Fix implicit string conversion breaking NativeStrings (#6035)

* Fix implicit string conversion breaking NativeStrings

* Allow string literals to coerce to either string type

* Add test for CPU string passing

---------

Co-authored-by: Yong He <yonghe@outlook.com>

Author: juliusikkala

source/slang/core.meta.slang

@@ -1463,6 +1463,11 @@ struct String
     __intrinsic_op($(kIROp_MakeString))
     __init(double val);
 
+    [require(cpp)]
+    __implicit_conversion($(kConversionCost_None))
+    __intrinsic_op($(kIROp_MakeString))
+    __init(NativeString value);
+
     /// Returns the length of the string.
     [require(cpp)]
     int64_t getLength();
@@ -1502,6 +1507,7 @@ struct NativeString
 
     property int length { [__unsafeForceInlineEarly] get{return getLength();} }
 
+    __implicit_conversion($(kConversionCost_None))
     __intrinsic_op($(kIROp_getNativeStr))
     __init(String value);
 };

source/slang/slang-check-conversion.cpp

@@ -785,8 +785,8 @@ bool SemanticsVisitor::_coerce(
         return true;
     }
 
-    // If both are string types we assume they are convertable in both directions
-    if (as<StringTypeBase>(fromType) && as<StringTypeBase>(toType))
+    // Assume string literals are convertible to any string type.
+    if (as<StringLiteralExpr>(fromExpr) && as<StringTypeBase>(toType))
     {
         if (outToExpr)
             *outToExpr = fromExpr;

tests/cpu-program/string-passing.slang

@@ -0,0 +1,17 @@
+//TEST:EXECUTABLE:
+
+void printer(NativeString string)
+{
+    printf(string);
+}
+
+void indirectPrinter(String string)
+{
+    printer(string);
+}
+
+export __extern_cpp int main()
+{
+    indirectPrinter("success\n");
+    return 0;
+}

tests/cpu-program/string-passing.slang.expected

@@ -0,0 +1,6 @@
+result code = 0
+standard error = {
+}
+standard output = {
+success
+}