Censor cfg_attr for attribute macros

This is not documented (and I discovered that from experimenting and looking at the compiler's source code), but cfg_attrs *on the same level* as the attribute macro should be processed before it is expanded. cfg_attrs *below* should not (and this is contrary to what happens with derive macros, where both should be processed).

crates/test-fixture/src/lib.rs

@@ -17,7 +17,7 @@ use hir_expand::{
     tt::{Leaf, TokenTree, TopSubtree, TopSubtreeBuilder, TtElement, TtIter},
     FileRange,
 };
-use intern::Symbol;
+use intern::{sym, Symbol};
 use rustc_hash::FxHashMap;
 use span::{Edition, EditionedFileId, FileId, Span};
 use stdx::itertools::Itertools;
@@ -511,6 +511,21 @@ pub fn issue_18898(_attr: TokenStream, input: TokenStream) -> TokenStream {
                 disabled: false,
             },
         ),
+        (
+            r#"
+#[proc_macro_attribute]
+pub fn disallow_cfg(_attr: TokenStream, input: TokenStream) -> TokenStream {
+    input
+}
+"#
+            .into(),
+            ProcMacro {
+                name: Symbol::intern("disallow_cfg"),
+                kind: ProcMacroKind::Attr,
+                expander: sync::Arc::new(DisallowCfgProcMacroExpander),
+                disabled: false,
+            },
+        ),
     ])
 }
 
@@ -865,3 +880,30 @@ impl ProcMacroExpander for Issue18898ProcMacroExpander {
         })
     }
 }
+
+// Reads ident type within string quotes, for issue #17479.
+#[derive(Debug)]
+struct DisallowCfgProcMacroExpander;
+impl ProcMacroExpander for DisallowCfgProcMacroExpander {
+    fn expand(
+        &self,
+        subtree: &TopSubtree,
+        _: Option<&TopSubtree>,
+        _: &Env,
+        _: Span,
+        _: Span,
+        _: Span,
+        _: Option<String>,
+    ) -> Result<TopSubtree, ProcMacroExpansionError> {
+        for tt in subtree.token_trees().flat_tokens() {
+            if let tt::TokenTree::Leaf(tt::Leaf::Ident(ident)) = tt {
+                if ident.sym == sym::cfg || ident.sym == sym::cfg_attr {
+                    return Err(ProcMacroExpansionError::Panic(
+                        "cfg or cfg_attr found in DisallowCfgProcMacroExpander".to_owned(),
+                    ));
+                }
+            }
+        }
+        Ok(subtree.clone())
+    }
+}