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github-merge-queue[bot] pushed a commit to branch 
gh-readonly-queue/main/pr-11533-b1eed0a891010f9844b4ba89177bda8d465fb1b1
in repository https://gitbox.apache.org/repos/asf/arrow-rs.git

commit d8691ea9f11cdf2c2578d93e6673291dfb1b8c71
Author: Jeffrey Vo <[email protected]>
AuthorDate: Wed Oct 14 11:15:50 2026 +0000

    fix: `UnionFields::from_iter` should validate input (#11533)
    
    # Which issue does this PR close?
    
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    enhancement/bug.
    
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    - None
    
    # Rationale for this change
    
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    helps reviewers understand your changes and offer better suggestions for
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    Please try to explain the problem you are trying to solve in terms of
    the user-visible
    behavior, rather than the implementation.
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    Make it impossible to create an invalid `UnionFields` via safe APIs.
    This makes it easier for downstream uses of `UnionFields` (in other
    parts of the codebase), since they can rely on the data being valid,
    instead of needing to check each time.
    
    # What changes are included in this PR?
    
    <!--
    There is no need to duplicate the description in the issue here but it
    is
    sometimes worth providing a summary of the individual changes in this
    PR.
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    Make sure `UnionFields::from_iter` validates its input before
    construction a `UnionFields`, similar to `UnionFields::try_new`
    
    # Are these changes tested?
    
    <!--
    We typically require tests for all PRs in order to:
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    Added new unit test.
    
    # Are there any significant user-facing changes?
    
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    - Significant change to existing functionality (e.g. function returns
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    required and to ensure we follow proper semantic versioning.
    
    DO NOT restate the changes of this PR; if a bug was fixed and already
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    `UnionFields::from_iter` can now panic on invalid input data. This can
    be reachable via a `collect()` into a `UnionFields` for example.
---
 arrow-schema/src/fields.rs | 38 ++++++++++++++++++++++++++++++++++----
 1 file changed, 34 insertions(+), 4 deletions(-)

diff --git a/arrow-schema/src/fields.rs b/arrow-schema/src/fields.rs
index e9362a0148..f7fa588c31 100644
--- a/arrow-schema/src/fields.rs
+++ b/arrow-schema/src/fields.rs
@@ -309,7 +309,12 @@ impl<'a> IntoIterator for &'a Fields {
     }
 }
 
-/// A cheaply cloneable, owned collection of [`FieldRef`] and their 
corresponding type ids
+/// A cheaply cloneable, owned collection of [`FieldRef`] and their 
corresponding
+/// type ids.
+///
+/// Invariants:
+/// - All type ids are unique
+/// - All type ids are non-negative
 #[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 #[cfg_attr(feature = "serde", serde(transparent))]
@@ -350,8 +355,9 @@ impl UnionFields {
     /// # Errors
     ///
     /// This function returns an error if:
+    /// - Iterators are not the same length
     /// - Any type_id appears more than once (duplicate type ids)
-    /// - The type_ids are duplicated
+    /// - There is a negative type_id
     ///
     /// # Examples
     ///
@@ -615,14 +621,26 @@ impl UnionFields {
                 output.push((field_type_id, from_field.clone()))
             }
         }
-        *self = output.into_iter().collect();
+        // TODO: see above comment; can we remove the need to go through 
try_new
+        //       and just validate in this function ourselves?
+        let (type_ids, fields): (Vec<i8>, Vec<FieldRef>) = 
output.into_iter().unzip();
+        *self = Self::try_new(type_ids, fields)?;
         Ok(())
     }
 }
 
 impl FromIterator<(i8, FieldRef)> for UnionFields {
+    /// # Panics
+    ///
+    /// This will panic on invalid inputs:
+    /// - Duplicate type IDs
+    /// - Negative type IDs
+    ///
+    /// Prefer using [`UnionFields::try_new`] for a fallible construction 
method.
     fn from_iter<T: IntoIterator<Item = (i8, FieldRef)>>(iter: T) -> Self {
-        Self(iter.into_iter().collect())
+        let iter = iter.into_iter();
+        let (type_ids, fields): (Vec<i8>, Vec<FieldRef>) = iter.unzip();
+        Self::try_new(type_ids, fields).expect("invalid arguments passed")
     }
 }
 
@@ -1002,4 +1020,16 @@ mod tests {
         assert!(res.is_ok());
         assert_eq!(res.unwrap().len(), 3);
     }
+
+    #[test]
+    #[should_panic(
+        expected = "invalid arguments passed: InvalidArgumentError(\"type ids 
must be non-negative: -1\")"
+    )]
+    fn test_union_fields_from_iter_should_validate() {
+        let fields: Vec<(i8, FieldRef)> = vec![
+            (0, Arc::new(Field::new("a", DataType::Int32, true))),
+            (-1, Arc::new(Field::new("a", DataType::Int32, true))),
+        ];
+        UnionFields::from_iter(fields);
+    }
 }

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