Remove 'NilLiteralExpr'

Sources/Core/AST/AST+Walk.swift

@@ -129,8 +129,6 @@ extension AST {
       traverse(self[n] as! MatchExpr, notifying: &o)
     case NameExpr.self:
       traverse(self[n] as! NameExpr, notifying: &o)
-    case NilLiteralExpr.self:
-      traverse(self[n] as! NilLiteralExpr, notifying: &o)
     case ParameterTypeExpr.self:
       traverse(self[n] as! ParameterTypeExpr, notifying: &o)
     case PragmaLiteralExpr.self:
@@ -544,11 +542,6 @@ extension AST {
     walk(roots: n.arguments.map(\.value), notifying: &o)
   }
 
-  /// Visits the children of `n` in pre-order, notifying `o` when a node is entered or left.
-  public func traverse<O: ASTWalkObserver>(
-    _ n: NilLiteralExpr, notifying o: inout O
-  ) {}
-
   /// Visits the children of `n` in pre-order, notifying `o` when a node is entered or left.
   public func traverse<O: ASTWalkObserver>(
     _ n: ParameterTypeExpr, notifying o: inout O

Sources/Core/AST/NodeIDs/NodeKind.swift

@@ -116,7 +116,6 @@ extension NodeKind {
     MapLiteralExpr.self,
     MatchExpr.self,
     NameExpr.self,
-    NilLiteralExpr.self,
     ParameterTypeExpr.self,
     PragmaLiteralExpr.self,
     RemoteExpr.self,

Sources/FrontEnd/Parse/Lexer.swift

@@ -109,7 +109,6 @@ public struct Lexer: IteratorProtocol, Sequence {
       case "let": token.kind = .`let`
       case "match": token.kind = .`match`
       case "namespace": token.kind = .`namespace`
-      case "nil": token.kind = .`nil`
       case "operator": token.kind = .`operator`
       case "postfix": token.kind = .`postfix`
       case "prefix": token.kind = .`prefix`

Sources/FrontEnd/Parse/Parser.swift

@@ -1661,12 +1661,6 @@ public enum Parser {
           site: head.site))
       return AnyExprID(expr)
 
-    case .nil:
-      // Nil literal.
-      _ = state.take()
-      let expr = state.insert(NilLiteralExpr(site: head.site))
-      return AnyExprID(expr)
-
     case .under:
       // Wildcard expression.
       _ = state.take()

Sources/FrontEnd/Parse/Token.swift

@@ -41,7 +41,6 @@ public struct Token {
     case `let`
     case `match`
     case `namespace`
-    case `nil`
     case `operator`
     case `poundElse`
     case `poundElseif`

Tests/HyloTests/LexerTests.swift

@@ -133,9 +133,9 @@ final class LexerTests: XCTestCase {
   func testKeywords() {
     let input: SourceFile = """
       any break catch conformance continue do else extension for fun if import in infix init inout
-      let match namespace nil operator postfix prefix property private public remote return set
-      sink some spawn static subscript trait try type typealias var where while yield yielded
-      #if #else #elseif #endif
+      let match namespace operator postfix prefix property private public remote return set sink
+      some spawn static subscript trait try type typealias var where while yield yielded #if #else
+      #elseif #endif
       """
 
     assert(
@@ -160,7 +160,6 @@ final class LexerTests: XCTestCase {
         TokenSpecification(.`let`, "let"),
         TokenSpecification(.`match`, "match"),
         TokenSpecification(.`namespace`, "namespace"),
-        TokenSpecification(.`nil`, "nil"),
         TokenSpecification(.`operator`, "operator"),
         TokenSpecification(.`postfix`, "postfix"),
         TokenSpecification(.`prefix`, "prefix"),

Tests/HyloTests/ParserTests.swift

@@ -1160,12 +1160,6 @@ final class ParserTests: XCTestCase {
     XCTAssertEqual(expr.value, "Hylo")
   }
 
-  func testNilLiteralExpr() throws {
-    let input: SourceFile = "nil"
-    let (exprID, _) = try input.parse(with: Parser.parseExpr(in:))
-    XCTAssertEqual(exprID?.kind, .init(NilLiteralExpr.self))
-  }
-
   func testPragmaLiteralExpr() throws {
     let input: SourceFile = "#file"
     let (exprID, ast) = try input.parse(with: Parser.parseExpr(in:))