rust-lang · bors · May 26, 2025 · May 23, 2025
diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -47,7 +47,7 @@ use rustc_infer::infer::relate::RelateResult;
 use rustc_infer::infer::{Coercion, DefineOpaqueTypes, InferOk, InferResult};
 use rustc_infer::traits::{
     IfExpressionCause, MatchExpressionArmCause, Obligation, PredicateObligation,
-    PredicateObligations,
+    PredicateObligations, SelectionError,
 };
 use rustc_middle::span_bug;
 use rustc_middle::ty::adjustment::{
@@ -677,7 +677,21 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                     return Err(TypeError::Mismatch);
                 }
 
-                // Dyn-compatibility violations or miscellaneous.
+                Err(SelectionError::TraitDynIncompatible(_)) => {
+                    // Dyn compatibility errors in coercion will *always* be due to the
+                    // fact that the RHS of the coercion is a non-dyn compatible `dyn Trait`
+                    // writen in source somewhere (otherwise we will never have lowered
+                    // the dyn trait from HIR to middle).
+                    //
+                    // There's no reason to emit yet another dyn compatibility error,
+                    // especially since the span will differ slightly and thus not be
+                    // deduplicated at all!
+                    self.fcx.set_tainted_by_errors(
+                        self.fcx
+                            .dcx()
+                            .span_delayed_bug(self.cause.span, "dyn compatibility during coercion"),
+                    );
+                }
                 Err(err) => {
                     let guar = self.err_ctxt().report_selection_error(
                         obligation.clone(),

diff --git a/tests/ui/async-await/dyn/mut-is-pointer-like.stderr b/tests/ui/async-await/dyn/mut-is-pointer-like.stderr
@@ -24,24 +24,6 @@ LL |     async fn async_dispatch(self: Pin<&mut Self>) -> Self::Output;
    |              ^^^^^^^^^^^^^^ ...because method `async_dispatch` is `async`
    = help: consider moving `async_dispatch` to another trait
 
-error[E0038]: the trait `AsyncTrait` is not dyn compatible
-  --> $DIR/mut-is-pointer-like.rs:35:56
-   |
-LL |         let x: Pin<&mut dyn AsyncTrait<Output = ()>> = f;
-   |                                                        ^ `AsyncTrait` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/mut-is-pointer-like.rs:16:14
-   |
-LL | trait AsyncTrait {
-   |       ---------- this trait is not dyn compatible...
-...
-LL |     async fn async_dispatch(self: Pin<&mut Self>) -> Self::Output;
-   |              ^^^^^^^^^^^^^^ ...because method `async_dispatch` is `async`
-   = help: consider moving `async_dispatch` to another trait
-   = note: required for the cast from `Pin<&mut {async block@$DIR/mut-is-pointer-like.rs:32:32: 32:37}>` to `Pin<&mut dyn AsyncTrait<Output = ()>>`
-
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to 1 previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0038`.
diff --git a/tests/ui/async-await/dyn/works.stderr b/tests/ui/async-await/dyn/works.stderr
@@ -7,24 +7,6 @@ LL | #![feature(async_fn_in_dyn_trait)]
    = note: see issue #133119 <https://github.com/rust-lang/rust/issues/133119> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-error[E0038]: the trait `AsyncTrait` is not dyn compatible
-  --> $DIR/works.rs:27:34
-   |
-LL |         let x: &dyn AsyncTrait = &"hello, world!";
-   |                                  ^^^^^^^^^^^^^^^^ `AsyncTrait` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/works.rs:14:14
-   |
-LL | trait AsyncTrait {
-   |       ---------- this trait is not dyn compatible...
-LL |     async fn async_dispatch(&self);
-   |              ^^^^^^^^^^^^^^ ...because method `async_dispatch` is `async`
-   = help: consider moving `async_dispatch` to another trait
-   = help: only type `&'static str` implements `AsyncTrait`; consider using it directly instead.
-   = note: required for the cast from `&&'static str` to `&dyn AsyncTrait`
-
 error[E0038]: the trait `AsyncTrait` is not dyn compatible
   --> $DIR/works.rs:27:16
    |
@@ -42,6 +24,6 @@ LL |     async fn async_dispatch(&self);
    = help: consider moving `async_dispatch` to another trait
    = help: only type `&'static str` implements `AsyncTrait`; consider using it directly instead.
 
