test-exercise: Detect Clippy failure by exit code, not by grep

The previous grep-based approach was too fragile:
On beta, Clippy isn't even installed.
Therefore, all attempts to run it simply show that Clippy wasn't
installed, without actually running Clippy.
This failure went completely undetected because the grep for `tests/`
isn't going to match that.

It's much less fragile to insert a `deny(clippy:all)` directive and let
the exit code speak.

As a side-effect, now Clippy *errors* for example solutions will cause
the CI to fail, whereas Clippy *warnings* in them will still be
accepted.
We can see this as a value-add, assuming that if Clippy defaults to
treating a particular lint as an error that it really should be fixed.
However this does require that some examples be fixed in this PR.

When merging this PR I plan to keep the commits separate, but will
remove the proof commit and its reversion before merging.

Author: petertseng

.github/workflows/tests.yml

@@ -138,17 +138,11 @@ jobs:
           toolchain: ${{ matrix.rust }}
           default: true
 
-      - name: Clippy tests
+      - name: Clippy
         env:
-          CLIPPY: tests/
+          CLIPPY: true
         run: ./_test/check-exercises.sh
 
-      # Our examples currently have too much Clippy output to consider this.
-      # But maybe in the future.
-      #- name: Clippy examples
-      #  env:
-      #    CLIPPY: src/lib.rs
-      #  run: ./_test/check-exercises.sh
   nightly-compilation:
     name: Check exercises on nightly (benchmark enabled)
     runs-on: ubuntu-latest

bin/test-exercise

@@ -83,10 +83,12 @@ done
 (
    cd $exercise
    if [ -n "$CLIPPY" ]; then
+     # Consider any Clippy to be an error in tests only.
+     # For now, not the example solutions since they have many Clippy warnings,
+     # and are not shown to students anyway.
+     sed -i -e '1i #![deny(clippy::all)]' tests/*.rs
      cargo clippy --all-targets --color always "$@" 2>clippy.log
-     # Only consider it a failure if output contains the string in CLIPPY.
-     # For example, if we're only looking in tests, CLIPPY contains tests/
-     if grep -q $CLIPPY clippy.log; then
+     if [ $? -ne 0 ]; then
        cat clippy.log
        exit 1
      else