chore(rust): Factor out ensure_is_constant_duration (#15733)

crates/polars-plan/src/dsl/function_expr/ewm_by.rs

@@ -10,13 +10,7 @@ pub(super) fn ewm_mean_by(
         _ => None,
     };
     polars_ensure!(!half_life.negative(), InvalidOperation: "half_life cannot be negative");
-    polars_ensure!(half_life.is_constant_duration(time_zone),
-        InvalidOperation: "expected `half_life` to be a constant duration \
-        (i.e. one independent of differing month durations or of daylight savings time), got {}.\n\
-        \n\
-        You may want to try:\n\
-        - using `'730h'` instead of `'1mo'`\n\
-        - using `'24h'` instead of `'1d'` if your series is time-zone-aware", half_life);
+    ensure_is_constant_duration(half_life, time_zone, "half_life")?;
     // `half_life` is a constant duration so we can safely use `duration_ns()`.
     let half_life = half_life.duration_ns();
     let values = &s[0];

crates/polars-time/src/windows/duration.rs

@@ -15,6 +15,7 @@ use polars_core::prelude::{
     datetime_to_timestamp_ms, datetime_to_timestamp_ns, datetime_to_timestamp_us, polars_bail,
     PolarsResult,
 };
+use polars_error::polars_ensure;
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 
@@ -947,6 +948,21 @@ fn new_datetime(
     Some(NaiveDateTime::new(date, time))
 }
 
+pub fn ensure_is_constant_duration(
+    duration: Duration,
+    time_zone: Option<&str>,
+    variable_name: &str,
+) -> PolarsResult<()> {
+    polars_ensure!(duration.is_constant_duration(time_zone), 
+        InvalidOperation: "expected `{}` to be a constant duration \
+            (i.e. one independent of differing month durations or of daylight savings time), got {}.\n\
+            \n\
+            You may want to try:\n\
+            - using `'730h'` instead of `'1mo'`\n\
+            - using `'24h'` instead of `'1d'` if your series is time-zone-aware", variable_name, duration);
+    Ok(())
+}
+
 #[cfg(test)]
 mod test {
     use super::*;