Use the attribute placeholder if the cfg_attr couldn't be parsed

compiler/rustc_expand/src/config.rs

@@ -269,6 +269,27 @@ impl<'a> StripUnconfigured<'a> {
         }
     }
 
+    /// `cfg` and `cfg_attr` gets replaced with an inert `rustc_cfg_placeholder` to keep the
+    /// attribute "spot" in the AST. This allows suggestions to remove an item to provide a
+    /// correct suggestion when `#[cfg_attr]`s are present.
+    fn mk_placeholder(&self, cfg_attr: &ast::Attribute) -> ast::Attribute {
+        let item = ast::AttrItem {
+            unsafety: ast::Safety::Default,
+            path: ast::Path::from_ident(rustc_span::symbol::Ident::with_dummy_span(
+                sym::rustc_cfg_placeholder,
+            )),
+            args: ast::AttrArgs::Empty,
+            tokens: None,
+        };
+        let kind = ast::AttrKind::Normal(P(ast::NormalAttr { item, tokens: None }));
+        ast::Attribute {
+            kind,
+            id: self.sess.psess.attr_id_generator.mk_attr_id(),
+            style: cfg_attr.style,
+            span: cfg_attr.span,
+        }
+    }
+
     /// Parse and expand a single `cfg_attr` attribute into a list of attributes
     /// when the configuration predicate is true, or otherwise expand into an
     /// empty list of attributes.
@@ -282,7 +303,7 @@ impl<'a> StripUnconfigured<'a> {
         let Some((cfg_predicate, expanded_attrs)) =
             rustc_parse::parse_cfg_attr(cfg_attr, &self.sess.psess)
         else {
-            return vec![];
+            return vec![self.mk_placeholder(cfg_attr)];
         };
 
         // Lint on zero attributes in source.
@@ -296,25 +317,7 @@ impl<'a> StripUnconfigured<'a> {
         }
 
         if !attr::cfg_matches(&cfg_predicate, &self.sess, self.lint_node_id, self.features) {
-            // `cfg` and `cfg_attr` gets replaced with an inert `rustc_cfg_placeholder` to keep the
-            // attribute "spot" in the AST. This allows suggestions to remove an item to provide a
-            // correct suggestion when `#[cfg_attr]`s are present.
-            let item = ast::AttrItem {
-                unsafety: ast::Safety::Default,
-                path: ast::Path::from_ident(rustc_span::symbol::Ident::with_dummy_span(
-                    sym::rustc_cfg_placeholder,
-                )),
-                args: ast::AttrArgs::Empty,
-                tokens: None,
-            };
-            let kind = ast::AttrKind::Normal(P(ast::NormalAttr { item, tokens: None }));
-            let attr: ast::Attribute = ast::Attribute {
-                kind,
-                id: self.sess.psess.attr_id_generator.mk_attr_id(),
-                style: ast::AttrStyle::Outer,
-                span: cfg_attr.span,
-            };
-            return vec![attr];
+            return vec![self.mk_placeholder(cfg_attr)];
         }
 
         if recursive {

tests/ui/removing-extern-crate-malformed-cfg.fixed

@@ -0,0 +1,15 @@
+//@ edition:2018
+//@ aux-build:removing-extern-crate.rs
+//@ run-rustfix
+
+#![warn(rust_2018_idioms)]
+
+ //~ WARNING unused `extern crate`
+ //~ WARNING unused `extern crate`
+
+mod another {
+     //~ WARNING unused `extern crate`
+     //~ WARNING unused `extern crate`
+}
+
+fn main() {}

tests/ui/removing-extern-crate-malformed-cfg.rs

@@ -0,0 +1,17 @@
+//@ edition:2018
+//@ aux-build:removing-extern-crate.rs
+//@ run-rustfix
+
+#![warn(rust_2018_idioms)]
+
+#[cfg_attr(test, "macro_use")] //~ ERROR expected
+extern crate removing_extern_crate as foo; //~ WARNING unused `extern crate`
+extern crate core; //~ WARNING unused `extern crate`
+
+mod another {
+    #[cfg_attr(test)] //~ ERROR expected
+    extern crate removing_extern_crate as foo; //~ WARNING unused `extern crate`
+    extern crate core; //~ WARNING unused `extern crate`
+}
+
+fn main() {}

tests/ui/removing-extern-crate-malformed-cfg.stderr

@@ -0,0 +1,76 @@
+error: expected identifier, found `"macro_use"`
+  --> $DIR/removing-extern-crate-malformed-cfg.rs:7:18
+   |
+LL | #[cfg_attr(test, "macro_use")]
+   |                  ^^^^^^^^^^^ expected identifier
+   |
+   = help: the valid syntax is `#[cfg_attr(condition, attribute, other_attribute, ...)]`
+   = note: for more information, visit <https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg_attr-attribute>
+
+error: expected one of `(`, `,`, `::`, or `=`, found `<eof>`
+  --> $DIR/removing-extern-crate-malformed-cfg.rs:12:16
+   |
+LL |     #[cfg_attr(test)]
+   |                ^^^^ expected one of `(`, `,`, `::`, or `=`
+   |
+   = help: the valid syntax is `#[cfg_attr(condition, attribute, other_attribute, ...)]`
+   = note: for more information, visit <https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg_attr-attribute>
+
+warning: unused `extern crate`
+  --> $DIR/removing-extern-crate-malformed-cfg.rs:8:1
+   |
+LL | extern crate removing_extern_crate as foo;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
+   |
+note: the lint level is defined here
+  --> $DIR/removing-extern-crate-malformed-cfg.rs:5:9
+   |
+LL | #![warn(rust_2018_idioms)]
+   |         ^^^^^^^^^^^^^^^^
+   = note: `#[warn(unused_extern_crates)]` implied by `#[warn(rust_2018_idioms)]`
+help: remove the unused `extern crate`
+   |
+LL - #[cfg_attr(test, "macro_use")]
+LL - extern crate removing_extern_crate as foo;
+LL +
+   |
+
+warning: unused `extern crate`
+  --> $DIR/removing-extern-crate-malformed-cfg.rs:9:1
+   |
+LL | extern crate core;
+   | ^^^^^^^^^^^^^^^^^^ unused
+   |
+help: remove the unused `extern crate`
+   |
+LL - extern crate core;
+LL +
+   |
+
+warning: unused `extern crate`
+  --> $DIR/removing-extern-crate-malformed-cfg.rs:13:5
+   |
+LL |     extern crate removing_extern_crate as foo;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
+   |
+help: remove the unused `extern crate`
+   |
+LL -     #[cfg_attr(test)]
+LL -     extern crate removing_extern_crate as foo;
+LL +
+   |
+
+warning: unused `extern crate`
+  --> $DIR/removing-extern-crate-malformed-cfg.rs:14:5
+   |
+LL |     extern crate core;
+   |     ^^^^^^^^^^^^^^^^^^ unused
+   |
+help: remove the unused `extern crate`
+   |
+LL -     extern crate core;
+LL +
+   |
+
+error: aborting due to 2 previous errors; 4 warnings emitted
+