Refactor CppWitness as CustomWitness #6491
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danakj
approved these changes
Dec 15, 2025
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| // A witness synthesized for an arbitrary construct. For example, a `Destroy` | ||
| // witness, or a C++ overloaded operator. | ||
| struct CustomWitness { |
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Maybe BuiltinWitness to explain its meaning and difference from a witness coming from carbon code a little more? And as opposed to "custom" as in user-customized.
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I think it's user-customized similar to CustomLayoutType. That is, both tracking the mapping of user code (references to members of a type) in a particular layout that Carbon expects. Neither instruction is directly modified by users, but both track things in a way distinct from how more standard types/witnesses behave (and could in theory support arbitrary things, including the more standard Carbon equivalent).
Note too, there's not much builtin here:
- The
Destroyinterface is user-defined. - For C++, it's a thin wrapper around a C++ destructor thunk.
- Equivalent for other things, like copy, move,
+, etc
- Equivalent for other things, like copy, move,
- For Carbon, it will eventually become a thin wrapper around a call to a vtable function for destroy.
- Perhaps only copy, move as equivalents (not sure how far this will go)
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This is in anticipation of using the same construct for all implementations of
Destroy, as well as other similar use-cases with language-defined interfaces.