tests: cargo clean before checking stubs compile

1.59.0-beta.6 (0426998f5 2022-02-02) runs into an internal compiler
error unless we do this.
(This has been reported to rustc in
rust-lang/rust#93131)

It may be a good idea to keep this `clean` in the long term anyway, so
as to ensure that nothing from the "Check exercises" step unduly affects
the "Ensure stubs compile" step.

Author: petertseng

.github/workflows/tests.yml

@@ -123,6 +123,9 @@ jobs:
         run: ./_test/check_exercises.sh
         continue-on-error: ${{ matrix.rust == 'beta' && matrix.deny_warnings == '1' }}
 
+      - name: Cargo clean (to prevent previous compilation from unintentionally interfering with later ones)
+        run: ./_test/cargo_clean_all.sh
+
       - name: Ensure stubs compile
         env:
           DENYWARNINGS: ${{ matrix.deny_warnings }}

_test/cargo_clean_all.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+status=0
+repo="$(cd "$(dirname "$0")/.." && pwd)"
+
+for ex in "$repo"/exercises/*/*/; do
+  name=$(grep '^name =' "$ex/Cargo.toml" | cut -d\" -f2)
+  if [ -z "$name" ]; then
+    echo "don't know name of $ex"
+    status=1
+    continue
+  fi
+  cargo clean --manifest-path "$ex/Cargo.toml" --package "$name"
+done
+
+exit $status