Allow dropping dyn principal

compiler/rustc_codegen_cranelift/src/unsize.rs

@@ -34,7 +34,9 @@ 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.
                 debug_assert!(
                     validate_trivial_unsize(fx.tcx, data_a, data_b),
                     "NOP unsize vtable changed principal trait ref: {data_a} -> {data_b}"

compiler/rustc_codegen_ssa/src/base.rs

@@ -147,7 +147,7 @@ pub fn validate_trivial_unsize<'tcx>(
                 infcx.leak_check(universe, None).is_ok()
             })
         }
-        (None, None) => true,
+        (_, None) => true,
         _ => false,
     }
 }
@@ -175,7 +175,8 @@ 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() {
                 // Codegen takes advantage of the additional assumption, where if the
                 // principal trait def id of what's being casted doesn't change,
                 // then we don't need to adjust the vtable at all. This

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -785,7 +785,8 @@ 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

@@ -1018,7 +1018,7 @@ 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

@@ -1153,6 +1153,10 @@ 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/impl-trait/unsized_coercion5.old.stderr

@@ -1,24 +1,12 @@
-error[E0308]: mismatched types
-  --> $DIR/unsized_coercion5.rs:16:32
-   |
-LL |         let y: Box<dyn Send> = x as Box<dyn Trait + Send>;
-   |                -------------   ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait `Send`, found trait `Trait + Send`
-   |                |
-   |                expected due to this
-   |
-   = note: expected struct `Box<dyn Send>`
-              found struct `Box<dyn Trait + Send>`
-
 error[E0277]: the size for values of type `impl Trait + ?Sized` cannot be known at compilation time
-  --> $DIR/unsized_coercion5.rs:16:32
+  --> $DIR/unsized_coercion5.rs:17:32
    |
 LL |         let y: Box<dyn Send> = x as Box<dyn Trait + Send>;
    |                                ^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `impl Trait + ?Sized`
    = note: required for the cast from `Box<impl Trait + ?Sized>` to `Box<dyn Trait + Send>`
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0277, E0308.
-For more information about an error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0277`.

tests/ui/impl-trait/unsized_coercion5.rs

@@ -3,6 +3,7 @@
 
 //@ revisions: next old
 //@[next] compile-flags: -Znext-solver
+//@[next] check-pass
 
 #![feature(trait_upcasting)]
 
@@ -15,7 +16,6 @@ fn hello() -> Box<impl Trait + ?Sized> {
         let x = hello();
         let y: Box<dyn Send> = x as Box<dyn Trait + Send>;
         //[old]~^ ERROR: the size for values of type `impl Trait + ?Sized` cannot be know
-        //~^^ ERROR: mismatched types
     }
     Box::new(1u32)
 }

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

@@ -0,0 +1,62 @@
+//@ run-pass
+//@ check-run-results
+
+use std::any::Any;
+
+const fn yeet_principal(x: Box<dyn Any + Send>) -> Box<dyn Send> {
+    x
+}
+
+trait Bar: Send + Sync {}
+
+impl<T: Send + Sync> Bar for T {}
+
+const 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);
+}
+
+// Test that upcast works in `const`
+
+const fn yeet_principal_3(x: &(dyn Any + Send + Sync)) -> &(dyn Send + Sync) {
+    x
+}
+
+#[used]
+pub static FOO: &(dyn Send + Sync) = yeet_principal_3(&false);
+
+const fn yeet_principal_4(x: &dyn Bar) -> &(dyn Send + Sync) {
+    x
+}
+
+#[used]
+pub static BAR: &(dyn Send + Sync) = yeet_principal_4(&false);

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

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