SIL: don't set the let flag on a SILGlobalVariable if it contains a struct with @_rawLayout

Even if the global declaration is a `let`.
Raw-layout storage may be mutated even for let-variables.
Treating such variables as non-let ensures that optimization don't assume that they are not mutated.

Author: eeckstein

lib/SIL/IR/SILGlobalVariable.cpp

@@ -43,6 +43,24 @@ SILGlobalVariable *SILGlobalVariable::create(SILModule &M, SILLinkage linkage,
   return var;
 }
 
+static bool isGlobalLet(SILModule &mod, VarDecl *decl, SILType type) {
+  if (!decl)
+    return false;
+
+  if (!decl->isLet())
+    return false;
+
+  // Raw-layout storage may be mutated even for let-variables. Therefore don't
+  // treat such variables as `let` in SIL.
+  auto teCtxt = TypeExpansionContext::maximal(mod.getSwiftModule(),
+                                              mod.isWholeModule());
+  auto typeProps = mod.Types.getTypeProperties(type, teCtxt);
+  if (typeProps.isOrContainsRawLayout())
+    return false;
+
+  return true;
+}
+
 SILGlobalVariable::SILGlobalVariable(SILModule &Module, SILLinkage Linkage,
                                      SerializedKind_t serializedKind,
                                      StringRef Name, SILType LoweredType,
@@ -53,7 +71,7 @@ SILGlobalVariable::SILGlobalVariable(SILModule &Module, SILLinkage Linkage,
       Linkage(unsigned(Linkage)), HasLocation(Loc.has_value()), VDecl(Decl) {
   setSerializedKind(serializedKind);
   IsDeclaration = isAvailableExternally(Linkage);
-  setLet(Decl ? Decl->isLet() : false);
+  setLet(isGlobalLet(Module, Decl, LoweredType));
   Module.silGlobals.push_back(this);
 }