RFR: 8254354: Add an asExact() VarHandle combinator

[Rémi Forax]

On Fri, 23 Oct 2020 17:47:36 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> Hi,
> 
> This patch adds an asExact() combinator to VarHandle, that will return a new VarHandle that performs exact type checks, similar to MethodHandle::invokeExact, to help developers catch inexact VarHandle usage, which can lead to performance degradation.
> 
> This is implemented using a boolean flag in VarForm. If the flag is set, the exact type of the invocation is checked against the exact type in the VarForm. If there is a mismatch, a WrongMethodTypeException is thrown.
> 
> Other than that, there is also an asGeneric() combinator added that does the inverse operation (thanks to Rémi for the suggestion). I've also added The `@Hidden` annotation to the VarHandleGuards methods, as well as a type-checking helper method called from the generic invocation lambda form, so that the stack trace we get points at the location where the VarHandle is being used.
> 
> Thanks,
> Jorn
> 
> CSR link: https://bugs.openjdk.java.net/browse/JDK-8255375

Looks good to me.

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PR: https://git.openjdk.java.net/jdk/pull/843