feat: add allow_missing in the struct_field

[Xyedo]

Fixes: #1146

Add #[allow_missing] in the field attribute for DeserializeRow macro both on enforce_order flavor and match_by_name flavor

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[github-actions]

cargo semver-checks found no API-breaking changes in this PR.
Checked commit: fd9db3f

[Copilot]

Pull Request Overview

This PR adds an allow_missing attribute to struct fields for both row and value deserialization, allowing fields to be initialized with Default::default() when missing from the database instead of failing. This feature provides compatibility with sqlx's #[sqlx(default)] attribute for smoother migration to scylla-rust-driver.

• Implements #[scylla(allow_missing)] attribute for both struct-level and field-level usage
• Adds support for the attribute in both row and value deserialization contexts
• Includes comprehensive test coverage for the new functionality

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
scylla-macros/src/lib.rs	Adds documentation for the new allow_missing attribute
scylla-macros/src/serialize/value.rs	Adds ignored allow_missing attribute parsing for value serialization
scylla-macros/src/serialize/row.rs	Adds ignored allow_missing attribute parsing for row serialization
scylla-macros/src/deserialize/value.rs	Implements allow_missing logic for value deserialization
scylla-macros/src/deserialize/row.rs	Implements allow_missing logic for row deserialization
scylla/tests/integration/macros/hygiene.rs	Adds hygiene tests for the new attribute
scylla-cql/src/deserialize/row_tests.rs	Adds comprehensive tests for allow_missing functionality

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Lorak-mmk]

Sorry for the delay! Please rebase on main to make semver checks work again.

[Lorak-mmk]

Please let's do one thing in this PR. Other field attributes can't be used on a struct, so let's remove this ability from this one as well, at least for now. It will also simplify this PR a lot.

[Xyedo]

sorry, i don't understand what your intention is, is it start with scylla[allow_missing] field attributes in this PR, and put scylla[allow_missing] in struct attributes in another (next) PR?

[wprzytula]

This is how understand this. Also, if we choose to allow this attribute to be put on a struct, then we want to allow others as well. Therefore, such another PR would necessarily by larger.
For now, not to impede the per-field attribute, let's strip this PR and get it merged.

[Lorak-mmk]

sorry, i don't understand what your intention is, is it start with scylla[allow_missing] field attributes in this PR, and put scylla[allow_missing] in struct attributes in another (next) PR?

In another PR, or maybe not at all - we will have to decide if we want to include such functionality in the driver.
I see "using field attributes on a struct" as a totally separate matter from "using allow_missing in DeserializeRow", so I think it should not be put in a single commit or PR.

[Xyedo]

Hi! sorry for the delay, removing struct attribute and add enforce_order functionalities based on DeserializeValue code is done in my latest commit

[Copilot]

Pull Request Overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Inconsistent capitalization: sentence should start with a capital letter. Change 'if set' to 'If set'.

Suggested change

	/// if set, implementation will not fail if some columns are missing.
	/// If set, implementation will not fail if some columns are missing.

[Copilot]

Inconsistent capitalization: sentence should start with a capital letter. Change 'if set' to 'If set'.

Suggested change

	/// if set, implementation will not fail if some columns are missing.
	/// If set, implementation will not fail if some columns are missing.

[Copilot]

The documentation is incorrect. For DeserializeValue, the allow_missing attribute should handle missing fields in the UDT definition, not null values. The description should say 'If the UDT definition does not contain this field' similar to the field-level documentation at line 528.

Suggested change

	/// If the value of the field received from DB is null, the field will be
	/// initialized with `Default::default()`.
	/// If the UDT definition does not contain this field, it will be initialized
	/// with `Default::default()`.

[wprzytula]

🌱 Hmm, perhaps Copilot is right? Perhaps a better name would be allow_null? cc @Lorak-mmk

[Xyedo]

this is allow_missing struct attributes docs that i forgot to delete, mb!