Option to pass flags to `rustc` while bootstrapping using a file

When bootstrapping rustc invocations can be rather large. For example this is an invocation I wanted to debug today (~10.9 KiB):

LD_LIBRARY_PATH="/loc/waffle/rust-h/build/x86_64-unknown-linux-gnu/stage1/lib:/loc/waffle/rust-h/build/x86_64-unknown-linux-gnu/stage1-std/release/deps" RUST_BACKTRACE="1" "/loc/waffle/rust-h/build/x86_64-unknown-linux-gnu/stage1/bin/rustc" "--crate-name" "compiler_builtins" "--edition=2021" "/usr/sec/waffle/.cargo/registry/src/index.crates.io-6f17d22bba15001f/compiler_builtins-0.1.138/src/[lib.rs](http://lib.rs/)" "--error-format=json" "--json=diagnostic-rendered-ansi,artifacts,future-incompat" "--diagnostic-width=204" "--crate-type" "lib" "--emit=dep-info,metadata,link" "-C" "opt-level=3" "-C" "embed-bitcode=no" "-C" "codegen-units=10000" "-C" "debuginfo=line-tables-only" "--cfg" "feature=\"compiler-builtins\"" "--cfg" "feature=\"core\"" "--cfg" "feature=\"default\"" "--cfg" "feature=\"mem\"" "--cfg" "feature=\"rustc-dep-of-std\"" "--check-cfg" "cfg(docsrs)" "--check-cfg" "cfg(feature, values(\"c\", \"cc\", \"compiler-builtins\", \"core\", \"default\", \"mangled-names\", \"mem\", \"no-asm\", \"no-f16-f128\", \"public-test-deps\", \"rustc-dep-of-std\"))" "-C" "metadata=8b1a123d4d7caf72" "-C" "extra-filename=-8b1a123d4d7caf72" "--out-dir" "/loc/waffle/rust-h/build/x86_64-unknown-linux-gnu/stage1-std/riscv32i-unknown-none-tasking/release/deps" "--target" "riscv32i-unknown-none-tasking" "-C" "linker=cc" "-L" "dependency=/loc/waffle/rust-h/build/x86_64-unknown-linux-gnu/stage1-std/riscv32i-unknown-none-tasking/release/deps" "-L" "dependency=/loc/waffle/rust-h/build/x86_64-unknown-linux-gnu/stage1-std/release/deps" "--extern" "core=/loc/waffle/rust-h/build/x86_64-unknown-linux-gnu/stage1-std/riscv32i-unknown-none-tasking/release/deps/librustc_std_workspace_core-2e8331c85d5501e5.rmeta" "--cap-lints" "warn" "-Csymbol-mangling-version=legacy" "--check-cfg=cfg(feature,values(any()))" "-Zunstable-options" "--check-cfg=cfg(bootstrap)" "-Zmacro-backtrace" "-Csplit-debuginfo=off" "-Cforce-frame-pointers=true" "-Cprefer-dynamic" "-Zinline-mir" "-Zinline-mir-preserve-debug" "-Clink-args=-Wl,-z,origin" "-Clink-args=-Wl,-rpath,$ORIGIN/../lib" "-Cembed-bitcode=yes" "-Clto=off" "-Cforce-unwind-tables=yes" "-Cforce-frame-pointers=yes" "-Zcrate-attr=doc(html_root_url=\[https://doc.rust-lang.org/nightly/\](https://doc.rust-lang.org/nightly/))" "-Z" "binary-dep-depinfo" "--cfg" "f16_enabled" "--cfg" "f128_enabled" "--cfg" "feature=\"unstable\"" "--cfg" "feature=\"force-soft-floats\"" "--cfg" "feature=\"mem\"" "--check-cfg" "cfg(__ashldi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__ashlsi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__ashrdi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__ashrsi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__bswapsi2, values(\"optimized-c\"))" "--check-cfg" "cfg(__bswapdi2, values(\"optimized-c\"))" "--check-cfg" "cfg(__bswapti2, values(\"optimized-c\"))" "--check-cfg" "cfg(__divdi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__divsi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__divmoddi4, values(\"optimized-c\"))" "--check-cfg" "cfg(__divmodsi4, values(\"optimized-c\"))" "--check-cfg" "cfg(__divmodsi4, values(\"optimized-c\"))" "--check-cfg" "cfg(__divmodti4, values(\"optimized-c\"))" "--check-cfg" "cfg(__lshrdi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__lshrsi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__moddi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__modsi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__muldi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__udivdi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__udivmoddi4, values(\"optimized-c\"))" "--check-cfg" "cfg(__udivmodsi4, values(\"optimized-c\"))" "--check-cfg" "cfg(__udivsi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__umoddi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__umodsi3, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas1_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas1_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas1_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas1_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas2_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas2_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas2_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas2_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas4_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas4_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas4_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas4_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas8_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas8_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas8_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas8_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas16_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas16_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas16_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_cas16_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd1_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd1_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd1_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd1_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd2_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd2_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd2_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd2_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd4_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd4_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd4_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd4_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd8_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd8_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd8_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldadd8_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr1_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr1_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr1_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr1_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr2_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr2_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr2_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr2_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr4_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr4_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr4_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr4_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr8_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr8_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr8_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldclr8_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor1_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor1_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor1_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor1_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor2_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor2_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor2_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor2_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor4_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor4_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor4_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor4_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor8_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor8_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor8_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldeor8_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset1_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset1_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset1_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset1_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset2_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset2_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset2_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset2_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset4_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset4_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset4_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset4_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset8_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset8_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset8_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_ldset8_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp1_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp1_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp1_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp1_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp2_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp2_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp2_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp2_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp4_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp4_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp4_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp4_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp8_relax, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp8_acq, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp8_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(__aarch64_swp8_acq_rel, values(\"optimized-c\"))" "--check-cfg" "cfg(target_feature, values(\"vis3\"))" "--check-cfg" "cfg(feature, values(\"checked\"))" "--check-cfg" "cfg(assert_no_panic)" "--check-cfg" "cfg(f16_enabled)" "--check-cfg" "cfg(f128_enabled)" "--check-cfg" "cfg(feature, values(\"unstable\", \"force-soft-floats\"))" "--check-cfg" "cfg(feature, values(\"mem-unaligned\"))" "--check-cfg" "cfg(thumb)" "--check-cfg" "cfg(thumb_1)" "--check-cfg" "cfg(kernel_user_helpers)" "-Wrust_2018_idioms" "-Wunused_lifetimes" "--sysroot" "/loc/waffle/rust-h/build/x86_64-unknown-linux-gnu/stage1" "-Z" "force-unstable-if-unmarked"

