4 files changed, 54 insertions, 7 deletions

diff --git a/crates/hir_expand/src/db.rs b/crates/hir_expand/src/db.rs
index e8f4af309..3ebe194e4 100644
--- a/crates/hir_expand/src/db.rs
+++ b/crates/hir_expand/src/db.rs
@@ -13,8 +13,8 @@ use syntax::{
 
 use crate::{
     ast_id_map::AstIdMap, hygiene::HygieneFrame, input::process_macro_input, BuiltinDeriveExpander,
-    BuiltinFnLikeExpander, HirFileId, HirFileIdRepr, MacroCallId, MacroCallLoc, MacroDefId,
-    MacroDefKind, MacroFile, ProcMacroExpander,
+    BuiltinFnLikeExpander, HirFileId, HirFileIdRepr, MacroCallId, MacroCallKind, MacroCallLoc,
+    MacroDefId, MacroDefKind, MacroFile, ProcMacroExpander,
 };
 
 /// Total limit on the number of tokens produced by any macro invocation.
@@ -377,7 +377,12 @@ fn expand_proc_macro(
         _ => unreachable!(),
     };
 
-    expander.expand(db, loc.krate, &macro_arg.0)
+    let attr_arg = match &loc.kind {
+        MacroCallKind::Attr { attr_args, .. } => Some(attr_args),
+        _ => None,
+    };
+
+    expander.expand(db, loc.krate, &macro_arg.0, attr_arg)
 }
 
 fn is_self_replicating(from: &SyntaxNode, to: &SyntaxNode) -> bool {
diff --git a/crates/hir_expand/src/input.rs b/crates/hir_expand/src/input.rs
index fe4790e7b..40116a479 100644
--- a/crates/hir_expand/src/input.rs
+++ b/crates/hir_expand/src/input.rs
@@ -28,6 +28,14 @@ pub(crate) fn process_macro_input(
 
             remove_derives_up_to(item, derive_attr_index as usize).syntax().clone()
         }
+        MacroCallKind::Attr { invoc_attr_index, .. } => {
+            let item = match ast::Item::cast(node.clone()) {
+                Some(item) => item,
+                None => return node,
+            };
+
+            remove_attr_invoc(item, invoc_attr_index as usize).syntax().clone()
+        }
     }
 }
 
@@ -46,6 +54,17 @@ fn remove_derives_up_to(item: ast::Item, attr_index: usize) -> ast::Item {
     item
 }
 
+/// Removes the attribute invoking an attribute macro from `item`.
+fn remove_attr_invoc(item: ast::Item, attr_index: usize) -> ast::Item {
+    let item = item.clone_for_update();
+    let attr = item
+        .attrs()
+        .nth(attr_index)
+        .unwrap_or_else(|| panic!("cannot find attribute #{}", attr_index));
+    attr.syntax().detach();
+    item
+}
+
 #[cfg(test)]
 mod tests {
     use base_db::fixture::WithFixture;
diff --git a/crates/hir_expand/src/lib.rs b/crates/hir_expand/src/lib.rs
index 90d8ae240..618f26b95 100644
--- a/crates/hir_expand/src/lib.rs
+++ b/crates/hir_expand/src/lib.rs
@@ -258,14 +258,29 @@ pub enum MacroCallKind {
         /// out-of-line modules, which may have attributes spread across 2 files!
         derive_attr_index: u32,
     },
+    Attr {
+        ast_id: AstId<ast::Item>,
+        attr_name: String,
+        attr_args: tt::Subtree,
+        /// Syntactical index of the invoking `#[attribute]`.
+        ///
+        /// Outer attributes are counted first, then inner attributes. This does not support
+        /// out-of-line modules, which may have attributes spread across 2 files!
+        invoc_attr_index: u32,
+    },
 }
 
+// FIXME: attribute indices do not account for `cfg_attr`, which means that we'll strip the whole
+// `cfg_attr` instead of just one of the attributes it expands to
+
 impl MacroCallKind {
     /// Returns the file containing the macro invocation.
     fn file_id(&self) -> HirFileId {
         match self {
             MacroCallKind::FnLike { ast_id, .. } => ast_id.file_id,
-            MacroCallKind::Derive { ast_id, .. } => ast_id.file_id,
+            MacroCallKind::Derive { ast_id, .. } | MacroCallKind::Attr { ast_id, .. } => {
+                ast_id.file_id
+            }
         }
     }
 
@@ -274,7 +289,7 @@ impl MacroCallKind {
             MacroCallKind::FnLike { ast_id, .. } => {
                 ast_id.with_value(ast_id.to_node(db).syntax().clone())
             }
-            MacroCallKind::Derive { ast_id, .. } => {
+            MacroCallKind::Derive { ast_id, .. } | MacroCallKind::Attr { ast_id, .. } => {
                 ast_id.with_value(ast_id.to_node(db).syntax().clone())
             }
         }
@@ -285,7 +300,9 @@ impl MacroCallKind {
             MacroCallKind::FnLike { ast_id, .. } => {
                 Some(ast_id.to_node(db).token_tree()?.syntax().clone())
             }
-            MacroCallKind::Derive { ast_id, .. } => Some(ast_id.to_node(db).syntax().clone()),
+            MacroCallKind::Derive { ast_id, .. } | MacroCallKind::Attr { ast_id, .. } => {
+                Some(ast_id.to_node(db).syntax().clone())
+            }
         }
     }
 
@@ -293,6 +310,7 @@ impl MacroCallKind {
         match self {
             MacroCallKind::FnLike { fragment, .. } => *fragment,
             MacroCallKind::Derive { .. } => FragmentKind::Items,
+            MacroCallKind::Attr { .. } => FragmentKind::Items, // is this always correct?
         }
     }
 }
diff --git a/crates/hir_expand/src/proc_macro.rs b/crates/hir_expand/src/proc_macro.rs
index d5643393a..dbe1b446e 100644
--- a/crates/hir_expand/src/proc_macro.rs
+++ b/crates/hir_expand/src/proc_macro.rs
@@ -28,11 +28,16 @@ impl ProcMacroExpander {
         Self { krate, proc_macro_id: None }
     }
 
+    pub fn is_dummy(&self) -> bool {
+        self.proc_macro_id.is_none()
+    }
+
     pub fn expand(
         self,
         db: &dyn AstDatabase,
         calling_crate: CrateId,
         tt: &tt::Subtree,
+        attr_arg: Option<&tt::Subtree>,
     ) -> Result<tt::Subtree, mbe::ExpandError> {
         match self.proc_macro_id {
             Some(id) => {
@@ -46,7 +51,7 @@ impl ProcMacroExpander {
                 // Proc macros have access to the environment variables of the invoking crate.
                 let env = &krate_graph[calling_crate].env;
 
-                proc_macro.expander.expand(&tt, None, &env).map_err(mbe::ExpandError::from)
+                proc_macro.expander.expand(&tt, attr_arg, &env).map_err(mbe::ExpandError::from)
             }
             None => Err(mbe::ExpandError::UnresolvedProcMacro),
         }