Repository: rust-lang/rust-analyzer (rust)

Issue:
Support builtin derive attributes helper attributes
The `Default` derive now has a helper attribute defined called `default`. We don't support those yet.
See https://doc.rust-lang.org/src/core/default.rs.html#182
```rs
#[rustc_builtin_macro(Default, attributes(default))]
#[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
#[allow_internal_unstable(core_intrinsics)]
pub macro Default($item:item) {
    /* compiler built-in */
}
```
we need to parse the `#[rustc_builtin_macro(Default, attributes(default))]` line

Pull request description:
Support builtin derive macro helper attributes
Closes #13244 

It's a bit wasteful for `Macro2Data` to have `helpers` field currently just for `Default` derive macro, but I tend to think it's okay for the time being given how rare macro2's are used.

Patch:
diff --git a/crates/hir-def/src/data.rs b/crates/hir-def/src/data.rs
index 0e7acda4a7..b78ab71ef2 100644
--- a/crates/hir-def/src/data.rs
+++ b/crates/hir-def/src/data.rs
@@ -13,7 +13,9 @@ use crate::{
     intern::Interned,
     item_tree::{self, AssocItem, FnFlags, ItemTree, ItemTreeId, ModItem, Param, TreeId},
     nameres::{
-        attr_resolution::ResolvedAttr, diagnostics::DefDiagnostic, proc_macro::ProcMacroKind,
+        attr_resolution::ResolvedAttr,
+        diagnostics::DefDiagnostic,
+        proc_macro::{parse_macro_name_and_helper_attrs, ProcMacroKind},
         DefMap,
     },
     type_ref::{TraitRef, TypeBound, TypeRef},
@@ -348,6 +350,10 @@ impl ImplData {
 pub struct Macro2Data {
     pub name: Name,
     pub visibility: RawVisibility,
+    // It's a bit wasteful as currently this is only for builtin `Default` derive macro, but macro2
+    // are rarely used in practice so I think it's okay for now.
+    /// Derive helpers, if this is a derive rustc_builtin_macro
+    pub helpers: Option<Box<[Name]>>,
 }
 
 impl Macro2Data {
@@ -356,9 +362,18 @@ impl Macro2Data {
         let item_tree = loc.id.item_tree(db);
         let makro = &item_tree[loc.id.value];
 
+        let helpers = item_tree
+            .attrs(db, loc.container.krate(), ModItem::from(loc.id.value).into())
+            .by_key("rustc_builtin_macro")
+            .tt_values()
+            .next()
+            .and_then(|attr| parse_macro_name_and_helper_attrs(&attr.token_trees))
+            .map(|(_, helpers)| helpers);
+
         Arc::new(Macro2Data {
             name: makro.name.clone(),
             visibility: item_tree[makro.visibility].clone(),
+            helpers,
         })
     }
 }
diff --git a/crates/hir-def/src/nameres/collector.rs b/crates/hir-def/src/nameres/collector.rs
index b0dd01f9db..f21d674f20 100644
--- a/crates/hir-def/src/nameres/collector.rs
+++ b/crates/hir-def/src/nameres/collector.rs
@@ -40,7 +40,7 @@ use crate::{
         diagnostics::DefDiagnostic,
         mod_resolution::ModDir,
         path_resolution::ReachedFixedPoint,
-        proc_macro::{ProcMacroDef, ProcMacroKind},
+        proc_macro::{parse_macro_name_and_helper_attrs, ProcMacroDef, ProcMacroKind},
         BuiltinShadowMode, DefMap, ModuleData, ModuleOrigin, ResolveMode,
     },
     path::{ImportAlias, ModPath, PathKind},
@@ -2005,6 +2005,7 @@ impl ModCollector<'_, '_> {
         let ast_id = InFile::new(self.file_id(), mac.ast_id.upcast());
 
