RUSTC_WRAPPER must be an absolute path

admin/coverage

@@ -3,6 +3,7 @@
 import os
 import glob
 import subprocess
+from os import path
 
 LLVM_PATH = glob.glob('/usr/lib/llvm-3.8/lib/clang/3.8.[0-9]/lib/linux/')[0]
 COVERAGE_OPTIONS = '-Ccodegen-units=1 -Clink-dead-code -Cpasses=insert-gcov-profiling -Zno-landing-pads -L%s -lclang_rt.profile-x86_64' % LLVM_PATH
@@ -25,7 +26,7 @@ def run(which):
 def rustc(*args):
     exe = ['cargo', 'rustc', '--all-features'] + list(args)
     env = dict(os.environ)
-    env.update(RUSTC_WRAPPER = './admin/coverage-rustc',
+    env.update(RUSTC_WRAPPER = path.abspath('./admin/coverage-rustc'),
                COVERAGE_OPTIONS = COVERAGE_OPTIONS)
     subprocess.check_call(exe, env = env)