cortex-m-rt Documentation: `-nostartfiles` linker option does not exist in `rust-lld`

[mfil]

The doc comments in cortex-m-rt/src/lib.rs say that you should use the following command to compile: cargo rustc --target thumbv7m-none-eabi -- -C link-arg=-nostartfiles -C link-arg=-Tlink.x. For me, this produces the following error:

$ cargo rustc --target thumbv7em-none-eabi -- -C link-arg=-nostartfiles -C link-arg=-Tlink.x 
   Compiling cortex-m-rt v0.7.5
   Compiling cortex_m_rt_test v0.1.0 (/home/max/test/cortex_m_rt_test)
error: linking with `rust-lld` failed: exit status: 1
  |
  = note:  "rust-lld" "-flavor" "gnu" "/tmp/rustcrMK7jK/symbols.o" "<3 object files omitted>" "--as-needed" "-Bstatic" "/home/max/test/cortex_m_rt_test/target/thumbv7em-none-eabi/debug/deps/libcortex_m_rt-eeb9b267930985b2.rlib" "<sysroot>/lib/rustlib/thumbv7em-none-eabi/lib/{libcore-*,libcompiler_builtins-*}.rlib" "-L" "/tmp/rustcrMK7jK/raw-dylibs" "-Bdynamic" "--eh-frame-hdr" "-z" "noexecstack" "-L" "/home/max/test/cortex_m_rt_test/target/thumbv7em-none-eabi/debug/build/cortex-m-rt-22da783ef07445d2/out" "-o" "/home/max/test/cortex_m_rt_test/target/thumbv7em-none-eabi/debug/deps/cortex_m_rt_test-39c54d44dafd8fd1" "--gc-sections" "-nostartfiles" "-Tlink.x"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: rust-lld: error: unknown argument '-nostartfiles'
          

error: could not compile `cortex_m_rt_test` (bin "cortex_m_rt_test") due to 1 previous error

It looks to me like -nostartfiles is a GCC-specific option. I can get it to work by adding -C linker=/usr/bin/arm-none-eabi-gcc to the command. But I'm wondering if that's necessary. When I leave out the -nostartfiles and use the default rust-lld linker, my simple test program still works.

[jonathanpallant]

That's because gcc is a compiler that can act as a linker driver and lld is a linker (like GNU ld is a linker).

The docs date from a time when you needed arm-none-eabi-gcc installed to act as a linker because we didn't ship lld - but gcc is a linker driver that will select some additional object code and then call the actual linker. That flag stops gcc selecting some extra object code.

We should update these docs.

[mfil]

Thanks for explaining! Should I make a pull request to remove the -nostartfiles or do you want to leave that for some larger doc update in the future?

[jonathanpallant]

Feel free to send a PR - I don't think a major update here is on anyone's roadmap.

Because cargo rustc is a bit unusual, I think I would swap:

- $ cargo rustc --target thumbv7m-none-eabi -- -C link-arg=-nostartfiles -C link-arg=-Tlink.x
+ $ RUSTFLAGS="-Clink-arg=-Tlink.x" cargo build --target thumbv7m-none-eabi