Added CHANGELOG Entries for the 0.4.0 Release

[InsertCreativityHere]

This PR adds CHANGELOG entries covering everything we did between 0.3.3 and 0.4.0.
And adds a link/date for the 0.4.0 release tomorrow.

[InsertCreativityHere]

Technically, slicec is both a binary and a library now (which I mention in the README).
But the fact a slicec library exists isn't interesting to users.
This is just a quirk of how projects are structured in Rust, and I don't want to emphasize it much.

The library is logically just an 'implementation detail' of the binary.

[InsertCreativityHere]

This tag doesn't exist yet, but will tomorrow morning.

[Copilot]

Pull request overview

Adds CHANGELOG entries describing changes between 0.3.3 and the upcoming 0.4.0 release, including the new release date, summary text, sectioned bullet list of changes, and the corresponding PR/version reference links.

Changes:

• Replaces the TBD placeholder with a 2026-6-2 release date and adds an introductory paragraph describing the library-to-binary transition.
• Adds Enhancements / Breaking / Changed bullet sections summarizing the work between 0.3.3 and 0.4.0.
• Adds PR reference link definitions (705–778) and the [0.4.0] release tag link.

---

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

[bernardnormier]

I think we should delete this file and put all this information under "What's Changed" in the GitHub release notes. See for example:
https://github.com/icerpc/icerpc-csharp/releases

[InsertCreativityHere]

I think we should delete this file and put all this information under "What's Changed" in the GitHub release notes

Yeah, we do the same for this repo too: https://github.com/icerpc/slicec/releases

So, why do I also have all this information duplicated in a CHANGELOG file? :)

• It lets us have one CHANGELOG per crate, whereas the GitHub releases are repo-wide.
  If you're only using slice-codec, what changed in these other crates is just noise.

• It's unsatisfying... but this is just what other crates do. For example, if you look at the tokio repo for async, they don't have a top-level CHANGELOG. But each subfolder (one per crate) has it's own CHANGELOG:

  • https://github.com/tokio-rs/tokio/tree/master/tokio
  • https://github.com/tokio-rs/tokio/tree/master/tokio-util
  • https://github.com/tokio-rs/tokio/tree/master/tokio-stream

• Or another very popular tokio crate named bytes: https://github.com/tokio-rs/bytes.
  They do like us, duplicate entries in their releases, and CHANGELOG

• Or to prove this isn't just tokio being weird, other crates we depend on which also do this:

  • test_case: https://github.com/frondeus/test-case/blob/master/CHANGELOG.md
  • in_definite: https://github.com/mrseanryan/in_definite/blob/master/CHANGELOG.md
  • clap: https://github.com/clap-rs/clap/blob/master/CHANGELOG.md

I'm not saying every single crate does this, but most of them do.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[externl]

What do other multi crate rust repos do wrt tagging and changelogs?

[bernardnormier]

We should do something similar to Tokio, but without these CHANGELOG files:

• release/tag = slicec-version, slice-rust-version (similar to tokio-1.47.5)
• release title = Slicec vX.Y.Z, Slice Rust vX.Y.Z
  with change description ("What's Changed") in these GitHub release notes. No CHANGELOG.

[externl]

I wonder if we should drop our changelog and just do these in GitHub releases.

[InsertCreativityHere]

What do other multi crate rust repos do wrt tagging and changelogs?

So, most of the multi-crate repos I've looked at do a single release, for the entire repo (not per crate).
Their releases and tags are the standard format.

The exceptions to this are tokio and tonic which do a single release per crate.
Their releases are just like Bernard described.
And their tags look like: tonic-types-v0.14.6, tonic-v0.14.6, tonic-prost-v0.14.6

There's also rust-crypto which also has separate tags for each of their crates.
But they don't use GitHub releases at all, so not as interesting on that front.

[externl]

We should markdown lint this file.

[bernardnormier]

We should remove this file. No CHANGELOG.