         // Case 1: builtin macros
+        let mut helpers_opt = None;
         let attrs = self.item_tree.attrs(self.def_collector.db, krate, ModItem::from(id).into());
         let expander = if attrs.by_key("rustc_builtin_macro").exists() {
             if let Some(expander) = find_builtin_macro(&mac.name) {
@@ -2013,6 +2014,25 @@ impl ModCollector<'_, '_> {
                     Either::Right(it) => MacroExpander::BuiltInEager(it),
                 }
             } else if let Some(expander) = find_builtin_derive(&mac.name) {
+                if let Some(attr) = attrs.by_key("rustc_builtin_macro").tt_values().next() {
+                    // NOTE: The item *may* have both `#[rustc_builtin_macro]` and `#[proc_macro_derive]`,
+                    // in which case rustc ignores the helper attributes from the latter, but it
+                    // "doesn't make sense in practice" (see rust-lang/rust#87027).
+                    if let Some((name, helpers)) =
+                        parse_macro_name_and_helper_attrs(&attr.token_trees)
+                    {
+                        // NOTE: rustc overrides the name if the macro name if it's different from the
+                        // macro name, but we assume it isn't as there's no such case yet. FIXME if
+                        // the following assertion fails.
+                        stdx::always!(
+                            name == mac.name,
+                            "built-in macro {} has #[rustc_builtin_macro] which declares different name {}",
+                            mac.name,
+                            name
+                        );
+                        helpers_opt = Some(helpers);
+                    }
+                }
                 MacroExpander::BuiltInDerive(expander)
             } else if let Some(expander) = find_builtin_attr(&mac.name) {
                 MacroExpander::BuiltInAttr(expander)
@@ -2037,6 +2057,12 @@ impl ModCollector<'_, '_> {
             macro_id,
             &self.item_tree[mac.visibility],
         );
+        if let Some(helpers) = helpers_opt {
+            self.def_collector
+                .def_map
+                .exported_derives
+                .insert(macro_id_to_def_id(self.def_collector.db, macro_id.into()), helpers);
+        }
     }
 
     fn collect_macro_call(&mut self, mac: &MacroCall, container: ItemContainerId) {
diff --git a/crates/hir-def/src/nameres/proc_macro.rs b/crates/hir-def/src/nameres/proc_macro.rs
index 52b79cd0fd..06b23392cf 100644
--- a/crates/hir-def/src/nameres/proc_macro.rs
+++ b/crates/hir-def/src/nameres/proc_macro.rs
@@ -37,45 +37,53 @@ impl Attrs {
             Some(ProcMacroDef { name: func_name.clone(), kind: ProcMacroKind::Attr })
         } else if self.by_key("proc_macro_derive").exists() {
             let derive = self.by_key("proc_macro_derive").tt_values().next()?;
+            let def = parse_macro_name_and_helper_attrs(&derive.token_trees)
+                .map(|(name, helpers)| ProcMacroDef { name, kind: ProcMacroKind::CustomDerive { helpers } });
 
