feat: show repartitioning info in validate-restore report

[thomasdudek]

Display auto-repartitioning details (strategy, source/target partition counts) in the dry-run validation report when repartitioning is configured for a topic. Also rewrites the CLI report printer to use dynamic box width with proper alignment.

• Add DryRunRepartitioningInfo struct to manifest types
• Populate repartitioning info from restore config in dry_run()
• Implement Display for RepartitioningStrategy
• Rewrite print_validation_report with dynamic-width box rendering

[sionsmith]

Review: Looks good — approve with minor suggestions

Build verification ✅

• cargo build — passes
• cargo test — 89 tests pass
• cargo clippy — clean

What works well ✅

1. Repartitioning info in dry-run — clean addition. DryRunRepartitioningInfo struct is well-placed, serde(skip_serializing_if) keeps JSON output clean, and the Display impl on RepartitioningStrategy is reusable.

2. Dynamic box width — big improvement over the old hardcoded-width approach, which had visible alignment issues (the original ║ lines didn't even align — compare the header/separator widths in the old code). The new code sizes the box to content correctly.

3. Repartitioning config lookup is correct — uses get(&target_topic) which matches the config convention (keyed by target topic name per config.rs:642).

4. #[non_exhaustive] on DryRunTopicReport — good forward-thinking. Since the only construction site is within kafka-backup-core (same crate), this has no current impact but protects against downstream breakage if the struct gains more fields.

5. Correct mapping of source_partitions — uses topic_backup.partitions.len() (actual source partition count from the backup manifest) rather than a config value.

Minor suggestions (non-blocking)

1. \x01 sentinel for centered lines — using a control character as an in-band signal works but is unconventional. A small enum like enum Line { Centered(String), Left(String) } or a Vec<(String, bool)> would be more self-documenting. Not a blocker since it's confined to one function.

2. Unicode display width — chars().count() counts code points, not terminal display columns. Characters like ✓, ✗, ⚠ may render as 1 or 2 columns wide depending on the terminal, causing slight box misalignment. A crate like unicode-width would fix this, but it's a pre-existing issue from the old code and only cosmetic. Not worth adding a dependency for.

3. Empty separator lines in topics section — lines.push(String::new()) at line 107 renders as a blank row inside the box (║ ║). This is fine aesthetically but slightly unusual. Consider if a visual separator like "─".repeat(...) or just omitting it would look better.

No issues found ✅

No bugs, no correctness problems, no breaking changes. Clean feature addition.

[sionsmith]

CI failure: #[non_exhaustive] breaks semver check

The Detect Breaking Changes check is failing because #[non_exhaustive] was added to DryRunTopicReport. cargo-semver-checks correctly flags this — adding #[non_exhaustive] to a previously exhaustive public struct is a breaking change (it prevents struct literal construction from outside the crate).

--- failure struct_marked_non_exhaustive: struct marked #[non_exhaustive] ---
  struct DryRunTopicReport in manifest.rs:843

Fix

Remove #[non_exhaustive] from DryRunTopicReport in crates/kafka-backup-core/src/manifest.rs:842:

 /// Per-topic dry-run report
 #[derive(Debug, Clone, Serialize, Deserialize)]
-#[non_exhaustive]
 pub struct DryRunTopicReport {

It's not needed here — the new repartitioning: Option<DryRunRepartitioningInfo> field already handles forward compatibility naturally (None default, serde skips on serialization). Everything else in the PR looks good.

Changing review from approve to request-changes since CI is blocked on this.

[thomasdudek]

Thanks for the thorough review!

Addressed suggestions

\x01 sentinel replaced with enum — Agreed this was unconventional. Replaced with a BoxLine enum (Left(String), Centered(String), Separator) which makes the intent self-documenting.

Empty separator lines — Replaced lines.push(String::new()) (blank row) with a BoxLine::Separator variant that renders a ─ line between topics. Also changed it to only appear between topics (not before the first one).

Unicode display width — Acknowledged as a pre-existing cosmetic issue. Agreed it's not worth adding a dependency for.

Regarding #[non_exhaustive] and semver CI

The semver check fails either way:

• With #[non_exhaustive]: struct_marked_non_exhaustive — adding the attribute to a previously exhaustive struct is a breaking change.
• Without #[non_exhaustive]: constructible_struct_adds_field — adding the repartitioning field to a constructible struct is also a breaking change.

Both are technically semver-major because external code could construct DryRunTopicReport via struct literal. The underlying issue is that adding any new field to a public struct without #[non_exhaustive] is a breaking change per semver rules.

Options:

1. Bump to 1.0.0 (or next major) — correct per semver, but seems premature for a minor feature addition.
2. Add #[non_exhaustive] to all public structs in manifest.rs as a separate semver-major release first, then this PR becomes a clean minor addition on top.

What's your preference? Or do I miss some other option?

[sionsmith]

Thanks for addressing the \x01 sentinel — the BoxLine enum is much cleaner!

The Detect Breaking Changes check is still failing though — #[non_exhaustive] on DryRunTopicReport is still present in manifest.rs:842. That's the only thing blocking CI. Just remove that one line and it should go green.

[thomasdudek]

The Detect Breaking Changes check is still failing though — #[non_exhaustive] on DryRunTopicReport is still present in manifest.rs:842. That's the only thing blocking CI. Just remove that one line and it should go green.

I removed it, but like mentioned in the comment above, this unfortunately brings up an other semver issue:

--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.46.0/src/lints/constructible_struct_adds_field.ron

this at least is equal to the one you've had in your feature PR https://github.com/osodevops/kafka-backup/actions/runs/22666759032/job/65700335839#step:4:74