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`__heap_base` and `__data_end` are exported for use by wasm-bindgen, which uses the wasm32-unknown-unknown target. On wasm32-wasi, as a step toward implementing the Canonical ABI, and as an aid to building speicalized WASI API polyfill wrappers, don't export `__heap_base` and `__data_end` on wasm32-wasi.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.