linker error when the "weak" feature is enabled

[japaric]

STR

$ cargo new --bin app && cd $_

$ edit Cargo.toml && tail -n4 $_
[dependencies.compiler_builtins]
branch = "rustbuild"
features = ["weak"]
git = "https://github.com/japaric/compiler-builtins"

$ edit src/main.rs && cat $_
#![feature(compiler_builtins_lib)]
#![feature(core_intrinsics)]
#![feature(lang_items)]
#![no_main]
#![no_std]

extern crate compiler_builtins;

use core::intrinsics;

#[no_mangle]
pub fn _start() {
    unsafe {
        intrinsics::copy_nonoverlapping(0x0 as *const u32, 0x20 as *mut u32, 0x20);
    }
}

#[no_mangle]
pub fn __aeabi_unwind_cpp_pr0() {}

#[lang = "panic_fmt"]
extern "C" fn panic_fmt() {}

$ xargo rustc --target thumbv6m-none-eabi -- -C link-args=-nostartfiles
error: linking with `arm-none-eabi-gcc` failed: exit code: 1
  |
  = note: "arm-none-eabi-gcc" "-L" "/home/japaric/.xargo/lib/rustlib/thumbv6m-none-eabi/lib" "/home/japaric/tmp/app/target/thumbv6m-none-eabi/debug/deps/app-6d9087d3fe6b93f5.0.o" "-o" "/home/japaric/tmp/app/target/thumbv6m-none-eabi/debug/deps/app-6d9087d3fe6b93f5" "-Wl,--gc-sections" "-nodefaultlibs" "-L" "/home/japaric/tmp/app/target/thumbv6m-none-eabi/debug/deps" "-L" "/home/japaric/tmp/app/target/debug/deps" "-L" "/home/japaric/.xargo/lib/rustlib/thumbv6m-none-eabi/lib" "-Wl,-Bstatic" "-Wl,-Bdynamic" "/home/japaric/tmp/app/target/thumbv6m-none-eabi/debug/deps/librlibc-ca4e4855ef16b6a5.rlib" "/home/japaric/.xargo/lib/rustlib/thumbv6m-none-eabi/lib/libcore-539305ece2e53820.rlib" "/home/japaric/tmp/app/target/thumbv6m-none-eabi/debug/deps/libcompiler_builtins-71942e6a7f812dde.rlib" "-nostartfiles"
  = note: /home/japaric/tmp/app/target/thumbv6m-none-eabi/debug/deps/libcompiler_builtins-71942e6a7f812dde.rlib(compiler_builtins-71942e6a7f812dde.0.o): In function `compiler_builtins::arm::__aeabi_memcpy4':
/home/japaric/.cargo/git/checkouts/compiler-builtins-299ea7fa6d11e5d0/b59c789/src/arm.rs:117: undefined reference to `memcpy'
/usr/lib/gcc/arm-none-eabi/6.2.0/../../../../arm-none-eabi/bin/ld: /home/japaric/tmp/app/target/thumbv6m-none-eabi/debug/deps/app-6d9087d3fe6b93f5: hidden symbol `memcpy' isn't defined
/usr/lib/gcc/arm-none-eabi/6.2.0/../../../../arm-none-eabi/bin/ld: final link failed: Bad value
collect2: error: ld returned 1 exit status

Note the order of the linker arguments:

• app.0.o
• librlibc.rlib
• libcore.rlib
• libcompiler_builtins.rlib

And the relevant bits of running nm over these object files:

$ arm-none-eabi-nm -g **/app*.0.o
         U __aeabi_memcpy4

$ arm-none-eabi-nm -g **/librlibc*.rlib
00000000 W memcmp
00000000 W memcpy
00000000 W memmove
00000000 W memset

$ arm-none-eabi-nm -g ~/.xargo/lib/rustlib/thumbv6m-none-eabi/lib/libcore*.rlib
         U __aeabi_memclr4
         U __aeabi_memcpy
         U __aeabi_memcpy4
         U __aeabi_memset

$ arm-none-eabi-nm -g **/libcompiler_builtins*.rlib
         U memcpy
         U memmove
         U memset
00000000 T __aeabi_memclr
00000000 T __aeabi_memclr4
00000000 T __aeabi_memclr8
00000000 T __aeabi_memcpy
00000000 T __aeabi_memcpy4
00000000 T __aeabi_memcpy8
00000000 T __aeabi_memmove
00000000 T __aeabi_memmove4
00000000 T __aeabi_memmove8
00000000 T __aeabi_memset
00000000 T __aeabi_memset4
00000000 T __aeabi_memset8

For this to actually work, librlibc.rlib should be passed to the linker after libcompiler_builtins.rlib but that's not possible because compiler-builtins is marked as #![compiler_builtins] and will always be the last rlib to be passed to the linker.

Not quite sure how to solve this other than copy paste rlibc's mem* functions into compiler-builtins or special case rlibc in rustc ...

May be worth to mention that this is not a problem in no_std programs that link to libc and not use the "weak" Cargo feature as the linker arguments would look look like: libcore.rlib .. libcompiler_builtins.rlib -lc. In that case -lc would provide memcpy et al.

cc @alexcrichton