-            match &*derive.token_trees {
-                // `#[proc_macro_derive(Trait)]`
-                [TokenTree::Leaf(Leaf::Ident(trait_name))] => Some(ProcMacroDef {
-                    name: trait_name.as_name(),
-                    kind: ProcMacroKind::CustomDerive { helpers: Box::new([]) },
-                }),
-
-                // `#[proc_macro_derive(Trait, attributes(helper1, helper2, ...))]`
-                [
-                    TokenTree::Leaf(Leaf::Ident(trait_name)),
-                    TokenTree::Leaf(Leaf::Punct(comma)),
-                    TokenTree::Leaf(Leaf::Ident(attributes)),
-                    TokenTree::Subtree(helpers)
-                ] if comma.char == ',' && attributes.text == "attributes" =>
-                {
-                    let helpers = helpers.token_trees.iter()
-                        .filter(|tt| !matches!(tt, TokenTree::Leaf(Leaf::Punct(comma)) if comma.char == ','))
-                        .map(|tt| {
-                            match tt {
-                                TokenTree::Leaf(Leaf::Ident(helper)) => Some(helper.as_name()),
-                                _ => None
-                            }
-                        })
-                        .collect::<Option<Box<[_]>>>()?;
-
-                    Some(ProcMacroDef {
-                        name: trait_name.as_name(),
-                        kind: ProcMacroKind::CustomDerive { helpers },
-                    })
-                }
-
-                _ => {
-                    tracing::trace!("malformed `#[proc_macro_derive]`: {}", derive);
-                    None
-                }
+            if def.is_none() {
+                tracing::trace!("malformed `#[proc_macro_derive]`: {}", derive);
             }
+
+            def
         } else {
             None
         }
     }
 }
+
+// This fn is intended for `#[proc_macro_derive(..)]` and `#[rustc_builtin_macro(..)]`, which have
+// the same strucuture.
+#[rustfmt::skip]
+pub(crate) fn parse_macro_name_and_helper_attrs(tt: &[TokenTree]) -> Option<(Name, Box<[Name]>)> {
+    match tt {
+        // `#[proc_macro_derive(Trait)]`
+        // `#[rustc_builtin_macro(Trait)]`
+        [TokenTree::Leaf(Leaf::Ident(trait_name))] => Some((trait_name.as_name(), Box::new([]))),
+
+        // `#[proc_macro_derive(Trait, attributes(helper1, helper2, ...))]`
+        // `#[rustc_builtin_macro(Trait, attributes(helper1, helper2, ...))]`
+        [
+            TokenTree::Leaf(Leaf::Ident(trait_name)),
+            TokenTree::Leaf(Leaf::Punct(comma)),
+            TokenTree::Leaf(Leaf::Ident(attributes)),
+            TokenTree::Subtree(helpers)
+        ] if comma.char == ',' && attributes.text == "attributes" =>
+        {
+            let helpers = helpers
+                .token_trees
+                .iter()
+                .filter(
+                    |tt| !matches!(tt, TokenTree::Leaf(Leaf::Punct(comma)) if comma.char == ','),
+                )
+                .map(|tt| match tt {
+                    TokenTree::Leaf(Leaf::Ident(helper)) => Some(helper.as_name()),
+                    _ => None,
+                })
+                .collect::<Option<Box<[_]>>>()?;
+
+            Some((trait_name.as_name(), helpers))
+        }
+
+        _ => None,
+    }
+}
diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index cbbcaebb42..5f36ce62f8 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -2349,12 +2349,14 @@ impl DeriveHelper {
 
     pub fn name(&self, db: &dyn HirDatabase) -> Name {
         match self.derive {
-            MacroId::Macro2Id(_) => None,
+            MacroId::Macro2Id(it) => {
+                db.macro2_data(it).helpers.as_deref().and_then(|it| it.get(self.idx)).cloned()
+            }
             MacroId::MacroRulesId(_) => None,
             MacroId::ProcMacroId(proc_macro) => db
                 .proc_macro_data(proc_macro)
                 .helpers
-                .as_ref()
+                .as_deref()
                 .and_then(|it| it.get(self.idx))
                 .cloned(),
         }


Test patch:
diff --git a/crates/hir-def/src/nameres/tests/macros.rs b/crates/hir-def/src/nameres/tests/macros.rs
index 3ece1379ad..fe0ad4f386 100644
--- a/crates/hir-def/src/nameres/tests/macros.rs
+++ b/crates/hir-def/src/nameres/tests/macros.rs
@@ -822,6 +822,28 @@ fn derive() {}
     );
 }
 
+#[test]
+fn resolves_derive_helper_rustc_builtin_macro() {
+    cov_mark::check!(resolved_derive_helper);
+    // This is NOT the correct usage of `default` helper attribute, but we don't resolve helper
+    // attributes on non mod items in hir nameres.
+    check(
+        r#"
+//- minicore: derive, default
+#[derive(Default)]
+#[default]
+enum E {
+    A,
+    B,
+}
+"#,
+        expect![[r#"
+            crate
+            E: t
+        "#]],
+    );
+}
+
 #[test]
 fn unresolved_attr_with_cfg_attr_hang() {
     // Another regression test for https://github.com/rust-lang/rust-analyzer/issues/8905
diff --git a/crates/test-utils/src/minicore.rs b/crates/test-utils/src/minicore.rs
index 40a330a6bd..013fd6b4fd 100644
--- a/crates/test-utils/src/minicore.rs
+++ b/crates/test-utils/src/minicore.rs
@@ -112,7 +112,7 @@ pub mod default {
         fn default() -> Self;
     }
     // region:derive
-    #[rustc_builtin_macro]
+    #[rustc_builtin_macro(Default, attributes(default))]
     pub macro Default($item:item) {}
     // endregion:derive
 }