Add hasFeature(X) to check for future features

Author: DougGregor

lib/Parse/ParseIfConfig.cpp

@@ -329,6 +329,16 @@ 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()) {
@@ -531,6 +541,9 @@ 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);

test/Frontend/future_feature.swift

@@ -1,9 +1,27 @@
 
+// Make sure that hasFeature(ConciseMagicFile) evaluates true when provided
+// explicitly.
+// RUN: %target-swift-frontend -typecheck -enable-future-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-future-feature UnknownFeature %s
+// RUN: %target-swift-frontend -typecheck -enable-future-feature ConciseMagicFile -enable-future-feature UnknownFeature %s
+// RUN: %target-swift-frontend -typecheck -enable-future-feature UnknownFeature -enable-future-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-future-feature ConciseMagicFile -swift-version 6 %s 2>&1 | %FileCheck %s
 // REQUIRES: asserts
 
 // CHECK: error: future 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/future_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