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4c002fce9d
This replaces the drop_in_place reference with null in vtables. On librustc_driver.so, this drops about ~17k dynamic relocations from the output, since many vtables can now be placed in read-only memory, rather than having a relocated pointer included. This makes a tradeoff by adding a null check at vtable call sites. That's hard to avoid without changing the vtable format (e.g., to use a pc-relative relocation instead of an absolute address, and avoid the dynamic relocation that way). But it seems likely that the check is cheap at runtime.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.