[SE-0362] Piecemeal adoption of upcoming language improvements

CHANGELOG.md

@@ -3,6 +3,17 @@ CHANGELOG
 
 _**Note:** This is in reverse chronological order, so newer entries are added to the top._
 
+## Swift 5.8
+
+* [SE-0362][]:
+
+  The compiler flag `-enable-upcoming-feature X` can now be used to enable a specific feature `X` that has been accepted by the evolution process, but whose introduction into the language is waiting for the next major version (e.g., version 6). The `X` is specified by any proposal that falls into this category:
+  * `ConciseMagicFile` enables the new `#file` semantics in [SE-0274][].
+  * `ForwardTrailingClosures` disables the "backward" scanning behavior of [SE-0286][].
+  * `BareSlashRegexLiterals` enables the regex literal syntax of [SE-0352][].
+
+  Features can be detected in source code with `#if hasFeature(X)`.
+
 ## Swift 5.7
 
 * The Swift compiler no longer warns about redundant requirements in generic declarations. For example,
@@ -9459,6 +9470,7 @@ Swift 1.0
 [SE-0267]: <https://github.com/apple/swift-evolution/blob/main/proposals/0267-where-on-contextually-generic.md>
 [SE-0268]: <https://github.com/apple/swift-evolution/blob/main/proposals/0268-didset-semantics.md>
 [SE-0269]: <https://github.com/apple/swift-evolution/blob/main/proposals/0269-implicit-self-explicit-capture.md>
+[SE-0274]: <https://github.com/apple/swift-evolution/blob/main/proposals/0274-magic-file.md>
 [SE-0276]: <https://github.com/apple/swift-evolution/blob/main/proposals/0276-multi-pattern-catch-clauses.md>
 [SE-0279]: <https://github.com/apple/swift-evolution/blob/main/proposals/0279-multiple-trailing-closures.md>
 [SE-0280]: <https://github.com/apple/swift-evolution/blob/main/proposals/0280-enum-cases-as-protocol-witnesses.md>
@@ -9509,6 +9521,7 @@ Swift 1.0
 [SE-0355]: <https://github.com/apple/swift-evolution/blob/main/proposals/0355-regex-syntax-run-time-construction.md>
 [SE-0357]: <https://github.com/apple/swift-evolution/blob/main/proposals/0357-regex-string-processing-algorithms.md>
 [SE-0358]: <https://github.com/apple/swift-evolution/blob/main/proposals/0358-primary-associated-types-in-stdlib.md>
+[SE-0362]: <https://github.com/apple/swift-evolution/blob/main/proposals/0362-piecemeal-future-features.md>
 
 [SR-75]: <https://bugs.swift.org/browse/SR-75>
 [SR-106]: <https://bugs.swift.org/browse/SR-106>

include/swift/AST/DiagnosticsFrontend.def

@@ -40,6 +40,10 @@ ERROR(error_experimental_feature_not_available, none,
       "experimental feature '%0' cannot be enabled in a production compiler",
       (StringRef))
 
+ERROR(error_upcoming_feature_on_by_default, none,
+      "upcoming feature '%0' is already enabled as of Swift version %1",
+      (StringRef, unsigned))
+
 ERROR(error_unknown_library_level, none,
       "unknown library level '%0', "
       "expected one of 'api', 'spi' or 'other'", (StringRef))

include/swift/Basic/Feature.h

@@ -53,7 +53,7 @@ inline bool featureImpliesFeature(Feature feature, Feature implied) {
 }
 
 /// Get the feature corresponding to this "future" feature, if there is one.
-llvm::Optional<Feature> getFutureFeature(llvm::StringRef name);
+llvm::Optional<Feature> getUpcomingFeature(llvm::StringRef name);
 
 /// Get the feature corresponding to this "experimental" feature, if there is
 /// one.

include/swift/Basic/Features.def

@@ -48,8 +48,8 @@
   LANGUAGE_FEATURE(FeatureName, SENumber, Description, Option)
 #endif
 
-#ifndef FUTURE_FEATURE
-#  define FUTURE_FEATURE(FeatureName, SENumber, Version)   \
+#ifndef UPCOMING_FEATURE
+#  define UPCOMING_FEATURE(FeatureName, SENumber, Version)   \
      LANGUAGE_FEATURE(FeatureName, SENumber, #FeatureName, \
                       langOpts.hasFeature(#FeatureName))
 #endif
@@ -90,9 +90,9 @@ SUPPRESSIBLE_LANGUAGE_FEATURE(PrimaryAssociatedTypes2, 346, "Primary associated
 SUPPRESSIBLE_LANGUAGE_FEATURE(UnavailableFromAsync, 0, "@_unavailableFromAsync", true)
 SUPPRESSIBLE_LANGUAGE_FEATURE(NoAsyncAvailability, 340, "@available(*, noasync)", true)
 
