[foreign-memaccess+abi] RFR: 8292918: Split FunctionDescriptor into an interface and implementation(s)

[Per Minborg]

On Fri, 26 Aug 2022 06:47:35 GMT, Athijegannathan Sundararajan <sundar at openjdk.org> wrote:

>> This PR splits the current FunctionDescription class into an interface and two internal implementing classes.
>
> src/java.base/share/classes/jdk/internal/foreign/FunctionDescriptorImpl.java line 219:
> 
>> 217:             for (MemoryLayout layout : addedLayouts) {
>> 218:                 requireNonNull(layout);
>> 219:             }
> 
> Do we need to enforce other problems (index, null) before throwing UnsupportedOperationException? Existing code unconditionally throws UnsupportedOperationException.

I think this is a philosophical question and how strictly we want to adhere to principles. Maybe this is overdoing it so I will revert this section.

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PR: https://git.openjdk.org/panama-foreign/pull/713