1 files changed, 87 insertions, 0 deletions

diff --git a/crates/hir_expand/src/input.rs b/crates/hir_expand/src/input.rs
new file mode 100644
index 000000000..8f652cd7c
--- /dev/null
+++ b/crates/hir_expand/src/input.rs
@@ -0,0 +1,87 @@
+//! Macro input conditioning.
+
+use syntax::{
+    ast::{self, AttrsOwner},
+    AstNode, SyntaxNode,
+};
+
+use crate::{db::AstDatabase, name::AsName, AttrId, LazyMacroId, MacroCallKind, MacroCallLoc};
+
+pub fn process_macro_input(db: &dyn AstDatabase, node: SyntaxNode, id: LazyMacroId) -> SyntaxNode {
+    let loc: MacroCallLoc = db.lookup_intern_macro(id);
+
+    match loc.kind {
+        MacroCallKind::FnLike { .. } => node,
+        MacroCallKind::Derive { derive_attr, .. } => {
+            let item = match ast::Item::cast(node.clone()) {
+                Some(item) => item,
+                None => return node,
+            };
+
+            remove_derives_up_to(item, derive_attr).syntax().clone()
+        }
+    }
+}
+
+/// Removes `#[derive]` attributes from `item`, up to `attr`.
+fn remove_derives_up_to(item: ast::Item, attr: AttrId) -> ast::Item {
+    let item = item.clone_for_update();
+    let idx = attr.0 as usize;
+    for attr in item.attrs().take(idx + 1) {
+        if let Some(name) =
+            attr.path().and_then(|path| path.as_single_segment()).and_then(|seg| seg.name_ref())
+        {
+            if name.as_name().to_string() == "derive" {
+                attr.syntax().detach();
+            }
+        }
+    }
+    item
+}
+
+#[cfg(test)]
+mod tests {
+    use base_db::fixture::WithFixture;
+    use base_db::SourceDatabase;
+    use expect_test::{expect, Expect};
+
+    use crate::test_db::TestDB;
+
+    use super::*;
+
+    fn test_remove_derives_up_to(attr: AttrId, ra_fixture: &str, expect: Expect) {
+        let (db, file_id) = TestDB::with_single_file(&ra_fixture);
+        let parsed = db.parse(file_id);
+
+        let mut items: Vec<_> =
+            parsed.syntax_node().descendants().filter_map(ast::Item::cast).collect();
+        assert_eq!(items.len(), 1);
+
+        let item = remove_derives_up_to(items.pop().unwrap(), attr);
+        expect.assert_eq(&item.to_string());
+    }
+
+    #[test]
+    fn remove_derive() {
+        test_remove_derives_up_to(
+            AttrId(2),
+            r#"
+#[allow(unused)]
+#[derive(Copy)]
+#[derive(Hello)]
+#[derive(Clone)]
+struct A {
+    bar: u32
+}
+        "#,
+            expect![[r#"
+#[allow(unused)]
+
+
+#[derive(Clone)]
+struct A {
+    bar: u32
+}"#]],
+        );
+    }
+}