-FUTURE_FEATURE(ConciseMagicFile, 274, 6)
-FUTURE_FEATURE(ForwardTrailingClosures, 286, 6)
-FUTURE_FEATURE(BareSlashRegexLiterals, 354, 6)
+UPCOMING_FEATURE(ConciseMagicFile, 274, 6)
+UPCOMING_FEATURE(ForwardTrailingClosures, 286, 6)
+UPCOMING_FEATURE(BareSlashRegexLiterals, 354, 6)
 
 EXPERIMENTAL_FEATURE(StaticAssert)
 EXPERIMENTAL_FEATURE(VariadicGenerics)
@@ -118,6 +118,6 @@ EXPERIMENTAL_FEATURE(AdditiveArithmeticDerivedConformances)
 EXPERIMENTAL_FEATURE(SendableCompletionHandlers)
 
 #undef EXPERIMENTAL_FEATURE
-#undef FUTURE_FEATURE
+#undef UPCOMING_FEATURE
 #undef SUPPRESSIBLE_LANGUAGE_FEATURE
 #undef LANGUAGE_FEATURE

include/swift/Basic/LangOptions.h

@@ -28,6 +28,7 @@
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/Support/Regex.h"

include/swift/Option/Options.td

@@ -696,6 +696,11 @@ def enable_experimental_feature :
   Flags<[FrontendOption]>,
   HelpText<"Enable an experimental feature">;
 
+def enable_upcoming_feature : Separate<["-"], "enable-upcoming-feature">,
+  Flags<[FrontendOption]>,
+  HelpText<"Enable a feature that will be introduced in an upcoming language "
+           "version">;
+
 def Rpass_EQ : Joined<["-"], "Rpass=">,
   Flags<[FrontendOption]>,
   HelpText<"Report performed transformations by optimization passes whose "

lib/Basic/LangOptions.cpp

@@ -240,7 +240,7 @@ bool LangOptions::hasFeature(Feature feature) const {
 }
 
 bool LangOptions::hasFeature(llvm::StringRef featureName) const {
-  if (auto feature = getFutureFeature(featureName))
+  if (auto feature = getUpcomingFeature(featureName))
     return hasFeature(*feature);
 
   if (auto feature = getExperimentalFeature(featureName))
@@ -434,10 +434,10 @@ bool swift::isSuppressibleFeature(Feature feature) {
   llvm_unreachable("covered switch");
 }
 
-llvm::Optional<Feature> swift::getFutureFeature(llvm::StringRef name) {
+llvm::Optional<Feature> swift::getUpcomingFeature(llvm::StringRef name) {
   return llvm::StringSwitch<Optional<Feature>>(name)
 #define LANGUAGE_FEATURE(FeatureName, SENumber, Description, Option)
-#define FUTURE_FEATURE(FeatureName, SENumber, Version) \
+#define UPCOMING_FEATURE(FeatureName, SENumber, Version) \
                    .Case(#FeatureName, Feature::FeatureName)
 #include "swift/Basic/Features.def"
                    .Default(None);
@@ -455,7 +455,7 @@ llvm::Optional<Feature> swift::getExperimentalFeature(llvm::StringRef name) {
 llvm::Optional<unsigned> swift::getFeatureLanguageVersion(Feature feature) {
   switch (feature) {
 #define LANGUAGE_FEATURE(FeatureName, SENumber, Description, Option)
-#define FUTURE_FEATURE(FeatureName, SENumber, Version) \
+#define UPCOMING_FEATURE(FeatureName, SENumber, Version) \
   case Feature::FeatureName: return Version;
 #include "swift/Basic/Features.def"
   default: return None;

lib/Driver/ToolChains.cpp

@@ -237,6 +237,7 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
   inputArgs.AddLastArg(arguments, options::OPT_warn_concurrency);
   inputArgs.AddLastArg(arguments, options::OPT_strict_concurrency);
   inputArgs.AddAllArgs(arguments, options::OPT_enable_experimental_feature);
+  inputArgs.AddAllArgs(arguments, options::OPT_enable_upcoming_feature);
   inputArgs.AddLastArg(arguments, options::OPT_warn_implicit_overrides);
   inputArgs.AddLastArg(arguments, options::OPT_typo_correction_limit);
   inputArgs.AddLastArg(arguments, options::OPT_enable_app_extension);

