fix: release removed and cleared slots on netstandard builds [patch]

[matt-edmondson]

Fixes #79

What was wrong

On the netstandard2.0 and netstandard2.1 builds, the slot-clearing guard fell back to !typeof(T).IsValueType. That check is false for every struct, including structs that hold references:

• ContiguousMap stores a struct Entry, so on those builds it never released a removed or cleared key or value.
• ContiguousCollection, ContiguousSet and RingBuffer leaked struct elements such as KeyValuePair<int, object> or (string, int).

Change

• New internal SlotClearing.IsNeeded<T>():
  • It uses RuntimeHelpers.IsReferenceOrContainsReferences<T>() wherever that API exists (netstandard2.1, netcoreapp2.0 and later).
  • On netstandard2.0 it always returns true. Clearing a slot is cheap, and skipping one that holds a reference leaks.
• All seven guard sites now call it: ContiguousMap Remove/Clear, ContiguousCollection RemoveAt/Clear, ContiguousSet Remove/Clear, and RingBuffer.Clear. The #if NET5_0_OR_GREATER blocks and their conditional using lines are gone.
• No public API changes. On net5+ the behaviour is unchanged.

Tests

New file: Containers.Test/SlotClearingTests.cs.

• WeakReference tests: values are collectable after ContiguousMap.Clear, after removing every key from a ContiguousMap, and after Clear/RemoveAt on a ContiguousCollection<KeyValuePair<int, object>> and Clear on a ContiguousSet<KeyValuePair<int, object>>.
• IsNeeded tests: true for object, KeyValuePair<int, object> and (string, int), and false for int.
• Containers_DoNotDecideSlotClearingWithAValueTypeCheck: a source guard that fails if any container goes back to an IsValueType check.
  • The test project only targets net10.0, so the behavioural tests can't reach the netstandard fallback. This guard is the in-suite regression check.
  • With the container changes reverted, this test fails and names all four files.

Manual netstandard2.0 check (the issue's acceptance criteria allow this when CI has no netstandard leg). I referenced the built netstandard2.0/ktsu.Containers.dll from a net10 console app and checked with a WeakReference after GC.Collect:

Case	Before	After
ContiguousMap<int, object>.Clear()	alive	collected
ContiguousMap<int, object>.Remove(key)	alive	collected
ContiguousCollection<KeyValuePair<int, object>>.Clear()	alive	collected

Full suite: 431 tests passed (dotnet test).

There is no overlap with the open RingBuffer PRs #99 and #100. They change PushBack and enumeration, not Clear.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NfxXtpcxX8hkLYrePfHkV4

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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