add a WARNING to rust.optimize option in config.toml.example

Author: Guanqun Lu

config.toml.example

@@ -258,10 +258,9 @@
 [rust]
 
 # Whether or not to optimize the compiler and standard library.
-#
-# Note: the slowness of the non optimized compiler compiling itself usually
-#       outweighs the time gains in not doing optimizations, therefore a
-#       full bootstrap takes much more time with `optimize` set to false.
+# WARNING: Building with optimize = false is NOT SUPPORTED. Due to bootstrapping,
+# building without optimizations takes much longer than optimizing. Further, some platforms
+# fail to build without this optimization (c.f. #65352).
 #optimize = true
 
 # Indicates that the build should be configured for debugging Rust. A