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/hir-def/src/macro_expansion_tests/proc_macros.rs

@@ -5,7 +5,7 @@
 //! in-memory macros.
 use expect_test::expect;
 
-use crate::macro_expansion_tests::check;
+use crate::macro_expansion_tests::{check, check_errors};
 
 #[test]
 fn attribute_macro_attr_censoring() {
@@ -216,3 +216,21 @@ struct S;
 #[doc = "doc attr"] struct S;"##]],
     );
 }
+
+#[test]
+fn cfg_evaluated_before_attr_macros() {
+    check_errors(
+        r#"
+//- proc_macros: disallow_cfg
+
+use proc_macros::disallow_cfg;
+
+#[disallow_cfg] #[cfg(false)] fn foo() {}
+// True cfg are kept.
+// #[disallow_cfg] #[cfg(true)] fn bar() {}
+#[disallow_cfg] #[cfg_attr(false, inline)] fn baz() {}
+#[disallow_cfg] #[cfg_attr(true, inline)] fn qux() {}
+    "#,
+        expect![[r#""#]],
+    );
+}

crates/hir-expand/src/cfg_process.rs

@@ -201,9 +201,6 @@ pub(crate) fn process_cfg_attrs(
         MacroDefKind::BuiltInAttr(_, expander) => expander.is_derive(),
         _ => false,
     };
-    if !is_derive {
-        return None;
-    }
     let mut remove = FxHashSet::default();
 
     let item = ast::Item::cast(node.clone())?;
@@ -220,13 +217,27 @@ pub(crate) fn process_cfg_attrs(
             }
         }
     }
+
+    if is_derive {
+        // Only derives get their code cfg-clean, normal attribute macros process only the cfg at their level
+        // (cfg_attr is handled above, cfg is handled in the def map).
         match item {
             ast::Item::Struct(it) => match it.field_list()? {
                 ast::FieldList::RecordFieldList(fields) => {
-                process_has_attrs_with_possible_comma(db, fields.fields(), loc.krate, &mut remove)?;
+                    process_has_attrs_with_possible_comma(
+                        db,
+                        fields.fields(),
+                        loc.krate,
+                        &mut remove,
+                    )?;
                 }
                 ast::FieldList::TupleFieldList(fields) => {
-                process_has_attrs_with_possible_comma(db, fields.fields(), loc.krate, &mut remove)?;
+                    process_has_attrs_with_possible_comma(
+                        db,
+                        fields.fields(),
+                        loc.krate,
+                        &mut remove,
+                    )?;
                 }
             },
             ast::Item::Enum(it) => {
@@ -243,6 +254,7 @@ pub(crate) fn process_cfg_attrs(
             // FIXME: Implement for other items if necessary. As we do not support #[cfg_eval] yet, we do not need to implement it for now
             _ => {}
         }
+    }
     Some(remove)
 }
 /// Parses a `cfg` attribute from the meta

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())
+    }
+}