[Concurrency] Skip Sendable checking for nonisolated(nonsending) -> @concurrent conversions

Converting a `nonisolated(nonsending)` function type to a
`@concurrent` one is the same as converting it to an actor-isolated
function type. `nonisolated(nonsending)` functions run on the
caller's actor without crossing an isolation boundary and so
no Sendable checking is necessary.

Resolves: swiftlang#87235
Resolves: rdar://170380869
(cherry picked from commit 5f9efd2)

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -2829,7 +2829,6 @@ namespace {
           case FunctionTypeIsolation::Kind::NonIsolated: {
             switch (fromIsolation.getKind()) {
             case FunctionTypeIsolation::Kind::Parameter:
-            case FunctionTypeIsolation::Kind::NonIsolatedNonsending:
             case FunctionTypeIsolation::Kind::Erased:
               diagnoseNonSendableParametersAndResult(
                   toFnType, version::Version::getFutureMajorLanguageVersion());
@@ -2841,13 +2840,13 @@ namespace {
               break;
             }
 
-            case FunctionTypeIsolation::Kind::NonIsolated: {
-              // nonisolated synchronous <-> @concurrent
-              if (fromFnType->isAsync() != toFnType->isAsync()) {
-                diagnoseNonSendableParametersAndResult(
-                    toFnType,
-                    version::Version::getFutureMajorLanguageVersion());
-              }
+            case FunctionTypeIsolation::Kind::NonIsolated:
+            case FunctionTypeIsolation::Kind::NonIsolatedNonsending: {
+              // Converting a `nonisolated(nonsending)` function type to a
+              // `@concurrent` one is the same as converting it to an
+              // actor-isolated function type. `nonisolated(nonsending)`
+              // functions run on the caller's actor without crossing an
+              // isolation boundary and so no Sendable checking is necessary.
               break;
             }
             }

test/Concurrency/attr_execution/conversions.swift

@@ -141,10 +141,9 @@ func testNonSendableDiagnostics(
   // expected-warning@-1 {{cannot convert '@isolated(any) @Sendable () async -> NonSendable' to '() async -> NonSendable' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
 
 
-  let _: @concurrent (NonSendable) async -> Void = caller1 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
-  // expected-warning@-1 {{cannot convert 'nonisolated(nonsending) @Sendable (NonSendable) async -> Void' to '(NonSendable) async -> Void' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
-  let _: @concurrent () async -> NonSendable = caller2 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}
-  // expected-warning@-1 {{cannot convert 'nonisolated(nonsending) @Sendable () async -> NonSendable' to '() async -> NonSendable' because crossing of an isolation boundary requires parameter and result types to conform to 'Sendable' protocol}}
+  // Calling `nonisolated(nonsending)` from `@concurrent` doesn't cross an isolation boundary.
+  let _: @concurrent (NonSendable) async -> Void = caller1 // Ok
+  let _: @concurrent () async -> NonSendable = caller2 // Ok
 
   let _: @MainActor (NonSendable) async -> Void = nonIsolated1 // Ok
   let _: @MainActor (NonSendable) async -> Void = nonIsolated2 // expected-note {{type 'NonSendable' does not conform to 'Sendable' protocol}}