Sema: Don't warn on @_export public imports not used in API

An `@_export public import` is meaningful on it's own as it declares a
relationship between two modules and how clients see them. As such that
import doesn't have to be referenced from API to be appropriate. Let's
not warn on any public import with an `@_export` attribute.

rdar://122032960

Author: xymus

lib/Sema/TypeCheckAccess.cpp

@@ -2472,6 +2472,10 @@ void swift::diagnoseUnnecessaryPublicImports(SourceFile &SF) {
     if (importedModule->getTopLevelModule() != importedModule)
       continue;
 
+    // Ignore @_exported as by themselves the import is meaningful.
+    if (import.options.contains(ImportFlags::Exported))
+      continue;
+
     AccessLevel levelUsed = SF.getMaxAccessLevelUsingImport(importedModule);
     if (import.accessLevel <= levelUsed)
       continue;

test/ModuleInterface/imports-swift6.swift

@@ -18,7 +18,7 @@
 //--- empty.swift
 
 //--- main.swift
-@_exported public import resilient // expected-warning {{public import of 'resilient' was not used in public declarations or inlinable code}}
+@_exported public import resilient
 public import B.B2 // expected-warning {{public import of 'B' was not used in public declarations or inlinable code}}
 
 public import func C.c // expected-warning {{public import of 'C' was not used in public declarations or inlinable code}}
@@ -40,7 +40,6 @@ public import NotSoSecret // expected-warning {{'NotSoSecret' inconsistently imp
 @_implementationOnly import NotSoSecret2 // expected-note {{imported as implementation-only here}}
 //--- clientWithError.swift
 @_exported public import nonResilient // expected-error {{module 'nonResilient' was not compiled with library evolution support; using it means binary compatibility for 'clientWithError' can't be guaranteed}}
-// expected-warning @-1 {{public import of 'nonResilient' was not used in public declarations or inlinable code}}
 
 // CHECK-6-NOT: import
 // CHECK-6: {{^}}public import A{{$}}

test/Sema/conflicting-import-restrictions.swift

@@ -49,11 +49,10 @@
 // RUN:   -experimental-spi-only-imports -verify
 
 //--- Public_Exported.swift
-@_exported public import Lib // expected-warning {{public import of 'Lib' was not used in public declarations or inlinable code}}
+@_exported public import Lib
 
 //--- Package_Exported.swift
 @_exported package import Lib // expected-error {{'@_exported' is incompatible with 'package'; it can only be applied to public imports}}
-// expected-warning @-1 {{package import of 'Lib' was not used in package declarations}}
 
 //--- Internal_Exported.swift
 @_exported internal import Lib // expected-error {{'@_exported' is incompatible with 'internal'; it can only be applied to public imports}}

test/Sema/implementation-only-import-suggestion.swift

@@ -159,4 +159,3 @@ public import FullyPrivateClang // expected-error{{private module 'FullyPrivateC
 public import LocalClang // expected-error{{private module 'LocalClang' is imported publicly from the public module 'MainLib'}}
 // expected-warning @-1 {{public import of 'LocalClang' was not used in public declarations or inlinable code}}
 @_exported public import MainLib // expected-warning{{private module 'MainLib' is imported publicly from the public module 'MainLib'}}
-// expected-warning @-1 {{public import of 'MainLib' was not used in public declarations or inlinable code}}

test/Sema/superfluously-public-imports.swift

@@ -20,6 +20,7 @@
 // RUN: %target-swift-frontend -emit-module %t/UnusedPackageImport.swift -o %t -I %t
 // RUN: %target-swift-frontend -emit-module %t/ImportNotUseFromAPI.swift -o %t -I %t
 // RUN: %target-swift-frontend -emit-module %t/ImportUsedInPackage.swift -o %t -I %t
+// RUN: %target-swift-frontend -emit-module %t/ExportedUnused.swift -o %t -I %t
 
 /// Check diagnostics.
 // RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t \
@@ -102,6 +103,8 @@ public func notAnAPIFunc() -> NotAnAPIType { return NotAnAPIType() }
 public struct PackageType {}
 public func packageFunc() -> PackageType { return PackageType() }
 
+//--- ExportedUnused.swift
+
 //--- Client_Swift5.swift
 /// No diagnostics should be raised on the implicit access level.
 import UnusedImport // expected-error {{ambiguous implicit access level for import of 'UnusedImport'; it is imported as 'public' elsewhere}}
@@ -133,6 +136,8 @@ package import UnusedPackageImport // expected-warning {{package import of 'Unus
 public import ImportNotUseFromAPI // expected-warning {{public import of 'ImportNotUseFromAPI' was not used in public declarations or inlinable code}} {{1-8=}}
 public import ImportUsedInPackage // expected-warning {{public import of 'ImportUsedInPackage' was not used in public declarations or inlinable code}} {{1-7=package}}
 
+@_exported public import ExportedUnused
+
 public func useInSignature(_ a: TypeUsedInSignature) {} // expected-remark {{struct 'TypeUsedInSignature' is imported via 'DepUsedInSignature'}}
 public func exportedTypeUseInSignature(_ a: ExportedType) {} // expected-remark {{struct 'ExportedType' is imported via 'Exporter', which reexports definition from 'Exportee'}}