7 files changed, 159 insertions, 14 deletions

diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index d3ef29db4..b43d61d0e 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -534,6 +534,18 @@ impl Module {
                                 Some(derive_name.clone()),
                             )
                         }
+                        MacroCallKind::Attr { ast_id, invoc_attr_index, attr_name, .. } => {
+                            let node = ast_id.to_node(db.upcast());
+                            let attr =
+                                node.attrs().nth((*invoc_attr_index) as usize).unwrap_or_else(
+                                    || panic!("cannot find attribute #{}", invoc_attr_index),
+                                );
+                            (
+                                ast_id.file_id,
+                                SyntaxNodePtr::from(AstPtr::new(&attr)),
+                                Some(attr_name.clone()),
+                            )
+                        }
                     };
                     sink.push(UnresolvedProcMacro {
                         file,
@@ -558,7 +570,9 @@ impl Module {
                             let node = ast_id.to_node(db.upcast());
                             (ast_id.file_id, SyntaxNodePtr::from(AstPtr::new(&node)))
                         }
-                        MacroCallKind::Derive { ast_id, .. } => {
+                        MacroCallKind::Derive { ast_id, .. }
+                        | MacroCallKind::Attr { ast_id, .. } => {
+                            // FIXME: point to the attribute instead, this creates very large diagnostics
                             let node = ast_id.to_node(db.upcast());
                             (ast_id.file_id, SyntaxNodePtr::from(AstPtr::new(&node)))
                         }
diff --git a/crates/hir_def/src/lib.rs b/crates/hir_def/src/lib.rs
index 9aa95720a..987485acc 100644
--- a/crates/hir_def/src/lib.rs
+++ b/crates/hir_def/src/lib.rs
@@ -55,6 +55,7 @@ use std::{
     sync::Arc,
 };
 
+use attr::Attr;
 use base_db::{impl_intern_key, salsa, CrateId};
 use hir_expand::{
     ast_id_map::FileAstId,
@@ -768,3 +769,42 @@ fn derive_macro_as_call_id(
         .into();
     Ok(res)
 }
+
+fn attr_macro_as_call_id(
+    item_attr: &AstIdWithPath<ast::Item>,
+    macro_attr: &Attr,
+    db: &dyn db::DefDatabase,
+    krate: CrateId,
+    resolver: impl Fn(path::ModPath) -> Option<MacroDefId>,
+) -> Result<MacroCallId, UnresolvedMacro> {
+    let def: MacroDefId = resolver(item_attr.path.clone())
+        .ok_or_else(|| UnresolvedMacro { path: item_attr.path.clone() })?;
+    let last_segment = item_attr
+        .path
+        .segments()
+        .last()
+        .ok_or_else(|| UnresolvedMacro { path: item_attr.path.clone() })?;
+    let mut arg = match &macro_attr.input {
+        Some(input) => match &**input {
+            attr::AttrInput::Literal(_) => tt::Subtree::default(),
+            attr::AttrInput::TokenTree(tt) => tt.clone(),
+        },
+        None => tt::Subtree::default(),
+    };
+    // The parentheses are always disposed here.
+    arg.delimiter = None;
+
+    let res = def
+        .as_lazy_macro(
+            db.upcast(),
+            krate,
+            MacroCallKind::Attr {
+                ast_id: item_attr.ast_id,
+                attr_name: last_segment.to_string(),
+                attr_args: arg,
+                invoc_attr_index: macro_attr.id.ast_index,
+            },
+        )
+        .into();
+    Ok(res)
+}
diff --git a/crates/hir_def/src/nameres/collector.rs b/crates/hir_def/src/nameres/collector.rs
index 6b41921ae..874a4ebb1 100644
--- a/crates/hir_def/src/nameres/collector.rs
+++ b/crates/hir_def/src/nameres/collector.rs
@@ -23,7 +23,7 @@ use syntax::ast;
 
