Impl !Send for MutexGuard

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1

I am fiddling with creation of synchronization primitives, and faced a thing i can't understand.
When i tried to make my custom MutexGuard not Send using "impl !Send for MutexGuard<'_, T> {}", compiler pointed this error and gently suggested to use marker type. Ok, no problem, bro. But why does original std mutex guard version contain these lines and feel fine then?

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Send for MutexGuard<'_, T> {}
2

The standard library uses that unstable feature and many, many more. It's developed in step with the compiler and takes advantage of other implementation details to boot (which a normal library can't soundly do).

Occasionally there's a push to make std less magical, but it sort of ebbs and flows.

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