expand: Change how `#![cfg(FALSE)]` behaves on crate root

Previously it removed all other attributes from the crate root.
Now it removes only attributes below itself.

So it becomes possible to configure some global crate properties even for fully unconfigured crates.

2 files changed, 21 insertions, 7 deletions

diff --git a/compiler/rustc_expand/src/config.rs b/compiler/rustc_expand/src/config.rs
index 690f80f6876..bcfa5313bde 100644
--- a/compiler/rustc_expand/src/config.rs
+++ b/compiler/rustc_expand/src/config.rs
@@ -197,9 +197,11 @@ pub fn pre_configure_attrs(sess: &Session, attrs: &[Attribute]) -> ast::AttrVec
         config_tokens: false,
         lint_node_id: ast::CRATE_NODE_ID,
     };
-    let attrs: ast::AttrVec =
-        attrs.iter().flat_map(|attr| strip_unconfigured.process_cfg_attr(attr)).collect();
-    if strip_unconfigured.in_cfg(&attrs) { attrs } else { ast::AttrVec::new() }
+    attrs
+        .iter()
+        .flat_map(|attr| strip_unconfigured.process_cfg_attr(attr))
+        .take_while(|attr| !is_cfg(attr) || strip_unconfigured.cfg_true(attr).0)
+        .collect()
 }
 
 #[macro_export]
diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
index dd8863df195..9850723a857 100644
--- a/compiler/rustc_expand/src/expand.rs
+++ b/compiler/rustc_expand/src/expand.rs
@@ -1039,7 +1039,12 @@ trait InvocationCollectorNode: HasAttrs + HasNodeId + Sized {
     ) -> Result<Self::OutputTy, Self> {
         Ok(noop_flat_map(node, collector))
     }
-    fn expand_cfg_false(&mut self, collector: &mut InvocationCollector<'_, '_>, span: Span) {
+    fn expand_cfg_false(
+        &mut self,
+        collector: &mut InvocationCollector<'_, '_>,
+        _pos: usize,
+        span: Span,
+    ) {
         collector.cx.emit_err(RemoveNodeNotSupported { span, descr: Self::descr() });
     }
 
@@ -1409,8 +1414,15 @@ impl InvocationCollectorNode for ast::Crate {
     fn noop_visit<V: MutVisitor>(&mut self, visitor: &mut V) {
         noop_visit_crate(self, visitor)
     }
-    fn expand_cfg_false(&mut self, collector: &mut InvocationCollector<'_, '_>, _span: Span) {
-        self.attrs.clear();
+    fn expand_cfg_false(
+        &mut self,
+        collector: &mut InvocationCollector<'_, '_>,
+        pos: usize,
+        _span: Span,
+    ) {
+        // Attributes above `cfg(FALSE)` are left in place, because we may want to configure
+        // some global crate properties even on fully unconfigured crates.
+        self.attrs.truncate(pos);
         // Standard prelude imports are left in the crate for backward compatibility.
         self.items.truncate(collector.cx.num_standard_library_imports);
     }
@@ -1804,7 +1816,7 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
                             continue;
                         }
 
-                        node.expand_cfg_false(self, span);
+                        node.expand_cfg_false(self, pos, span);
                         continue;
                     }
                     sym::cfg_attr => {