Expose 'trust_ci' as a feature flag under CI

$CI was chosen instead of $TRAVIS and the ! -z check was done such that
this can work on all CI platforms (at least GitLab CI, Travis CI, AppVeyor,
and Circle CI).

Author: Susurrus

README.md

@@ -46,6 +46,14 @@ This is an overview of what must / can be changed:
 
 - the "package phase". Tweak what goes into the release tarball / zipfile.
 
+### Managing test failures
+
+Some tests will fail when run in the `qemu` emulated environment even though
+they succeed on the actual hardware. In this case the `trust_ci` feature flag
+has been exposed to the build, test, and run stages. You can therefore disable
+some tests by putting `#[cfg_attr(trust_ci, ignore)]` before them. These tests
+will show up as 'ignored' in the test output.
+
 ### Generate binary releases
 
 You only need to push an **annotated** tag to kick off the build process.

ci/script.sh

@@ -4,6 +4,11 @@ set -ex
 
 # TODO This is the "test phase", tweak it as you see fit
 main() {
+    # Add a cfg spec to allow disabling specific tests under CI.
+    if [ ! -z $CI ]; then
+        export RUSTFLAGS=--cfg=trust_ci
+    fi
+
     cross build --target $TARGET
     cross build --target $TARGET --release
 

src/lib.rs

@@ -20,4 +20,10 @@ pub fn hello() {
 mod tests {
     #[test]
     fn it_works() {}
+
+    #[test]
+    #[cfg_attr(trust_ci, ignore)]
+    fn ignore_in_ci() {
+        assert!(false);
+    }
 }