stdlib: add a semantic attribute on the String.utf8CString and define it as "readonly"

Needed for StringOptimization.

Author: eeckstein

include/swift/AST/SemanticAttrs.def

@@ -26,6 +26,7 @@
 
 SEMANTICS_ATTR(STRING_EQUALS, "string.equals")
 SEMANTICS_ATTR(STRING_MAKE_UTF8, "string.makeUTF8")
+SEMANTICS_ATTR(STRING_GET_UTF8_CSTRING, "string.getUTF8CString")
 SEMANTICS_ATTR(STRING_ESCAPE_PERCENT_GET, "string.escapePercent.get")
 SEMANTICS_ATTR(STRING_CONCAT, "string.concat")
 SEMANTICS_ATTR(STRING_APPEND, "string.append")

stdlib/public/core/StringUTF8View.swift

@@ -251,13 +251,16 @@ extension String {
   ///     }
   ///     // Prints "6"
   public var utf8CString: ContiguousArray<CChar> {
-    if _fastPath(_guts.isFastUTF8) {
-      var result = _guts.withFastCChar { ContiguousArray($0) }
-      result.append(0)
-      return result
-    }
+    @_effects(readonly) @_semantics("string.getUTF8CString")
+    get {
+      if _fastPath(_guts.isFastUTF8) {
+        var result = _guts.withFastCChar { ContiguousArray($0) }
+        result.append(0)
+        return result
+      }
 
-    return _slowUTF8CString()
+      return _slowUTF8CString()
+    }
   }
 
   @usableFromInline @inline(never) // slow-path