 use crate::{
     attr::{Attr, AttrId, AttrInput, Attrs},
-    builtin_attr,
+    attr_macro_as_call_id, builtin_attr,
     db::DefDatabase,
     derive_macro_as_call_id,
     intern::Interned,
@@ -223,7 +223,7 @@ struct MacroDirective {
 enum MacroDirectiveKind {
     FnLike { ast_id: AstIdWithPath<ast::MacroCall>, fragment: FragmentKind },
     Derive { ast_id: AstIdWithPath<ast::Item>, derive_attr: AttrId },
-    Attr { ast_id: AstIdWithPath<ast::Item>, attr: AttrId, mod_item: ModItem },
+    Attr { ast_id: AstIdWithPath<ast::Item>, attr: Attr, mod_item: ModItem },
 }
 
 struct DefData<'a> {
@@ -419,7 +419,7 @@ impl DefCollector<'_> {
         let mut unresolved_macros = std::mem::replace(&mut self.unresolved_macros, Vec::new());
         let pos = unresolved_macros.iter().position(|directive| {
             if let MacroDirectiveKind::Attr { ast_id, mod_item, attr } = &directive.kind {
-                self.skip_attrs.insert(ast_id.ast_id.with_value(*mod_item), *attr);
+                self.skip_attrs.insert(ast_id.ast_id.with_value(*mod_item), attr.id);
 
                 let file_id = ast_id.ast_id.file_id;
                 let item_tree = self.db.file_item_tree(file_id);
@@ -1050,7 +1050,7 @@ impl DefCollector<'_> {
                                 let file_id = ast_id.ast_id.file_id;
                                 let item_tree = self.db.file_item_tree(file_id);
                                 let mod_dir = self.mod_dirs[&directive.module_id].clone();
-                                self.skip_attrs.insert(InFile::new(file_id, *mod_item), *attr);
+                                self.skip_attrs.insert(InFile::new(file_id, *mod_item), attr.id);
                                 ModCollector {
                                     def_collector: &mut *self,
                                     macro_depth: directive.depth,
@@ -1068,7 +1068,51 @@ impl DefCollector<'_> {
                     }
 
                     // Not resolved to a derive helper, so try to resolve as a macro.
-                    // FIXME: not yet :)
+                    match attr_macro_as_call_id(
+                        ast_id,
+                        attr,
+                        self.db,
+                        self.def_map.krate,
+                        &resolver,
+                    ) {
+                        Ok(call_id) => {
+                            let loc: MacroCallLoc = self.db.lookup_intern_macro(call_id);
+                            if let MacroDefKind::ProcMacro(exp, ..) = &loc.def.kind {
+                                if exp.is_dummy() {
+                                    // Proc macros that cannot be expanded are treated as not
+                                    // resolved, in order to fall back later.
+                                    self.def_map.diagnostics.push(
+                                        DefDiagnostic::unresolved_proc_macro(
+                                            directive.module_id,
+                                            loc.kind,
+                                        ),
+                                    );
+
+                                    let file_id = ast_id.ast_id.file_id;
+                                    let item_tree = self.db.file_item_tree(file_id);
+                                    let mod_dir = self.mod_dirs[&directive.module_id].clone();
+                                    self.skip_attrs
+                                        .insert(InFile::new(file_id, *mod_item), attr.id);
+                                    ModCollector {
+                                        def_collector: &mut *self,
+                                        macro_depth: directive.depth,
+                                        module_id: directive.module_id,
+                                        file_id,
+                                        item_tree: &item_tree,
+                                        mod_dir,
+                                    }
+                                    .collect(&[*mod_item]);
+
+                                    // Remove the macro directive.
+                                    return false;
+                                }
+                            }
+                            resolved.push((directive.module_id, call_id, directive.depth));
+                            res = ReachedFixedPoint::No;
+                            return false;
+                        }
+                        Err(UnresolvedMacro { .. }) => (),
+                    }
                 }
             }
 
@@ -1628,7 +1672,7 @@ impl ModCollector<'_, '_> {
                 self.def_collector.unresolved_macros.push(MacroDirective {
                     module_id: self.module_id,
                     depth: self.macro_depth + 1,
-                    kind: MacroDirectiveKind::Attr { ast_id, attr: attr.id, mod_item },
+                    kind: MacroDirectiveKind::Attr { ast_id, attr: attr.clone(), mod_item },
                 });
 
                 return Err(());
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),
         }