Improve error when generated function is not pure (#39568)

I hadn't realized that this included generators so it could help others.

Author: nalimilan

src/method.c

@@ -477,7 +477,7 @@ JL_DLLEXPORT jl_code_info_t *jl_code_for_staged(jl_method_instance_t *linfo)
                     ptls->in_pure_callback = 0;
                     jl_toplevel_eval(def->module, (jl_value_t*)func);
                 }
-                jl_error("The function body AST defined by this @generated function is not pure. This likely means it contains a closure or comprehension.");
+                jl_error("The function body AST defined by this @generated function is not pure. This likely means it contains a closure, a comprehension or a generator.");
             }
         }
 

test/core.jl

@@ -5000,7 +5000,8 @@ gVararg(a::fVararg(Int)) = length(a)
     false
 catch e
     (e::ErrorException).msg
-end == "The function body AST defined by this @generated function is not pure. This likely means it contains a closure or comprehension."
+end == "The function body AST defined by this @generated function is not pure. " *
+       "This likely means it contains a closure, a comprehension or a generator."
 
 let x = 1
     global g18444