libstd.dylib always depends on liballoc_jemalloc (and libpanic_unwind) #43637
Description
I am trying to use xargo to build a libstd without jemalloc. (That should enable cross-compiling libstd without cross-compiling any C code.)
I have the following Xargo.toml:
[dependencies]
std = {features = ["panic_abort", "force_alloc_system"]}
Now xargo build -v shows:
...
Running `rustc --crate-name std /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/lib.rs --crate-type dylib --crate-type rlib --emit=dep-info,link -C prefer-dynamic -C opt-level=3 --cfg 'feature="force_alloc_system"' --cfg 'feature="panic_abort"' -C metadata=f2ad81e4aae1da77 --out-dir /tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps --target x86_64-unknown-linux-gnu -L dependency=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps -L dependency=/tmp/xargo.j52xO5VAig3n/target/release/deps --extern rustc_tsan=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/librustc_tsan-b7ca73d041615ffd.rlib --extern alloc=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/liballoc-210591387bfdd319.rlib --extern panic_abort=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/libpanic_abort-2640770c1084be03.rlib --extern core=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/libcore-c98bfbbe49faadd5.rlib --extern std_unicode=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/libstd_unicode-c551e0da6902f65e.rlib --extern libc=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/liblibc-a777dc7e09579a84.rlib --extern unwind=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/libunwind-bfa0f42576422ec8.rlib --extern compiler_builtins=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/libcompiler_builtins-77a37c3c1af692d9.rlib --extern rustc_asan=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/librustc_asan-1b93e3a38c735df5.rlib --extern collections=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/libcollections-9a131e253f0f0440.rlib --extern rand=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/librand-5667d73b4da05dc4.rlib --extern alloc_system=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/liballoc_system-f1890d93ca3ab3ce.rlib --extern rustc_msan=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/librustc_msan-e33a58e222e716d7.rlib --extern rustc_lsan=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/deps/librustc_lsan-ad0c635ae24b3237.rlib --sysroot /home/r/.xargo/HOST -l dl -l rt -l pthread -L native=/tmp/xargo.j52xO5VAig3n/target/x86_64-unknown-linux-gnu/release/build/compiler_builtins-7685fddded0d8535/out`
error[E0463]: can't find crate for `alloc_jemalloc`
error: aborting due to previous error
error: Could not compile `std`.
This shows that the correct feature flags are passed to rustc. However, for some reason, rustc behaves as if libstd contained extern crate alloc_jemalloc, which it does not. Is there magic going on here that adds alloc_jemalloc nevertheless? I did some grepping of the code but found nothing.
(EDIT: There seems to be similar magic behavior for panic_unwind.)
If somehow it is not possible to get rid of this magic "link against liballoc_jemalloc", then (a) libstd/Cargo.toml should be changed to reflect this fact by always depending on liballoc_jemalloc, and (b) I hope there's a chance we can get a feature flag on liballoc_jemalloc itself to enable "dummy_jemalloc" mode (currently controlled by build.rs), and have libstd set that flag when force_alloc_system is set.
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