Fix bug in validate_cast_and_convert_metadata

TODO: Message body

.github/workflows/ci.yml

@@ -149,8 +149,17 @@ jobs:
         sudo apt-get install gcc-multilib
       if: contains(matrix.target, 'i686')
 
+      # On nightly, we always pass `--features
+      # __internal_use_only_use_nightly_features_in_tests`. This allows tests to
+      # enable nightly-only functionality, and has no effect on non-test code.
     - name: Run tests
-      run: ./cargo.sh +${{ matrix.toolchain }} test --package ${{ matrix.crate }} --target ${{ matrix.target }} ${{ matrix.features }} --verbose
+      run: |
+        ./cargo.sh +${{ matrix.toolchain }} test \
+          --package ${{ matrix.crate }} \
+          --target ${{ matrix.target }} \
+          ${{ matrix.features }} \
+          ${{ matrix.toolchain == 'nightly' && '' || '--features __internal_use_only_use_nightly_features_in_tests'  }} \
+          --verbose
       # Only run tests when targetting x86 (32- or 64-bit) - we're executing on
       # x86_64, so we can't run tests for any non-x86 target.
       #

Cargo.toml

@@ -39,6 +39,10 @@ simd-nightly = ["simd"]
 # We make no stability guarantees about this feature; it may be modified or
 # removed at any time.
 __internal_use_only_features_that_work_on_stable = ["alloc", "derive", "simd"]
+# Instructs tests that they are running using a nightly compiler, and should
+# make use of nightly features to enable more functionality. This is meant to be
+# invoked by CI.
+__internal_use_only_use_nightly_features_in_tests = []
 
 [dependencies]
 zerocopy-derive = { version = "=0.7.3", path = "zerocopy-derive", optional = true }