Sema: Set ApplyExpr::isNoAsync() for 'reasync' function calls that are known not to 'await'

Author: slavapestov

include/swift/AST/Expr.h

@@ -335,10 +335,11 @@ class alignas(8) Expr {
     NumCaptures : 32
   );
 
-  SWIFT_INLINE_BITFIELD(ApplyExpr, Expr, 1+1+1,
+  SWIFT_INLINE_BITFIELD(ApplyExpr, Expr, 1+1+1+1,
     ThrowsIsSet : 1,
     Throws : 1,
-    ImplicitlyAsync : 1
+    ImplicitlyAsync : 1,
+    NoAsync : 1
   );
 
   SWIFT_INLINE_BITFIELD_FULL(CallExpr, ApplyExpr, 1+1+16,
@@ -4359,6 +4360,7 @@ class ApplyExpr : public Expr {
     assert(validateArg(Arg) && "Arg is not a permitted expr kind");
     Bits.ApplyExpr.ThrowsIsSet = false;
     Bits.ApplyExpr.ImplicitlyAsync = false;
+    Bits.ApplyExpr.NoAsync = false;
   }
 
 public:
@@ -4397,6 +4399,15 @@ class ApplyExpr : public Expr {
     Bits.ApplyExpr.Throws = throws;
   }
 
+  /// Is this a 'rethrows' function that is known not to throw?
+  bool isNoThrows() const { return !throws(); }
+
+  /// Is this a 'reasync' function that is known not to 'await'?
+  bool isNoAsync() const { return Bits.ApplyExpr.NoAsync; }
+  void setNoAsync(bool noAsync) {
+    Bits.ApplyExpr.NoAsync = noAsync;
+  }
+
   /// Is this application _implicitly_ required to be an async call?
   /// Note that this is _not_ a check for whether the callee is async!
   /// Only meaningful after complete type-checking.

lib/Sema/TypeCheckEffects.cpp

@@ -2266,12 +2266,17 @@ class CheckEffectsCoverage : public EffectsHandlingWalker<CheckEffectsCoverage>
 
     checkThrowAsyncSite(E, /*requiresTry*/ true, classification);
 
-    // HACK: functions can get queued multiple times in
-    // definedFunctions, so be sure to be idempotent.
-    if (!E->isThrowsSet() && !classification.isInvalid()) {
-      auto throwsKind = classification.getConditionalKind(EffectKind::Throws);
-      E->setThrows(throwsKind == ConditionalEffectKind::Conditional ||
-                   throwsKind == ConditionalEffectKind::Always);
+    if (!classification.isInvalid()) {
+      // HACK: functions can get queued multiple times in
+      // definedFunctions, so be sure to be idempotent.
+      if (!E->isThrowsSet()) {
+        auto throwsKind = classification.getConditionalKind(EffectKind::Throws);
+        E->setThrows(throwsKind == ConditionalEffectKind::Conditional ||
+                     throwsKind == ConditionalEffectKind::Always);
+      }
+
+      auto asyncKind = classification.getConditionalKind(EffectKind::Async);
+      E->setNoAsync(asyncKind == ConditionalEffectKind::None);
     }
 
     // If current apply expression did not type-check, don't attempt