Don't allow test revisions that conflict with built in cfgs

src/tools/compiletest/src/common.rs

@@ -376,6 +376,7 @@ pub struct Config {
     pub only_modified: bool,
 
     pub target_cfgs: OnceLock<TargetCfgs>,
+    pub builtin_cfg_names: OnceLock<HashSet<String>>,
 
     pub nocapture: bool,
 
@@ -440,6 +441,11 @@ impl Config {
         self.target_cfg().panic == PanicStrategy::Unwind
     }
 
+    /// Get the list of builtin, 'well known' cfg names
+    pub fn builtin_cfg_names(&self) -> &HashSet<String> {
+        self.builtin_cfg_names.get_or_init(|| builtin_cfg_names(self))
+    }
+
     pub fn has_threads(&self) -> bool {
         // Wasm targets don't have threads unless `-threads` is in the target
         // name, such as `wasm32-wasip1-threads`.
@@ -651,6 +657,18 @@ pub enum Endian {
     Big,
 }
 
+fn builtin_cfg_names(config: &Config) -> HashSet<String> {
+    rustc_output(
+        config,
+        &["--print=check-cfg", "-Zunstable-options", "--check-cfg=cfg()"],
+        Default::default(),
+    )
+    .lines()
+    .map(|l| if let Some((name, _)) = l.split_once('=') { name.to_string() } else { l.to_string() })
+    .chain(std::iter::once(String::from("test")))
+    .collect()
+}
+
 fn rustc_output(config: &Config, args: &[&str], envs: HashMap<String, String>) -> String {
     let mut command = Command::new(&config.rustc_path);
     add_dylib_path(&mut command, iter::once(&config.compile_lib_path));

src/tools/compiletest/src/lib.rs

@@ -356,6 +356,7 @@ pub fn parse_config(args: Vec<String>) -> Config {
         force_rerun: matches.opt_present("force-rerun"),
 
         target_cfgs: OnceLock::new(),
+        builtin_cfg_names: OnceLock::new(),
 
         nocapture: matches.opt_present("nocapture"),
 

src/tools/compiletest/src/runtest.rs

@@ -478,6 +478,9 @@ impl<'test> TestCx<'test> {
                     "error: redundant cfg argument `{normalized_revision}` is already created by the revision"
                 );
             }
+            if self.config.builtin_cfg_names().contains(&normalized_revision) {
+                panic!("error: revision `{normalized_revision}` collides with a builtin cfg");
+            }
             cmd.args(cfg_arg);
         }
 

tests/ui/lint/lint-missing-doc-expect.rs

@@ -1,10 +1,10 @@
 // Make sure that `#[expect(missing_docs)]` is always correctly fulfilled.
 
 //@ check-pass
-//@ revisions: lib bin test
+//@ revisions: lib bin test_
 //@ [lib]compile-flags: --crate-type lib
 //@ [bin]compile-flags: --crate-type bin
-//@ [test]compile-flags: --test
+//@ [test_]compile-flags: --test
 
 #[expect(missing_docs)]
 pub fn foo() {}