Try __libc_ section prefix

[dtolnay]

Per #41 (comment). Recent versions of lld recognize section names with a __libc_ prefix as being retained even without rustc support for llvm.used.

Relevant to #41 and #40. Sadly as far as I can tell I don't think this fully fixes either one.

[dtolnay]

macOS: no change.

---- declaration::test_functions stdout ----
thread 'declaration::test_functions' panicked at 'assertion failed: !FUNCTIONS.is_empty()', tests/custom_linkme_path.rs:21:9

---- declaration::test_slice stdout ----
thread 'declaration::test_slice' panicked at 'assertion failed: !SLICE.is_empty()', tests/custom_linkme_path.rs:12:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Cortex-M:

error: linking with `rust-lld` failed: exit status: 1
  |
  = note: "rust-lld" "-flavor" "gnu" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/cortex_m_linkme_test-16abada556882b6c.cortex_m_linkme_test.11f206d2-cgu.0.rcgu.o" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/cortex_m_linkme_test-16abada556882b6c.cortex_m_linkme_test.11f206d2-cgu.1.rcgu.o" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/cortex_m_linkme_test-16abada556882b6c.cortex_m_linkme_test.11f206d2-cgu.2.rcgu.o" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/cortex_m_linkme_test-16abada556882b6c.cortex_m_linkme_test.11f206d2-cgu.3.rcgu.o" "--as-needed" "-L" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps" "-L" "/home/runner/work/linkme/linkme/target/release/deps" "-L" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/build/cortex-m-linkme-test-fd2f9025fa6ab5f3/out" "-L" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/build/cortex-m-dc8ce5e5692e1185/out" "-L" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/build/cortex-m-rt-146748f53fc7675e/out" "-L" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/build/cortex-m-semihosting-06c5e7e29f999ff6/out" "-L" "/usr/share/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/thumbv7m-none-eabi/lib" "-Bstatic" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/liblinkme-f7db9041ae6efe5a.rlib" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libcortex_m_rt-66869b94f8d0a046.rlib" "--start-group" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libpanic_semihosting-23f6673becf7b019.rlib" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libcortex_m_semihosting-d41a19e13b83fd58.rlib" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libcortex_m-d0a9c8248f754074.rlib" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libembedded_hal-d3d0fba186ae51c5.rlib" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libvoid-9ff3a42907c50f26.rlib" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libnb-a64e30147a73fe33.rlib" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libnb-722d2d7ea921a817.rlib" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libvolatile_register-8f58f7ecc06a6493.rlib" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libvcell-bf2fa04b84d99d34.rlib" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/libbare_metal-943465c25f42e4e6.rlib" "/usr/share/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/thumbv7m-none-eabi/lib/librustc_std_workspace_core-ba626387a2e50982.rlib" "/usr/share/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/thumbv7m-none-eabi/lib/libcore-bc639632082e340c.rlib" "--end-group" "/usr/share/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/thumbv7m-none-eabi/lib/libcompiler_builtins-4ec3ed03bf6f473e.rlib" "-Bdynamic" "--eh-frame-hdr" "-znoexecstack" "-L" "/usr/share/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/thumbv7m-none-eabi/lib" "-o" "/home/runner/work/linkme/linkme/target/thumbv7m-none-eabi/release/deps/cortex_m_linkme_test-16abada556882b6c" "--gc-sections" "-O1" "-Tlink.x"
  = note: rust-lld: error: no memory region specified for section '__libc_SHENANIGANS'
          rust-lld: error: no memory region specified for section '__libc_EMPTY'

[mystor]

It would make sense that macOS would be unchanged, as the code I linked in #41 (comment) is in the /ELF/ subdirectory, and therefore wouldn't be used for Mach-O binaries. The change for Mach-O is probably caused because on macOS #[used] changed meaning to llvm.compiler.used rather than llvm.used. I don't see a trick for working around that in https://github.com/llvm/llvm-project/blob/abbbc480a152c1afbac215064f22ae07c87ed964/lld/MachO/MarkLive.cpp, other than marking the symbols correctly (which I expect requires rustc support).

You can actually see the different generated code fairly easily in the compiler explorer: https://godbolt.org/z/4sTrKzxYn. When building with rustc 1.55.0, there is a .no_dead_strip example::FOO directive in the generated assembly, whereas in nightly that directive is missing.

I believe the original motivation for switching to llvm.compiler.used over llvm.used was that it aligned with clang better, but it appears as though clang specifies .no_dead_strip for both __attribute__((used)) and __attribute__((retain)) right now (https://godbolt.org/z/6Tc3xcjcr), so perhaps that only applies to ELF targets?

In contrast, I don't see any change in the generated assembly for armv7m-none-eabi, they look the same before and after: https://godbolt.org/z/bsP9E8jWq. The new linker errors also suggest that there may be some other place in lld which checks for the __libc_ prefix and is screwing things up there.