[cwouyang]

Closes #48.

Summary

Three static per-type caches held their Type keys strongly: the shared-type classification (SharedTypes), the holds-references cache of the deep copy behind Old<T>() (DeepCopier), and the metadata cache of EnsureAssignable<T>() (MemberComparison). A collectible type that went through Old<T>() or EnsureAssignable<T>() (a plugin in a collectible AssemblyLoadContext, a RunAndCollect dynamic assembly) could therefore never be collected. Its load context could never unload either.

Changes

• The three caches now go through the new internal TypeCache<TValue>, which has two tiers:
  • a ConcurrentDictionary for a type whose Type.IsCollectible is false;
  • a ConditionalWeakTable<Type, StrongBox<TValue>> for a collectible type, so the cache does not keep it alive.
• IsCollectible is also true for a constructed type that is collectible only through a generic argument or an array element type, such as a List<PluginType>. That type's Assembly is System.Private.CoreLib, so the tier is not chosen with Assembly.IsCollectible.
• A lookup reads the dictionary first. A hit proves the type is not collectible, so IsCollectible is read only on a miss.
• A hit allocates nothing in either tier. Measured once in a Release microbenchmark (JIT, one machine), before and after the final fix of this branch:

Cache hit	Before	After
Not collectible	14.5–18.1 ns, 0 B	5.1–5.4 ns, 0 B
Collectible	22–27 ns, 88 B	19–20 ns, 0 B

• Documentation: the XML doc of EnsureAssignable now names both tiers. ADR-0024 records the decision, the options and the measurements, and docs/adr/README.md lists it. CHANGELOG.md [Unreleased] has an entry under Fixed.

Not changed: comparison results, copies, exceptions and messages; PublicAPI.*.txt; trimming annotations.

Testing

• All existing tests pass (dotnet test): 1239 of 1239 at the last commit.
• New tests added for new functionality:
  • CollectibleTypeCacheTests: one collection test per cache, each failing before the fix because the type stayed alive. It also covers a List<T> and an array of a collectible type, and checks that the results for collectible and non-collectible types are the same.
  • TypeCacheTests: a hit for a non-collectible type allocates 0 bytes. This test failed at 24 bytes before its fix.
• No mixed structural/behavioral changes. The extraction and the rename are separate refactor commits.
• The Native AOT smoke program was published with -warnaserror (win-x64). All 107 checks pass with postconditions on. The DBC_POST=off variant was left to CI.
• Reviews:
  • an independent review per task;
  • a risk review of the shared caches, both of the change and of its final fix;
  • a review of the whole branch.

Left open

• No unit test pins that a hit for a collectible type allocates nothing, or that a dictionary hit skips IsCollectible. Only the microbenchmark shows them.
• No test exercises the collectible tier from several threads. Its thread safety comes from ConditionalWeakTable.
• The collection tests depend on the GC: at most 10 collections.
• Unloading a real collectible AssemblyLoadContext is not tested. The tests use RunAndCollect assemblies.

Related Issues

Closes #48