Illustrate how Semantics<'db, DB: ?Sized> can't actually work

Author: regexident

crates/hir/src/semantics.rs

@@ -159,23 +159,27 @@ pub struct SemanticsImpl<'db> {
     macro_call_cache: RefCell<FxHashMap<InFile<ast::MacroCall>, MacroCallId>>,
 }
 
-impl<DB> fmt::Debug for Semantics<'_, DB> {
+impl<DB: ?Sized> fmt::Debug for Semantics<'_, DB> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "Semantics {{ ... }}")
     }
 }
 
-impl<'db, DB> ops::Deref for Semantics<'db, DB> {
+impl<'db, DB: ?Sized> ops::Deref for Semantics<'db, DB> {
     type Target = SemanticsImpl<'db>;
 
     fn deref(&self) -> &Self::Target {
         &self.imp
     }
 }
 
-impl<DB: HirDatabase> Semantics<'_, DB> {
+impl<DB: HirDatabase + ?Sized> Semantics<'_, DB> {
     pub fn new(db: &DB) -> Semantics<'_, DB> {
+        // This doesn't work:
         let impl_ = SemanticsImpl::new(db);
+        // Neither does this:
+        let impl_ = SemanticsImpl::new(db as &(dyn HirDatabase));
+
         Semantics { db, imp: impl_ }
     }
 
@@ -2387,3 +2391,16 @@ impl RenameConflictsVisitor<'_> {
         self.body.walk_child_exprs(expr, |expr| self.rename_conflicts(expr));
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use hir_ty::db::HirDatabase;
+
+    use super::*;
+
+    #[test]
+    fn semantics_with_dyn_db() {
+        let maybe_db: Option<&dyn HirDatabase> = None;
+        let maybe_sema = maybe_db.map(|db| Semantics::new(db));
+    }
+}