Dtype selective build: fail if not xplat, if portable/optimized not in kernel_deps

Differential Revision: D75027794

Pull Request resolved: #10990

Author: lucylq

shim_et/xplat/executorch/codegen/codegen.bzl

@@ -641,6 +641,21 @@ def executorch_generated_lib(
     if not aten_mode and not functions_yaml_target and not custom_ops_yaml_target:
         fail("At least one of functions_yaml_target, custom_ops_yaml_target needs to be provided")
 
+    if dtype_selective_build:
+        if (not "//executorch/kernels/portable:operators" in kernel_deps) and (not "//executorch/kernels/optimized:optimized_operators" in kernel_deps):
+            fail("""
+            !!WARNING!! Dtype selective build is available for the portable and optimized kernel libraries.
+            If you are using those, please add them to `kernel_deps` in `executorch_generated_lib`:
+            //executorch/kernels/portable:operators
+            //executorch/kernels/optimized:optimized_operators
+            This will tell the build system to rebuild portable/optimized with the dtype selective build header.
+            For examples, see: //executorch/examples/selective_build/targets.bzl
+            Currently, kernel_deps contains {}.
+
+            If you have a custom kernel library, please remove `dtype_selective_build=True`
+            and use regular selective build.
+            """.format(kernel_deps))
+
     aten_suffix = "_aten" if aten_mode else ""
 
     # merge functions.yaml with fallback yaml