‘rustc’ fails with verbose output when target executable exists as a directory

[Icelandjack]

% cat foo.rs
fn main() { println!("Hello, world!\n"); }
% mkdir foo
% rustc foo.rs 
error: linking with `cc` failed: exit code: 1
note: cc '-m32' '-L' '/usr/local/lib/rustlib/i686-unknown-linux-gnu/lib' '-o' 'foo' 'foo.o' '-Wl,--whole-archive' '-lmorestack' '-Wl,--no-w\
hole-archive' '-nodefaultlibs' '-Wl,--gc-sections' '-Wl,--as-needed' '/usr/local/lib/rustlib/i686-unknown-linux-gnu/lib/libnative-1fb5e2c0-\
0.11.0-pre.rlib' '/usr/local/lib/rustlib/i686-unknown-linux-gnu/lib/libstd-59beb4f7-0.11.0-pre.rlib' '/usr/local/lib/rustlib/i686-unknown-l\
inux-gnu/lib/libsync-305341d2-0.11.0-pre.rlib' '/usr/local/lib/rustlib/i686-unknown-linux-gnu/lib/librustrt-d8560cb2-0.11.0-pre.rlib' '/usr\
/local/lib/rustlib/i686-unknown-linux-gnu/lib/librand-2ea8f361-0.11.0-pre.rlib' '/usr/local/lib/rustlib/i686-unknown-linux-gnu/lib/libcolle\
ctions-d412c0c4-0.11.0-pre.rlib' '/usr/local/lib/rustlib/i686-unknown-linux-gnu/lib/liballoc-1085c790-0.11.0-pre.rlib' '/usr/local/lib/rust\
lib/i686-unknown-linux-gnu/lib/libcore-c5ed6fb4-0.11.0-pre.rlib' '/usr/local/lib/rustlib/i686-unknown-linux-gnu/lib/liblibc-4f9a876d-0.11.0\
-pre.rlib' '-L' '/tmp/rusterr/.rust' '-L' '/tmp/rusterr' '-Wl,-Bdynamic' '-ldl' '-lpthread' '-lgcc_s' '-lpthread' '-lc' '-lm' '-Wl,-rpath,$\
ORIGIN/../../usr/local/lib/rustlib/i686-unknown-linux-gnu/lib' '-Wl,-rpath,/usr/local/lib/rustlib/i686-unknown-linux-gnu/lib' '-lcompiler-r\
t'
note: /usr/bin/ld: cannot open output file foo: Is a directory
collect2: ld returned 1 exit status

error: aborting due to previous error

[huonw]

Dupe of #13098. (Thanks for filing!)