Note that type aliases cannot be recursive

compiler/rustc_query_system/src/query/mod.rs

@@ -29,24 +29,53 @@ pub struct QueryStackFrame {
     pub name: &'static str,
     pub description: String,
     span: Option<Span>,
+    /// The `DefKind` this query frame is associated with, if applicable.
+    ///
+    /// We can't use `rustc_hir::def::DefKind` because `rustc_hir` is not
+    /// available in `rustc_query_system`. Instead, we have a simplified
+    /// custom version of it, called [`SimpleDefKind`].
+    def_kind: Option<SimpleDefKind>,
     /// This hash is used to deterministically pick
     /// a query to remove cycles in the parallel compiler.
     #[cfg(parallel_compiler)]
     hash: u64,
 }
 
+/// A simplified version of `rustc_hir::def::DefKind`.
+///
+/// It was added to help improve cycle errors caused by recursive type aliases.
+/// As of August 2021, `rustc_query_system` cannot depend on `rustc_hir`
+/// because it would create a dependency cycle. So, instead, a simplified
+/// version of `DefKind` was added to `rustc_query_system`.
+///
+/// `DefKind`s are converted to `SimpleDefKind`s in `rustc_query_impl`.
+#[derive(Debug, Copy, Clone)]
+pub enum SimpleDefKind {
+    Struct,
+    Enum,
+    Union,
+    Trait,
+    TyAlias,
+    TraitAlias,
+
+    // FIXME: add more from `rustc_hir::def::DefKind` and then remove `Other`
+    Other,
+}
+
 impl QueryStackFrame {
     #[inline]
     pub fn new(
         name: &'static str,
         description: String,
         span: Option<Span>,
+        def_kind: Option<SimpleDefKind>,
         _hash: impl FnOnce() -> u64,
     ) -> Self {
         Self {
             name,
             description,
             span,
+            def_kind,
             #[cfg(parallel_compiler)]
             hash: _hash(),
         }