Temporarily put uses of "Span" and "RawSpan" behind an experimental feature flag

While Span is present, we don't yet have an official way to create Span
instances. Until then, put uses of Span and RawSpan behind an
experimental feature flag (`Span`) that must be set to use these.

Addresses rdar://139308307.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -7912,6 +7912,10 @@ ERROR(noncopyable_cannot_have_read_set_accessor,none,
 ERROR(nonescapable_types_attr_disabled,none,
       "attribute requires '-enable-experimental-feature NonescapableTypes'", ())
 
+ERROR(span_requires_feature_flag,none,
+      "'%0' requires -enable-experimental-feature Span",
+      (StringRef))
+
 //------------------------------------------------------------------------------
 // MARK: Init accessors
 //------------------------------------------------------------------------------

include/swift/Basic/Features.def

@@ -234,6 +234,7 @@ EXPERIMENTAL_FEATURE(MacrosOnImports, true)
 EXPERIMENTAL_FEATURE(TupleConformances, false)
 EXPERIMENTAL_FEATURE(FullTypedThrows, false)
 EXPERIMENTAL_FEATURE(SameElementRequirements, false)
+EXPERIMENTAL_FEATURE(Span, true)
 
 // Whether to enable @_used and @_section attributes
 EXPERIMENTAL_FEATURE(SymbolLinkageMarkers, true)

lib/AST/FeatureSet.cpp

@@ -197,6 +197,7 @@ UNINTERESTING_FEATURE(GroupActorErrors)
 UNINTERESTING_FEATURE(SameElementRequirements)
 UNINTERESTING_FEATURE(UnspecifiedMeansMainActorIsolated)
 UNINTERESTING_FEATURE(GenerateForceToMainActorThunks)
+UNINTERESTING_FEATURE(Span)
 
 static bool usesFeatureSendingArgsAndResults(Decl *decl) {
   auto isFunctionTypeWithSending = [](Type type) {

lib/Sema/TypeCheckType.cpp

@@ -1719,6 +1719,32 @@ static void diagnoseGenericArgumentsOnSelf(const TypeResolution &resolution,
   }
 }
 
+/// Diagnose when this is one of the Span types, which currently requires
+/// an experimental feature to use.
+static void diagnoseSpanType(TypeDecl *typeDecl, SourceLoc loc,
+                             const DeclContext *dc) {
+  if (loc.isInvalid())
+    return;
+
+  if (!typeDecl->isStdlibDecl())
+    return;
+
+  ASTContext &ctx = typeDecl->getASTContext();
+  if (ctx.LangOpts.hasFeature(Feature::Span))
+    return;
+
+  auto nameString = typeDecl->getName().str();
+  if (nameString != "Span" && nameString != "RawSpan")
+    return;
+
+  // Don't require this in the standard library or _Concurrency library.
+  auto module = dc->getParentModule();
+  if (module->isStdlibModule() || module->getName().str() == "_Concurrency")
+    return;
+
+  ctx.Diags.diagnose(loc, diag::span_requires_feature_flag, nameString);
+}
+
 /// Resolve the given identifier type representation as an unqualified type,
 /// returning the type it references.
 /// \param silContext Used to look up generic parameters in SIL mode.
@@ -1854,6 +1880,9 @@ resolveUnqualifiedIdentTypeRepr(const TypeResolution &resolution,
       repr->setInvalid();
       return ErrorType::get(ctx);
     }
+
+    diagnoseSpanType(currentDecl, repr->getLoc(), DC);
+
     repr->setValue(currentDecl, currentDC);
     return current;
   }
@@ -2074,6 +2103,8 @@ static Type resolveQualifiedIdentTypeRepr(const TypeResolution &resolution,
     member = memberTypes.back().Member;
     inferredAssocType = memberTypes.back().InferredAssociatedType;
     repr->setValue(member, nullptr);
+
+    diagnoseSpanType(member, repr->getLoc(), DC);
   }
 
   return maybeDiagnoseBadMemberType(member, memberType, inferredAssocType);

test/stdlib/span_requires_feature.swift

@@ -0,0 +1,12 @@
+// RUN: %target-typecheck-verify-swift -verify-additional-prefix missing-
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature Span
+// REQUIRES: swift_feature_Span
+
+@available(SwiftStdlib 6.1, *)
+func f(_: Span<Int>) { }
+// expected-missing-error@-1{{'Span' requires -enable-experimental-feature Span}}
+
+
+@available(SwiftStdlib 6.1, *)
+func g(_: RawSpan) { }
+// expected-missing-error@-1{{'RawSpan' requires -enable-experimental-feature Span}}