Fix symbols::tests::test_symbols

    ---- symbols::tests::test_symbols stdout ----
    thread 'symbols::tests::test_symbols' panicked at library/proc_macro/src/bridge/client.rs:311:17:
    procedural macro API is used outside of a procedural macro

compiler/rustc_macros/src/symbols.rs

@@ -257,6 +257,14 @@ fn symbols_with_errors(input: TokenStream) -> (TokenStream, Vec<syn::Error>) {
             Value::SameAsName | Value::String(_) | Value::Unsupported(_) => continue,
         };
 
+        if !proc_macro::is_available() {
+            errors.error(
+                Span::call_site(),
+                "proc_macro::tracked_env is not available in unit test".to_owned(),
+            );
+            break;
+        }
+
         let value = match proc_macro::tracked_env::var(env_var.value()) {
             Ok(value) => value,
             Err(err) => {

compiler/rustc_macros/src/symbols/tests.rs

@@ -27,7 +27,7 @@ fn test_symbols() {
 
     let body_tokens = m.mac.tokens.clone();
 
-    test_symbols_macro(body_tokens, &[]);
+    test_symbols_macro(body_tokens, &["proc_macro::tracked_env is not available in unit test"]);
 }
 
 fn test_symbols_macro(input: TokenStream, expected_errors: &[&str]) {