rustup errors with a confusing message if run from a non-existing path

[lu-zero]

To reproduce:

# shell a
mkdir /tmp/testme
cd $_
# shell b
rm -fr /tmp/testme
# shell a
rustup
error: couldn't find value of CARGO_HOME
info: caused by: No such file or directory (os error 2)

from the strace I see that the getcwd failure is not managed.

[lu-zero]

mmap(NULL, 4190208, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f70d2501000
munmap(0x7f70d2501000, 1044480)         = 0
munmap(0x7f70d2800000, 1048576)         = 0
open("/sys/devices/system/cpu/online", O_RDONLY|O_CLOEXEC) = 3
read(3, "0-7\n", 8192)                  = 4
close(3)                                = 0
rt_sigaction(SIGPIPE, {SIG_IGN, [PIPE], SA_RESTORER|SA_RESTART, 0x7f70d2e3e3c0}, {SIG_DFL, [], 0}, 8) = 0
mmap(NULL, 2097152, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f70d2400000
open("/proc/self/maps", O_RDONLY|O_CLOEXEC) = 3
getrlimit(RLIMIT_STACK, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0
fstat(3, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
read(3, "557b49762000-557b49d5c000 r-xp 0"..., 1024) = 1024
read(3, "64/libm-2.23.so\n7f70d2e0b000-7f7"..., 1024) = 1024
read(3, "ib64/libpthread-2.23.so\n7f70d33d"..., 1024) = 1024
read(3, "2.8\n7f70d39f0000-7f70d39f1000 rw"..., 1024) = 843
close(3)                                = 0
sched_getaffinity(9765, 32, [0 1 2 3 4 5 6 7]) = 8
mmap(0x7ffe644db000, 4096, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7ffe644db000
rt_sigaction(SIGSEGV, {0x557b49bd1d00, [], SA_RESTORER|SA_STACK|SA_SIGINFO, 0x7f70d33cbf70}, NULL, 8) = 0
rt_sigaction(SIGBUS, {0x557b49bd1d00, [], SA_RESTORER|SA_STACK|SA_SIGINFO, 0x7f70d33cbf70}, NULL, 8) = 0
sigaltstack(NULL, {ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=0}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f70d3c11000
sigaltstack({ss_sp=0x7f70d3c11000, ss_flags=0, ss_size=8192}, NULL) = 0
getcwd(0x7f70d242a000, 512)             = -1 ENOENT (No such file or directory)
getcwd(0x7f70d242a000, 512)             = -1 ENOENT (No such file or directory)

Some strace.

[lu-zero]

The problem is in src/rustup-utils/src/utils.rs in cargo_home()

    let cwd = try!(env::current_dir().chain_err(|| ErrorKind::CargoHome));
    let cargo_home = env_var.clone().map(|home| {
        cwd.join(home)
    });

It should not try to get cwd if cargo_home isn't set and even in that case not error out with that message.