All the changes look correct. Let me review all five files one more time systematically:

1. identity-transform-stream.c++: cancel() takes by value, but exception is used after the cancel() call (via kj::mv(exception)). Adding kj::cp() makes the copy explicit. Correct.

2. writable-sink-adapter-test.c++: isErrored() returns kj::Maybe<const kj::Exception&>, so KJ_ASSERT_NONNULL returns const kj::Exception&. Using auto& instead of auto avoids an implicit copy. Correct.

3. exception.h: The three macros catch by non-const ref (was const kj::Exception& e) and now use throw kj::mv(e) instead of throw e. This avoids a copy on rethrow. The & e spacing (with space before &) is just clang-format style. Correct.

4. test-fixture.c++: Already catching by non-const ref (kj::Exception& e); throw kj::mv(e) avoids a copy on rethrow. Correct.

5. util/test.h: kj::cp(expException) makes the copy explicit. The NOLINTNEXTLINE(performance-unnecessary-copy-initialization) comment was already there acknowledging the copy, and now the copy is explicit. Correct.

All changes are straightforward, mechanically correct, and well-motivated by making the upstream kj::Exception copy constructor explicit. No behavioral changes, no backward compatibility concerns.

LGTM

github run

FWIW I would be in favor of adding a .clone() method to exceptions and then removing the copy constructor entirely.