lib/Frontend/CompilerInvocation.cpp

@@ -15,6 +15,7 @@
 
 #include "ArgsToFrontendOptionsConverter.h"
 #include "swift/AST/DiagnosticsFrontend.h"
+#include "swift/Basic/Feature.h"
 #include "swift/Basic/Platform.h"
 #include "swift/Option/Options.h"
 #include "swift/Option/SanitizerOptions.h"
@@ -637,6 +638,26 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
+  // Map historical flags over to future features.
+  for (const Arg *A : Args.filtered(OPT_enable_upcoming_feature)) {
+    // Ignore unknown features.
+    auto feature = getUpcomingFeature(A->getValue());
+    if (!feature)
+      continue;
+
+    // Check if this feature was introduced already in this language version.
+    if (auto firstVersion = getFeatureLanguageVersion(*feature)) {
+      if (Opts.isSwiftVersionAtLeast(*firstVersion)) {
+        Diags.diagnose(SourceLoc(), diag::error_upcoming_feature_on_by_default,
+                       A->getValue(), *firstVersion);
+        continue;
+      }
+    }
+
+    // Add the feature.
+    Opts.Features.insert(*feature);
+  }
+
   // Map historical flags over to experimental features. We do this for all
   // compilers because that's how existing experimental feature flags work.
   if (Args.hasArg(OPT_enable_experimental_variadic_generics))

lib/Parse/ParseIfConfig.cpp

@@ -133,6 +133,13 @@ static Expr *getSingleSubExp(ArgumentList *args, StringRef kindName,
   return nullptr;
 }
 
+/// Returns \c true if the condition is a version check.
+static bool isVersionIfConfigCondition(Expr *Condition);
+
+/// Evaluate the condition.
+/// \c true if success, \c false if failed.
+static bool evaluateIfConfigCondition(Expr *Condition, ASTContext &Context);
+
 /// The condition validator.
 class ValidateIfConfigCondition :
   public ExprVisitor<ValidateIfConfigCondition, Expr*> {
@@ -202,7 +209,7 @@ class ValidateIfConfigCondition :
 
     // We will definitely be consuming at least one operator.
     // Pull out the prospective RHS and slice off the first two elements.
-    Expr *RHS = validate(S[1]);
+    Expr *RHS = S[1];
     S = S.slice(2);
 
     while (true) {
@@ -226,7 +233,7 @@ class ValidateIfConfigCondition :
 
       OpName = NextOpName;
       Op = S[0];
-      RHS = validate(S[1]);
+      RHS = S[1];
       S = S.slice(2);
     }
 
@@ -329,6 +336,16 @@ class ValidateIfConfigCondition :
       return E;
     }
 
+    if (*KindName == "hasFeature") {
+      if (!getDeclRefStr(Arg, DeclRefKind::Ordinary)) {
+        D.diagnose(E->getLoc(), diag::unsupported_platform_condition_argument,
+                   "feature name");
+        return nullptr;
+      }
+
+      return E;
+    }
+
     // ( 'os' | 'arch' | '_endian' | '_runtime' ) '(' identifier ')''
     auto Kind = getPlatformConditionKind(*KindName);
     if (!Kind.hasValue()) {
@@ -416,14 +433,37 @@ class ValidateIfConfigCondition :
     return E;
   }
 
+  Expr *visitBinaryExpr(BinaryExpr *E) {
+    auto OpName = getDeclRefStr(E->getFn(), DeclRefKind::BinaryOperator);
+    if (auto lhs = validate(E->getLHS())) {
+      // If the left-hand side is a versioned condition, skip evaluation of
+      // the right-hand side if it won't ever affect the result.
+      if (OpName && isVersionIfConfigCondition(lhs)) {
+        assert(*OpName == "&&" || *OpName == "||");
+        bool isLHSTrue = evaluateIfConfigCondition(lhs, Ctx);
+        if (isLHSTrue && *OpName == "||")
+          return lhs;
+        if (!isLHSTrue && *OpName == "&&")
+          return lhs;
+      }
+
+      E->getArgs()->setExpr(0, lhs);
+    }
+
+    if (auto rhs = validate(E->getRHS()))
+      E->getArgs()->setExpr(1, rhs);
+
+    return E;
+  }
+
   // Fold sequence expression for non-Swift3 mode.
   Expr *visitSequenceExpr(SequenceExpr *E) {
     ArrayRef<Expr*> Elts = E->getElements();
-    Expr *foldedExpr = validate(Elts[0]);
+    Expr *foldedExpr = Elts[0];
     Elts = Elts.slice(1);
     foldedExpr = foldSequence(foldedExpr, Elts);
     assert(Elts.empty());
-    return foldedExpr;
+    return validate(foldedExpr);
   }
 
