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Mult Panic

This is a minimal reproducer for a bug where the compiler inserts a call to a non-existent panic function. The following things need to be true to reproduce the bug:

  • target must be wasm32-unknown-unknown
  • overflow checks must be enabled
  • link time optimization must be enabled
  • core must be recompiled using -Z build-std
  • The compiler needs to insert a call to memcpy

The non existent imported function will be then called by the overflow checks in the memcpy compiler intrinsic.

Steps to reproduce

I tested these steps with rustc 1.58.0-nightly (936f2600b 2021-11-22). Version 2021-11-17 and less seem to be unaffected. Therefore the first version to have this bug is 2021-11-18.

cargo +nightly build --release --target wasm32-unknown-unknown -Z build-std
wasm2wat target/wasm32-unknown-unknown/release/mult_panic.wasm | less

If you inspect the resulting wasm file you see that it imports some none existing panic function:

(import "env" "_ZN4core9panicking5panic17h80a3410ec4f43255E" (func $_ZN4core9panicking5panic17h80a3410ec4f43255E (type 0)))

This function is called from the compiler intrinsic on integer overflow. If either lto or integer overflow is disabled, the error goes away.