remove arg that is only ever used as NPY_UNSAFE_CASTING; remove code this renders unreachable (pandas-dev#18546)

Author: jbrockmendel

pandas/_libs/src/datetime.pxd

@@ -7,13 +7,6 @@ from cpython cimport PyUnicode_Check, PyUnicode_AsASCIIString
 cdef extern from "numpy/ndarrayobject.h":
     ctypedef int64_t npy_datetime
 
-    ctypedef enum NPY_CASTING:
-        NPY_NO_CASTING
-        NPY_EQUIV_CASTING
-        NPY_SAFE_CASTING
-        NPY_SAME_KIND_CASTING
-        NPY_UNSAFE_CASTING
-
 cdef extern from "numpy/npy_common.h":
     ctypedef unsigned char npy_bool
 
@@ -45,7 +38,6 @@ cdef extern from "datetime/np_datetime.h":
 
 cdef extern from "datetime/np_datetime_strings.h":
     int parse_iso_8601_datetime(char *str, int len, PANDAS_DATETIMEUNIT unit,
-                                NPY_CASTING casting,
                                 pandas_datetimestruct *out,
                                 int *out_local, int *out_tzoffset,
                                 PANDAS_DATETIMEUNIT *out_bestunit,
@@ -75,7 +67,6 @@ cdef inline int _cstring_to_dts(char *val, int length,
         int result
 
     result = parse_iso_8601_datetime(val, length, PANDAS_FR_ns,
-                                     NPY_UNSAFE_CASTING,
                                      dts, out_local, out_tzoffset,
                                      &out_bestunit, &special)
     return result

pandas/_libs/src/datetime/np_datetime.c

@@ -679,44 +679,6 @@ int convert_datetimestruct_to_datetime(pandas_datetime_metadata *meta,
     return 0;
 }
 
-/*
- * This provides the casting rules for the DATETIME data type units.
- *
- * Notably, there is a barrier between 'date units' and 'time units'
- * for all but 'unsafe' casting.
- */
-npy_bool can_cast_datetime64_units(PANDAS_DATETIMEUNIT src_unit,
-                                   PANDAS_DATETIMEUNIT dst_unit,
-                                   NPY_CASTING casting) {
-    switch (casting) {
-        /* Allow anything with unsafe casting */
-        case NPY_UNSAFE_CASTING:
-            return 1;
-
-        /*
-         * Only enforce the 'date units' vs 'time units' barrier with
-         * 'same_kind' casting.
-         */
-        case NPY_SAME_KIND_CASTING:
-            return (src_unit <= PANDAS_FR_D && dst_unit <= PANDAS_FR_D) ||
-                   (src_unit > PANDAS_FR_D && dst_unit > PANDAS_FR_D);
-
-        /*
-         * Enforce the 'date units' vs 'time units' barrier and that
-         * casting is only allowed towards more precise units with
-         * 'safe' casting.
-         */
-        case NPY_SAFE_CASTING:
-            return (src_unit <= dst_unit) &&
-                   ((src_unit <= PANDAS_FR_D && dst_unit <= PANDAS_FR_D) ||
-                    (src_unit > PANDAS_FR_D && dst_unit > PANDAS_FR_D));
-
-        /* Enforce equality with 'no' or 'equiv' casting */
-        default:
-            return src_unit == dst_unit;
-    }
-}
-
 /*
  * Converts a datetime based on the given metadata into a datetimestruct
  */

pandas/_libs/src/datetime/np_datetime.h

@@ -125,17 +125,6 @@ int cmp_pandas_datetimestruct(const pandas_datetimestruct *a,
 void
 add_minutes_to_datetimestruct(pandas_datetimestruct *dts, int minutes);
 
-/*
- * This provides the casting rules for the TIMEDELTA data type units.
- *
- * Notably, there is a barrier between the nonlinear years and
- * months units, and all the other units.
- */
-npy_bool
-can_cast_datetime64_units(PANDAS_DATETIMEUNIT src_unit,
-                          PANDAS_DATETIMEUNIT dst_unit,
-                          NPY_CASTING casting);
-
 
 int
 convert_datetime_to_datetimestruct(pandas_datetime_metadata *meta,