   // Other expression types are unsupported.
@@ -484,6 +524,7 @@ class EvaluateIfConfigCondition :
     bool isKnownFeature = llvm::StringSwitch<bool>(Name)
 #define LANGUAGE_FEATURE(FeatureName, SENumber, Description, Option) \
         .Case("$" #FeatureName, Option)
+#define UPCOMING_FEATURE(FeatureName, SENumber, Version)
 #include "swift/Basic/Features.def"
         .Default(false);
 
@@ -530,6 +571,9 @@ class EvaluateIfConfigCondition :
       ImportPath::Module::Builder builder(Ctx, Str, /*separator=*/'.',
                                           Arg->getStartLoc());
       return Ctx.canImportModule(builder.get(), version, underlyingModule);
+    } else if (KindName == "hasFeature") {
+      auto featureName = getDeclRefStr(Arg);
+      return Ctx.LangOpts.hasFeature(featureName);
     }
 
     auto Val = getDeclRefStr(Arg);
@@ -594,7 +638,6 @@ class IsVersionIfConfigCondition :
   bool visitExpr(Expr *E) { return false; }
 };
 
-/// Returns \c true if the condition is a version check.
 static bool isVersionIfConfigCondition(Expr *Condition) {
   return IsVersionIfConfigCondition().visit(Condition);
 }

test/Frontend/upcoming_feature.swift

@@ -0,0 +1,26 @@
+// Make sure that hasFeature(ConciseMagicFile) evaluates true when provided
+// explicitly.
+// RUN: %target-swift-frontend -typecheck -enable-upcoming-feature ConciseMagicFile %s
+
+// Make sure that hasFeature(ConciseMagicFile) evaluates true in Swift 6.
+// RUN: %target-swift-frontend -typecheck -swift-version 6 %s
+
+// Make sure that hasFeature(ConciseMagicFile) is off prior to Swift 6
+// RUN: %target-typecheck-verify-swift %s
+
+// It's fine to provide a feature that we don't know about
+// RUN: %target-swift-frontend -typecheck -enable-upcoming-feature ConciseMagicFile -enable-upcoming-feature UnknownFeature %s
+// RUN: %target-swift-frontend -typecheck -enable-upcoming-feature UnknownFeature -enable-upcoming-feature ConciseMagicFile %s
+
+
+// It's not fine to provide a feature that's in the specified language version.
+// RUN: not %target-swift-frontend -typecheck -enable-upcoming-feature ConciseMagicFile -swift-version 6 %s 2>&1 | %FileCheck %s
+// REQUIRES: asserts
+
+// CHECK: error: upcoming feature 'ConciseMagicFile' is already enabled as of Swift version 6
+
+#if hasFeature(ConciseMagicFile)
+let x = 0
+#else
+let y = boom // expected-error{{'boom'}}
+#endif

test/Parse/unknown_platform.swift

@@ -0,0 +1,28 @@
+// RUN: %target-typecheck-verify-swift
+
+// expected-error@+1{{unexpected platform condition}}
+#if hasGreeble(blah)
+#endif
+
+// Future compiler, short-circuit right-hand side
+#if compiler(>=10.0) && hasGreeble(blah)
+#endif
+
+// Current compiler, short-circuit right-hand side
+#if compiler(<10.0) || hasGreeble(blah)
+#endif
+
+// This compiler, don't short-circuit.
+// expected-error@+1{{unexpected platform condition}}
+#if compiler(>=5.7) && hasGreeble(blah)
+#endif
+
+// This compiler, don't short-circuit.
+// expected-error@+1{{unexpected platform condition}}
+#if compiler(<5.8) || hasGreeble(blah)
+#endif
+
+// Not a "version" check, so don't short-circuit.
+// expected-error@+1{{unexpected platform condition}}
+#if os(macOS) && hasGreeble(blah)
+#endif

test/Parse/upcoming_feature.swift

@@ -0,0 +1,5 @@
+// RUN: %target-typecheck-verify-swift
+
+// expected-error@+1{{unexpected platform condition argument: expected feature name}}
+#if hasFeature(17)
+#endif

test/Sema/diag_mismatched_magic_literals_swift6.swift

@@ -1,5 +1,6 @@
-// The future "Swift 6 mode" behavior is staged in behind  `-enable-experimental-concise-pound-file`.
+// The upcoming "Swift 6 mode" behavior is staged in behind  `-enable-experimental-concise-pound-file`.
 // RUN: %target-typecheck-verify-swift -enable-experimental-concise-pound-file
+// RUN: %target-typecheck-verify-swift -enable-upcoming-feature ConciseMagicFile
 
 // And is also available in Swift 6 mode on asserts compilers.
 // RUN: %target-typecheck-verify-swift -swift-version 6