Passing -Z embed-bitcode doesn't embed bitcode for debug builds of external dependencies

[davehylands]

I'm not 100% sure if this is a cargo bug or a rustc bug, but it looks like a rustc bug.

I tested using rustc 1.41.0-nightly (412f43ac5 2019-11-24) running on MacOSX Catalina.

I put my files in a small repository, which you can find here: https://github.com/davehylands/simple-lib

What I noticed is that if I build using cargo normally (i.e. without using --release) then the object files for external crates do not get bitcode embedded. If I build using cargo with --release when the object files for external creates do get bitcode embedded. In either case, cargo is being run by using RUSTFLAGS="-Z embed-bitcode"

You can pass --release to the build.sh script to get the --release behaviour, and pass no flags to get the debug behaviour. The build.sh script runs otool over the objects included in libsimple_lib.a to show whether bitcode is included or not.

If you modify the for loop to use

for obj in rand_core*.o; do

rather than:

for obj in *.o; do

then you'll get simpler output.

Normally, I use a rust toolchain that has the rust standard libraries built with bitcode, so I realize that these objects won't have bitcode using the nightly compiler.

Cargo.toml

[package]
name = "simple_lib"
version = "0.1.0"
authors = ["Dave Hylands <davehylands@fullstory.com>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
rand = "0.7"

[lib]
name = "simple_lib"
# rustc cannont create dylibs for iOS.
# https://github.com/rust-lang/rust/issues/21727#issuecomment-424026535
crate-type = ["staticlib"]

src/lib.rs

use rand;

pub fn test_func2() -> String {
    let x = rand::random::<u16>();
    format!("Your number is {}", x)
}

build.sh

#!/bin/sh
set -e
if [ "$1" == "--release" ]; then
  BUILD_TYPE="release"
  CARGO_ARGS="--release"
else
  BUILD_TYPE="debug"
fi
TARGET_DIR=target
rm -rf ${TARGET_DIR}
RUSTFLAGS="-Z embed-bitcode" cargo build -v ${CARGO_ARGS}

cd ${TARGET_DIR}/${BUILD_TYPE}
ar x libsimple_lib.a

for obj in *.o; do
  if otool -l ${obj} | grep bitcode > /dev/null ; then
    echo "${obj} has bitcode"
  else
    echo "${obj} DOES NOT have bitcode"
  fi
done

[workingjubilee]

Note: Apple deprecated bitcode support in Xcode 14. This is still a meaningful bug for this flag as either we should remove this flag or we should make it work correctly for whatever other nefarious purposes developers want to put it to.