/SwiftParser/Expressions.swift

@@ -1689,12 +1689,18 @@ extension Parser {
             arena: self.arena))
         } while keepGoing != nil && loopProgress.evaluate(currentToken)
       }
+      // We were promised a right square bracket, so we're going to get it.
+      var unexpectedNodes = [RawSyntax]()
+      while !self.at(.eof) && !self.at(.rightSquareBracket) && !self.at(.inKeyword) {
+        unexpectedNodes.append(RawSyntax(self.consumeAnyToken()))
+      }
       let (unexpectedBeforeRSquare, rsquare) = self.expect(.rightSquareBracket)
+      unexpectedNodes.append(contentsOf: unexpectedBeforeRSquare?.elements ?? [])
 
       captures = RawClosureCaptureSignatureSyntax(
         leftSquare: lsquare,
         items: elements.isEmpty ? nil : RawClosureCaptureItemListSyntax(elements: elements, arena: self.arena),
-        unexpectedBeforeRSquare,
+        unexpectedNodes.isEmpty ? nil : RawUnexpectedNodesSyntax(elements: unexpectedNodes, arena: self.arena),
         rightSquare: rsquare, arena: self.arena)
     } else {
       captures = nil
@@ -1781,7 +1787,7 @@ extension Parser {
           }
           specifiers.append(self.expectWithoutLookahead(.rightParen))
         }
-      } else if (self.currentToken.isIdentifier || self.at(.selfKeyword)) {
+      } else if self.currentToken.isIdentifier || self.at(.selfKeyword) {
         let next = self.peek()
         // "x = 42", "x," and "x]" are all strong captures of x.
         guard next.tokenKind == .equal || next.tokenKind == .comma

Tests/SwiftParserTest/Types.swift

@@ -42,6 +42,15 @@ final class TypeTests: XCTestCase {
 
     AssertParse("""
                 { [weak a, unowned(safe) self, b = 3] (a: Int, b: Int, _: Int) -> Int in }
-                """)
+                """,
+                { $0.parseClosureExpression() })
+
+    AssertParse("{[#^DIAG_1^#class]in#^DIAG_2^#",
+                { $0.parseClosureExpression() },
+                diagnostics: [
+                  DiagnosticSpec(locationMarker: "DIAG_1", message: "Expected '' in closure capture item"),
+                  DiagnosticSpec(locationMarker: "DIAG_1", message: "Unexpected text 'class' found in closure capture signature"),
+                  DiagnosticSpec(locationMarker: "DIAG_2", message: "Expected '}' to end closure"),
+                ])
   }
 }