[SE-0458] Suppress "no unsafe operations" warnings outside of strict mode

This is a stop-gap solution to prevent spurious warnings when "unsafe"
expressions and for..in loops are used without strict memory safety.

The full answer is probably to determine where unsafe code is all the time,
so that we can still (correctly) diagnose "no unsafe operations" even outside
of strict memory safety mode.

Author: DougGregor

lib/Sema/TypeCheckEffects.cpp

@@ -4309,7 +4309,8 @@ class CheckEffectsCoverage : public EffectsHandlingWalker<CheckEffectsCoverage>
         Ctx.Diags.diagnose(S->getForLoc(), diag::for_unsafe_without_unsafe)
           .fixItInsert(insertionLoc, "unsafe ");
       }
-    } else if (S->getUnsafeLoc().isValid()) {
+    } else if (S->getUnsafeLoc().isValid() &&
+               Ctx.LangOpts.hasFeature(Feature::StrictMemorySafety)) {
       // Extraneous "unsafe" on the sequence.
       Ctx.Diags.diagnose(S->getUnsafeLoc(), diag::no_unsafe_in_unsafe_for)
         .fixItRemove(S->getUnsafeLoc());
@@ -4343,6 +4344,9 @@ class CheckEffectsCoverage : public EffectsHandlingWalker<CheckEffectsCoverage>
   }
 
   void diagnoseRedundantUnsafe(UnsafeExpr *E) const {
+    if (!Ctx.LangOpts.hasFeature(Feature::StrictMemorySafety))
+      return;
+
     if (auto *SVE = SingleValueStmtExpr::tryDigOutSingleValueStmtExpr(E)) {
       // For an if/switch expression, produce a tailored warning.
       Ctx.Diags.diagnose(E->getUnsafeLoc(),
@@ -4351,6 +4355,7 @@ class CheckEffectsCoverage : public EffectsHandlingWalker<CheckEffectsCoverage>
         .highlight(E->getUnsafeLoc());
       return;
     }
+
     Ctx.Diags.diagnose(E->getUnsafeLoc(), diag::no_unsafe_in_unsafe);
   }
 

test/Unsafe/unsafe_nonstrict.swift

@@ -0,0 +1,18 @@
+// RUN: %target-typecheck-verify-swift -print-diagnostic-groups
+
+@unsafe func unsafeFunc() { }
+
+@unsafe
+struct UnsafeType { }
+
+protocol P { }
+
+struct X: @unsafe P { }
+
+func acceptP<T: P>(_: T) { }
+
+func testItAll(ut: UnsafeType, x: X, i: Int) {
+  _ = unsafe ut
+  unsafe acceptP(x)
+  _ = unsafe i
+}