[Compile Time Values] Add a new experimental feature and the parsing of the '@const' attribute

Author: artemcm

include/swift/AST/DeclAttr.def

@@ -547,7 +547,12 @@ DECL_ATTR(execution, Execution,
   166)
 DECL_ATTR_FEATURE_REQUIREMENT(Execution, ExecutionAttribute)
 
-LAST_DECL_ATTR(Execution)
+SIMPLE_DECL_ATTR(const, ConstVal,
+    OnParam | OnVar | OnFunc | ABIStableToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove,
+  167)
+DECL_ATTR_FEATURE_REQUIREMENT(ConstVal, CompileTimeValues)
+
+LAST_DECL_ATTR(ConstVal)
 
 #undef DECL_ATTR_ALIAS
 #undef CONTEXTUAL_DECL_ATTR_ALIAS

include/swift/Basic/Features.def

@@ -476,6 +476,9 @@ EXPERIMENTAL_FEATURE(IsolatedConformances, true)
 /// Syntax sugar features for concurrency.
 EXPERIMENTAL_FEATURE(ConcurrencySyntaxSugar, true)
 
+/// Allow declaration of compile-time values
+EXPERIMENTAL_FEATURE(CompileTimeValues, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/ASTDumper.cpp

@@ -4811,6 +4811,7 @@ class PrintAttribute : public AttributeVisitor<PrintAttribute, void, Label>,
   TRIVIAL_ATTR_PRINTER(Borrowed, borrowed)
   TRIVIAL_ATTR_PRINTER(Borrowing, borrowing)
   TRIVIAL_ATTR_PRINTER(CompileTimeLiteral, compile_time_literal)
+  TRIVIAL_ATTR_PRINTER(ConstVal, compile_time_value)
   TRIVIAL_ATTR_PRINTER(CompilerInitialized, compiler_initialized)
   TRIVIAL_ATTR_PRINTER(Consuming, consuming)
   TRIVIAL_ATTR_PRINTER(Convenience, convenience)

lib/AST/FeatureSet.cpp

@@ -354,6 +354,10 @@ static bool usesFeatureConcurrencySyntaxSugar(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureCompileTimeValues(Decl *decl) {
+  return decl->getAttrs().hasAttribute<ConstValAttr>();
+}
+
 static bool usesFeatureMemorySafetyAttributes(Decl *decl) {
   if (decl->getAttrs().hasAttribute<SafeAttr>() ||
       decl->getAttrs().hasAttribute<UnsafeAttr>())

lib/ASTGen/Sources/ASTGen/DeclAttrs.swift

@@ -199,6 +199,7 @@ extension ASTGenVisitor {
         .atRethrows,
         .borrowed,
         .compilerInitialized,
+        .constVal,
         .dynamicCallable,
         .eagerMove,
         .exported,

lib/Sema/TypeCheckAttr.cpp

@@ -434,6 +434,7 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   void visitFinalAttr(FinalAttr *attr);
   void visitMoveOnlyAttr(MoveOnlyAttr *attr);
   void visitCompileTimeLiteralAttr(CompileTimeLiteralAttr *attr) {}
+  void visitConstValAttr(ConstValAttr *attr) {}
   void visitIBActionAttr(IBActionAttr *attr);
   void visitIBSegueActionAttr(IBSegueActionAttr *attr);
   void visitLazyAttr(LazyAttr *attr);

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1725,6 +1725,7 @@ namespace  {
 
     UNINTERESTING_ATTR(NoMetadata)
     UNINTERESTING_ATTR(CompileTimeLiteral)
+    UNINTERESTING_ATTR(ConstVal)
 
     UNINTERESTING_ATTR(BackDeployed)
     UNINTERESTING_ATTR(KnownToBeLocal)

test/Parse/const.swift

@@ -0,0 +1,31 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature CompileTimeValues
+// REQUIRES: swift_feature_CompileTimeValues
+@const let x: Int = 42
+
+// FIXME: Only allow 'let' for `@const` properties, even in protocol requirements
+protocol ConstUserProto {
+	@const static var v: String { get }
+}
+
+class ConstFanClassWrong: ConstUserProto {
+	  @const static let v: String = ""
+    // FIXME: Only allow 'let' for `@const` properties
+    @const static var B: String = ""
+}
+
+func takeIntConst(@const _ a: Int) {}
+
+@const func constFunc(_ a: Int) {}
+
+struct Article {
+  let id: String
+}
+@const let keypath = \Article.id
+
+func LocalConstVarUser() -> Int {
+		@const let localConst = 3
+		return localConst + 1
+}
+
+// FIXME: This should be diagnosed
+@const let a: Bool = Bool.random()

test/Parse/const_no_feature.swift

@@ -0,0 +1,2 @@
+// RUN: %target-typecheck-verify-swift
+@const let x: Int = 42 // expected-error{{'const' attribute is only valid when experimental feature CompileTimeValues is enabled}}