[Strict memory safety] Fix "unsafe" checking for the for..in loop

The `$generator` variable we create for the async for..in loop is
`nonisolated(unsafe)`, so ensure that we generate an `unsafe`
expression when we use it. This uncovered some inconsistencies in how
we do `unsafe` checking for for..in loops, so fix those.

Fixes rdar://154775389.

lib/Sema/CSGen.cpp

@@ -4728,10 +4728,15 @@ generateForEachStmtConstraints(ConstraintSystem &cs, DeclContext *dc,
     }
 
     // Wrap the 'next' call in 'unsafe', if the for..in loop has that
-    // effect.
-    if (stmt->getUnsafeLoc().isValid()) {
-      nextCall = new (ctx) UnsafeExpr(
-          stmt->getUnsafeLoc(), nextCall, Type(), /*implicit=*/true);
+    // effect or if the loop is async (in which case the iterator variable
+    // is nonisolated(unsafe).
+    if (stmt->getUnsafeLoc().isValid() ||
+        (isAsync &&
+         ctx.LangOpts.StrictConcurrencyLevel == StrictConcurrency::Complete)) {
+      SourceLoc loc = stmt->getUnsafeLoc();
+      if (loc.isInvalid())
+        loc = stmt->getForLoc();
+      nextCall = new (ctx) UnsafeExpr(loc, nextCall, Type(), /*implicit=*/true);
     }
 
     // The iterator type must conform to IteratorProtocol.

lib/Sema/TypeCheckEffects.cpp

@@ -2457,7 +2457,7 @@ private:
       return ShouldRecurse;
     }
     ShouldRecurse_t checkUnsafe(UnsafeExpr *E) {
-      return E->isImplicit() ? ShouldRecurse : ShouldNotRecurse;
+      return ShouldNotRecurse;
     }
     ShouldRecurse_t checkTry(TryExpr *E) {
       return ShouldRecurse;
@@ -4626,10 +4626,6 @@ private:
           diagnoseUnsafeUse(unsafeUse);
         }
       }
-    } else if (S->getUnsafeLoc().isValid()) {
-      // Extraneous "unsafe" on the sequence.
-      Ctx.Diags.diagnose(S->getUnsafeLoc(), diag::no_unsafe_in_unsafe_for)
-        .fixItRemove(S->getUnsafeLoc());
     }
 
     return ShouldRecurse;
@@ -4689,7 +4685,10 @@ private:
       return;
     }
 
-    Ctx.Diags.diagnose(E->getUnsafeLoc(), diag::no_unsafe_in_unsafe)
+    Ctx.Diags.diagnose(E->getUnsafeLoc(),
+                       forEachNextCallExprs.contains(E)
+                           ? diag::no_unsafe_in_unsafe_for
+                           : diag::no_unsafe_in_unsafe)
       .fixItRemove(E->getUnsafeLoc());
   }
 

test/Unsafe/safe.swift

@@ -98,6 +98,8 @@ func testUnsafeAsSequenceForEach() {
   for _ in unsafe uas { } // expected-warning{{for-in loop uses unsafe constructs but is not marked with 'unsafe'}}{{documentation-file=strict-memory-safety}}{{7-7=unsafe }}
 
   for unsafe _ in unsafe uas { } // okay
+
+  for unsafe _ in [1, 2, 3] { } // expected-warning{{no unsafe operations occur within 'unsafe' for-in loop}}
 }
 
 func testForInUnsafeAmbiguity(_ integers: [Int]) {

test/Unsafe/unsafe_concurrency.swift

@@ -55,3 +55,10 @@ open class SyntaxVisitor {
 open class SyntaxAnyVisitor: SyntaxVisitor {
   override open func visit(_ token: TokenSyntax) { }
 }
+
+@available(SwiftStdlib 5.1, *)
+func testMemorySafetyWithForLoop() async {
+  let (stream, continuation) = AsyncStream<Int>.makeStream()
+  for await _ in stream {}
+  _ = continuation
+}