Nightly with MIRI and FFI

[jorgecarleitao]

While trying to unpin nightly (2021-03-25) to latest nightly, I have hit an internal compiler error.

Complete build log

It emerged between 2021-03-25 and today's build.

All the details are in the build log. In particular,

• rustc 1.55.0-nightly (24bdc6d73 2021-06-12) running on x86_64-unknown-linux-gnu

The error seems to emerge when compiling a transitive dependency, so it is not very easy to reproduce with a minimal example.

git clone git@github.com:jorgecarleitao/arrow2.git
cd arrow2
rustup toolchain install nightly
rustup default nightly
rustup component add rustfmt miri

cargo miri setup
cargo clean
cargo miri build

Regressed between nightly-2021-06-08 and nightly-2021-06-09, which is e4a6032...ed597e7, which corresponds to rust-lang/miri@5dde0fe...c8713c2 (which includes rust-lang/miri#1776 that touches exported_symbols 🤔).

[RalfJung]

@hyd-dev found out that the issue boils down to this strange bit of code

#[no_mangle]
#[cfg(feature="std")]
use std::{thread,panic, io, boxed, any, string};

Does it make any sense to put no_mangle on a use item...??

[syvb]

@RalfJung It doesn't -- it even generates a warning:

warning: attribute should be applied to a function or static
 --> src/ffi/mod.rs:3:1
  |
3 | #[no_mangle]
  | ^^^^^^^^^^^^
4 | #[cfg(feature="std")]
5 | use std::{thread,panic, io, boxed, any, string};
  | ------------------------------------------------ not a function or static
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!

[syvb]

@rustbot claim