feat: implement FieldGroupReader for Option<Vec<T>> in multipart form…

[fasilmveloor]

PR Type

Feature

PR Checklist

• Tests added or updated
• Documentation updated
• Changelog entry added
• Ensure CI builds and tests pass
• Rebased on top of the latest master branch
• Code formatted with rustfmt
• No breaking changes introduced

Overview

This PR adds support for using Option<Vec<T>> with the MultipartForm extractor in actix-multipart. It enables multipart fields that may be completely absent or may appear multiple times, while preserving the distinction between missing fields and empty collections.

Behavior

• Field absent: None
• Field present one or more times: Some(Vec<T>), where each occurrence is parsed via T: FieldReader<'t>

This allows optional repeated multipart fields to be represented ergonomically in MultipartForm-derived structs.

Example

#[derive(MultipartForm)]
struct MyForm {
    tags: Option<Vec<String>>,
}

• No tags parts → tags == None
• Multiple tags parts → tags == Some(vec![ ... ])

Motivation

Issue #3576 identified a gap in multipart extraction:

• Vec<T> could collect multiple values but did not distinguish absence from an empty list.
• Option<T> expressed absence but allowed only a single value.

Supporting Option<Vec<T>> resolves this by allowing both optionality and repetition.

Implementation Details

• Adds an implementation of FieldGroupReader<'t> for Option<Vec<T>> in
  actix-multipart/src/form/mod.rs.

• handle_field collects all occurrences of the field into a vector stored in extractor state.

• from_state returns:

  • Some(Vec<T>) if values were collected,
  • None if the field never appeared.

• The change follows existing internal patterns and maintains full backward compatibility.

Changelog

To be added under the Unreleased section of actix-multipart/CHANGES.md:

* Added support for `Option<Vec<T>>` in MultipartForm, enabling optional repeated multipart fields.

Semver

This is a backward-compatible enhancement and should be released as a semver-minor update.

Related Issues

• actix_multipart::form::FieldGroupReader is not Implemented For Option<Vec<T>>> #3576