Add default for Seg6Header

[Aperence]

Derive default for the Seg6Header structure. As the structure is tagged with non_exhaustive, there is currently no way to create a Seg6Header structure. By defining the Default trait for Seg6Header, this patch should allow to create Seg6Header instances.

[gemini-code-assist]

Summary of Changes

Hello @Aperence, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the usability of the Seg6Header structure by implementing the Default trait. Previously, the non_exhaustive attribute on Seg6Header prevented its direct instantiation. By providing a default implementation, developers can now easily create Seg6Header instances, streamlining code that interacts with Segment Routing over IPv6 (SRv6) headers.

Highlights

• Seg6Header Default Implementation: The Default trait has been derived for the Seg6Header struct, allowing for its instantiation with default values. This addresses the previous limitation where the non_exhaustive tag prevented direct creation.
• Seg6Mode Default Variant: The Default trait has also been derived for the Seg6Mode enum, with the Encap variant explicitly marked as the default.

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[gemini-code-assist]

Code Review

This pull request aims to make Seg6Header constructible from outside the crate by deriving the Default trait for it and its mode field of type Seg6Mode. This is a good approach for #[non_exhaustive] types. However, the chosen default Seg6Mode::Encap for Seg6Mode causes Seg6Header::default() to create an invalid instance that panics during serialization. My review includes a critical comment explaining the issue and suggesting a fix, which is to change the default mode to Seg6Mode::Inline.

[gemini-code-assist]

Deriving Default here is the correct approach to make Seg6Header constructible. However, it currently leads to a panic because of the Default implementation of Seg6Mode.

Seg6Header::default() will be created with mode: Seg6Mode::Encap (the current default) and an empty segments vector. When this default instance is serialized via emit_value(), the number_segments will be 0. This causes an integer underflow and a panic in debug builds on lines 182-183 when calculating segments_left and first_segment (0 - 1).

It appears Encap mode requires at least one segment to be valid. To fix this, Seg6Mode::Inline should be the default, as it handles an empty segment list correctly.

Please move the #[default] attribute from the Encap variant to the Inline variant in the Seg6Mode enum definition.

[Aperence]

Changed from a default implementation to a constructor returning a Result<Seg6Header, ...>.
This should prevent the usage of an invalid Seg6Header containing no segment, thus making
the underflow issue impossible.