Stop forwarding -L "" to compiler drivers; reject or ignore instead

[rprichard]

The current Rust build system passes -L "" to rustc when LLVM_STDCPP_LOCATION_<XXX> is blank (which seems to be the default situation). -L "" does not appear to do anything useful. rustc forwards -L "" to the compiler driver correctly, but both the gcc and clang compiler drivers forward it to ld in a way that breaks the subsequent ld command-line option. (They convert -L "" to just -L.)

Perhaps the rustc driver should reject -L "" instead. It already rejects -l "".

Alternatively, it could quietly ignore -L "". Another alternative is to fix gcc and clang.

I discovered this issue while investigating a FreeBSD build failure (#23287).

[rprichard]

Copying-and-pasting the example I posted in the FreeBSD issue:

mkdir subdir
echo "void dummy(void){}" > subdir/dummy.c
gcc -shared subdir/dummy.c -o subdir/libdummy.so
echo  >main.rs '#[link(name = "dummy", kind = "dylib")]'
echo >>main.rs 'extern { fn dummy(); }'
echo >>main.rs 'fn main() { unsafe { dummy(); } }'
rustc main.rs -L "" -Lsubdir

Produces:

error: linking with `cc` failed: exit code: 1
note: "cc" "-Wl,--as-needed" "-m64" "-L" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-o" "./main" "./main.o" "-Wl,--whole-archive" "-lmorestack" "-Wl,--no-whole-archive" "-Wl,--gc-sections" "-pie" "-nodefaultlibs" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcollections-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunicode-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/librand-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-4e7c5e5c.rlib" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-4e7c5e5c.rlib" "-L" "" "-L" "subdir" "-L" "/usr/local/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-L" "/home/rprichard/mess/dyncrate/.rust/lib/x86_64-unknown-linux-gnu" "-L" "/home/rprichard/mess/dyncrate/lib/x86_64-unknown-linux-gnu" "-Wl,--whole-archive" "-Wl,-Bstatic" "-Wl,--no-whole-archive" "-Wl,-Bdynamic" "-ldummy" "-ldl" "-lpthread" "-lrt" "-lgcc_s" "-lpthread" "-lc" "-lm" "-lcompiler-rt"
note: /usr/bin/ld: cannot find -ldummy
collect2: error: ld returned 1 exit status

Remove the -L "", or move it to the end of the command-line, and it works. ld is seeing -L -Lsubdir, so it adds the non-existent -Lsubdir directory to the search path, and it doesn't search inside subdir.