compiletest: add enable-by-default check-cfg

Author: Urgau

src/tools/compiletest/src/runtest.rs

@@ -1028,12 +1028,31 @@ impl<'test> TestCx<'test> {
     }
 
     fn set_revision_flags(&self, cmd: &mut Command) {
+        // Normalize revisions to be lowercase and replace `-`s with `_`s.
+        // Otherwise the `--cfg` flag is not valid.
+        let normalize_revision = |revision: &str| revision.to_lowercase().replace("-", "_");
+
         if let Some(revision) = self.revision {
-            // Normalize revisions to be lowercase and replace `-`s with `_`s.
-            // Otherwise the `--cfg` flag is not valid.
-            let normalized_revision = revision.to_lowercase().replace("-", "_");
+            let normalized_revision = normalize_revision(revision);
             cmd.args(&["--cfg", &normalized_revision]);
         }
+
+        if !self.props.no_auto_check_cfg {
+            let mut check_cfg = String::with_capacity(25);
+
+            // Generate `cfg(FALSE, REV1, ..., REVN)` (for all possible revisions)
+            //
+            // For compatibility reason we consider the `FALSE` cfg to be expected
+            // since it is extensively used in the testsuite.
+            check_cfg.push_str("cfg(FALSE");
+            for revision in &self.props.revisions {
+                check_cfg.push_str(",");
+                check_cfg.push_str(&normalize_revision(&revision));
+            }
+            check_cfg.push_str(")");
+
+            cmd.args(&["--check-cfg", &check_cfg]);
+        }
     }
 
     fn typecheck_source(&self, src: String) -> ProcRes {

tests/rustdoc-ui/argfile/commandline-argfile.rs

@@ -1,13 +1,13 @@
 // Check to see if we can get parameters from an @argsfile file
 //
 //@ check-pass
-//@ compile-flags: --cfg cmdline_set @{{src-base}}/argfile/commandline-argfile.args
+//@ compile-flags: --cfg cmdline_set --check-cfg=cfg(cmdline_set,unbroken)
+//@ compile-flags: @{{src-base}}/argfile/commandline-argfile.args
 
 #[cfg(not(cmdline_set))]
 compile_error!("cmdline_set not set");
 
 #[cfg(not(unbroken))]
 compile_error!("unbroken not set");
 
-fn main() {
-}
+fn main() {}