Rollup merge of #131857 - WaffleLapkin:dyn-drop-principal-3, r=compil…

…er-errors

Allow dropping dyn principal

Revival of #126660, which was a revival of #114679. Fixes #126313.

Allows dropping principal when coercing trait objects, e.g. `dyn Debug + Send` -> `dyn Send`.

cc `@compiler-errors` `@Jules-Bertholet`
r? `@lcnr`

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(

src/tools/miri/tests/pass/dyn-upcast.rs

@@ -9,6 +9,7 @@ fn main() {
  struct_();
  replace_vptr();
  vtable_nop_cast();
+ drop_principal();
 }
 
 fn vtable_nop_cast() {
@@ -430,3 +431,53 @@ fn replace_vptr() {
  let s = S(42);
  invoke_outer(&s);
 }
+
+fn drop_principal() {
+ use std::{alloc::Layout, 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");
+ }
+
+ let x = Box::new(CallMe::new(goodbye)) as Box<dyn Any + Send>;
+ let x_layout = Layout::for_value(&*x);
+ let y = yeet_principal(x);
+ let y_layout = Layout::for_value(&*y);
+ assert_eq!(x_layout, y_layout);
+ println!("before");
+ drop(y);
+
+ let x = Box::new(CallMe::new(goodbye)) as Box<dyn Bar>;
+ let x_layout = Layout::for_value(&*x);
+ let y = yeet_principal_2(x);
+ let y_layout = Layout::for_value(&*y);
+ assert_eq!(x_layout, y_layout);
+ println!("before");
+ drop(y);
+}

src/tools/miri/tests/pass/dyn-upcast.stdout

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

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 +1,68 @@
+//@ run-pass
+//@ check-run-results
+
+use std::{alloc::Layout, 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 x_layout = Layout::for_value(&*x);
+ let y = yeet_principal(x);
+ let y_layout = Layout::for_value(&*y);
+ assert_eq!(x_layout, y_layout);
+ println!("before");
+ drop(y);
+
+ let x = Box::new(CallMe::new(goodbye)) as Box<dyn Bar>;
+ let x_layout = Layout::for_value(&*x);
+ let y = yeet_principal_2(x);
+ let y_layout = Layout::for_value(&*y);
+ assert_eq!(x_layout, y_layout);
+ 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

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

@@ -0,0 +1,12 @@
+#![feature(dyn_star)]
+#![allow(incomplete_features)]
+
+trait Trait {}
+impl Trait for usize {}
+
+fn main() {
+ // We allow &dyn Trait + Send -> &dyn Send (i.e. dropping principal),
+ // but we don't (currently?) allow the same for dyn*
+ let x: dyn* Trait + Send = 1usize;
+ x as dyn* Send; //~ error: `dyn* Trait + Send` needs to have the same ABI as a pointer
+}

tests/ui/traits/dyn-star-drop-principal.stderr

@@ -0,0 +1,11 @@
+error[E0277]: `dyn* Trait + Send` needs to have the same ABI as a pointer
+ --> $DIR/dyn-star-drop-principal.rs:11:5
+ |
+LL | x as dyn* Send;
+ | ^ `dyn* Trait + Send` needs to be a pointer-like type
+ |
+ = help: the trait `PointerLike` is not implemented for `dyn* Trait + Send`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.