Make CI verify that proc_macro_span works in latest nightly

Author: dtolnay

.github/workflows/ci.yml

@@ -57,6 +57,9 @@ jobs:
           components: rust-src
       - name: Enable type layout randomization
         run: echo RUSTFLAGS=${RUSTFLAGS}\ -Zrandomize-layout >> $GITHUB_ENV
+      - run: cargo check
+        env:
+          RUSTFLAGS: --cfg procmacro2_nightly_testing ${{env.RUSTFLAGS}}
       - run: cargo test
       - run: cargo test --no-default-features
       - run: cargo test --no-default-features --test features -- --ignored make_sure_no_proc_macro # run the ignored test to make sure the `proc-macro` feature is disabled

src/lib.rs

@@ -122,6 +122,15 @@ compile_error! {"\
     build script as well.
 "}
 
+#[cfg(all(
+    procmacro2_nightly_testing,
+    feature = "proc-macro",
+    not(proc_macro_span)
+))]
+compile_error! {"\
+    Build script probe failed to compile.
+"}
+
 extern crate alloc;
 
 #[cfg(feature = "proc-macro")]