[WASM][ST] Use Queue instead of ConcurrentQueue in single-threaded thread pool

[pavelsavara]

Motivation: performance and size improvements for single-threaded WASM

• introduce FEATURE_SINGLE_THREAD define symbol
• use Queue instead of ConcurrentQueue in single-threaded thread pool
• protect ConcurrentQueue ctor from trimming when used with JsonSerializer.Deserialize

[eerhardt]

Why is this necessary? We try really hard to not use DynamicDependency unless it is a last resort.

[pavelsavara]

Until now the runtime itself was using ConcurrentQueue ctor and so it was never trimmed.

If we want to keep it friction-less this is one option how.
If we are OK to force users to start doing it themself, we could avoid this DynamicDependency.
Obviously only browser/wasi people will be impacted as thread pool on other platforms will keep it alive anyway.

[pavelsavara]

See src\libraries\System.Text.Json\tests\System.Text.Json.Tests\TrimmingTests\StackOrQueueNotRootedTest.cs

[pavelsavara]

Not a big deal, who needs to deserialize into ConcurrentQueue anyway ?
😉

• I can disable this test on the browser.
• Or I can add DynamicDependency in the test, I'm not sure if that would be defeating the test purpose slightly.

I think it's testing trimming of non-generic collections too, which can keep working.

Please let me know @eerhardt, I don't have strong opinion.

[eerhardt]

I would just work around it in the tests, and not change the product code for it. If you are using the JsonSerializer with trimming/native AOT, the source generator is the only support route.