Allow dropping dyn principal

compiler/rustc_codegen_cranelift/src/unsize.rs

@@ -33,7 +33,8 @@ pub(crate) fn unsized_info<'tcx>(
         {
             let old_info =
                 old_info.expect("unsized_info: missing old info for trait upcasting coercion");
-            if data_a.principal_def_id() == data_b.principal_def_id() {
+            let b_principal_def_id = data_b.principal_def_id();
+            if data_a.principal_def_id() == b_principal_def_id || b_principal_def_id.is_none() {
                 // A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables.
                 return old_info;
             }

compiler/rustc_codegen_ssa/src/base.rs

@@ -156,7 +156,8 @@ pub fn unsized_info<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         {
             let old_info =
                 old_info.expect("unsized_info: missing old info for trait upcasting coercion");
-            if data_a.principal_def_id() == data_b.principal_def_id() {
+            let b_principal_def_id = data_b.principal_def_id();
+            if data_a.principal_def_id() == b_principal_def_id || b_principal_def_id.is_none() {
                 // A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables.
                 return old_info;
             }

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -733,7 +733,8 @@ where
         let mut responses = vec![];
         // If the principal def ids match (or are both none), then we're not doing
         // trait upcasting. We're just removing auto traits (or shortening the lifetime).
-        if a_data.principal_def_id() == b_data.principal_def_id() {
+        let b_principal_def_id = b_data.principal_def_id();
+        if a_data.principal_def_id() == b_principal_def_id || b_principal_def_id.is_none() {
             responses.extend(self.consider_builtin_upcast_to_principal(
                 goal,
                 CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -998,7 +998,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // #2 (region bounds).
                 let principal_def_id_a = a_data.principal_def_id();
                 let principal_def_id_b = b_data.principal_def_id();
-                if principal_def_id_a == principal_def_id_b {
+                if principal_def_id_a == principal_def_id_b || principal_def_id_b.is_none() {
                     // We may upcast to auto traits that are either explicitly listed in
                     // the object type's bounds, or implied by the principal trait ref's
                     // supertraits.

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1140,6 +1140,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
                 let iter = data_a
                     .principal()
+                    .filter(|_| {
+                        // optionally drop the principal, if we're unsizing to no principal
+                        data_b.principal().is_some()
+                    })
                     .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
                     .into_iter()
                     .chain(

tests/ui/traits/dyn-drop-principal.rs

@@ -0,0 +1,46 @@
+//@ run-pass
+//@ check-run-results
+
+use std::any::Any;
+
+fn yeet_principal(x: Box<dyn Any + Send>) -> Box<dyn Send> {
+    x
+}
+
+trait Bar: Send {}
+
+impl<T: Send> Bar for T {}
+
+fn yeet_principal_2(x: Box<dyn Bar>) -> Box<dyn Send> {
+    x
+}
+
+struct CallMe<F: FnOnce()>(Option<F>);
+
+impl<F: FnOnce()> CallMe<F> {
+    fn new(f: F) -> Self {
+        CallMe(Some(f))
+    }
+}
+
+impl<F: FnOnce()> Drop for CallMe<F> {
+    fn drop(&mut self) {
+        (self.0.take().unwrap())();
+    }
+}
+
+fn goodbye() {
+    println!("goodbye");
+}
+
+fn main() {
+    let x = Box::new(CallMe::new(goodbye)) as Box<dyn Any + Send>;
+    let y = yeet_principal(x);
+    println!("before");
+    drop(y);
+
+    let x = Box::new(CallMe::new(goodbye)) as Box<dyn Bar>;
+    let y = yeet_principal_2(x);
+    println!("before");
+    drop(y);
+}

tests/ui/traits/dyn-drop-principal.run.stdout

@@ -0,0 +1,4 @@
+before
+goodbye
+before
+goodbye