This causes shells to melt (good luck editing a command when it doesn't render), which is very annoying when you want to change something about the command, or even simply run it through gdb (NB: you need to insert gdb --args in between the env vars and the rest of the command, which is mostly impossible when the command doesn't render!!).

One of the things I'd wish for is a way for cargo to pass check-cfg without repeating --check-cfg ~139 times. Something like --check-cfg "all(cfg(uwu), cfg(owo))" (or "cfg(uwu); cfg(owo)") maybe?

The second thing is for bootstrap to have an option (cli, config) to pass options to the rustc binary via a file. Note that rustc supports reading command line arguments from a file with @file:

: ~; echo '--version' > ./args.txt
: ~; rustc @./args.txt
rustc 1.86.0-nightly (48a426eca 2025-01-12)

Bootstrap could create a file in /tmp or in ./build/.. with the arguments and pass that to rustc. That would make it much easier to re-run commands run by bootstrap, which is sometimes useful when debugging.

cc @jieyouxu

Start by separating the two requested capabilities: reducing repeated --check-cfg arguments and letting bootstrap pass rustc options through a file. Review the existing cargo check-cfg handling and rustc read-file support mentioned in the issue, then define how bootstrap configuration or CLI options should express this. Done means both requested workflows work without requiring users to edit the generated command manually.