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to 1 previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0038`.
diff --git a/tests/ui/async-await/dyn/wrong-size.stderr b/tests/ui/async-await/dyn/wrong-size.stderr
@@ -7,24 +7,6 @@ LL | #![feature(async_fn_in_dyn_trait)]
    = note: see issue #133119 <https://github.com/rust-lang/rust/issues/133119> for more information
    = note: `#[warn(incomplete_features)]` on by default
 
-error[E0038]: the trait `AsyncTrait` is not dyn compatible
-  --> $DIR/wrong-size.rs:21:30
-   |
-LL |     let x: &dyn AsyncTrait = &"hello, world!";
-   |                              ^^^^^^^^^^^^^^^^ `AsyncTrait` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/wrong-size.rs:9:14
-   |
-LL | trait AsyncTrait {
-   |       ---------- this trait is not dyn compatible...
-LL |     async fn async_dispatch(&self);
-   |              ^^^^^^^^^^^^^^ ...because method `async_dispatch` is `async`
-   = help: consider moving `async_dispatch` to another trait
-   = help: only type `&'static str` implements `AsyncTrait`; consider using it directly instead.
-   = note: required for the cast from `&&'static str` to `&dyn AsyncTrait`
-
 error[E0038]: the trait `AsyncTrait` is not dyn compatible
   --> $DIR/wrong-size.rs:21:12
    |
@@ -42,6 +24,6 @@ LL |     async fn async_dispatch(&self);
    = help: consider moving `async_dispatch` to another trait
    = help: only type `&'static str` implements `AsyncTrait`; consider using it directly instead.
 
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to 1 previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0038`.
diff --git a/tests/ui/dyn-compatibility/almost-supertrait-associated-type.rs b/tests/ui/dyn-compatibility/almost-supertrait-associated-type.rs
@@ -6,7 +6,6 @@ use std::marker::PhantomData;
 fn transmute<T, U>(t: T) -> U {
     (&PhantomData::<T> as &dyn Foo<T, U>).transmute(t)
     //~^ ERROR the trait `Foo` is not dyn compatible
-    //~| ERROR the trait `Foo` is not dyn compatible
 }
 
 struct ActuallySuper;

diff --git a/tests/ui/dyn-compatibility/almost-supertrait-associated-type.stderr b/tests/ui/dyn-compatibility/almost-supertrait-associated-type.stderr
@@ -1,12 +1,12 @@
 error[E0038]: the trait `Foo` is not dyn compatible
-  --> $DIR/almost-supertrait-associated-type.rs:21:20
+  --> $DIR/almost-supertrait-associated-type.rs:20:20
    |
 LL | impl<T, U> Dyn for dyn Foo<T, U> + '_ {
    |                    ^^^^^^^^^^^^^^^^^^ `Foo` is not dyn compatible
    |
 note: for a trait to be dyn compatible it needs to allow building a vtable
       for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/almost-supertrait-associated-type.rs:33:34
+  --> $DIR/almost-supertrait-associated-type.rs:32:34
    |
 LL | trait Foo<T, U>: Super<ActuallySuper, Assoc = T>
    |       --- this trait is not dyn compatible...
@@ -23,7 +23,7 @@ LL |     (&PhantomData::<T> as &dyn Foo<T, U>).transmute(t)
    |
 note: for a trait to be dyn compatible it needs to allow building a vtable
       for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/almost-supertrait-associated-type.rs:33:34
+  --> $DIR/almost-supertrait-associated-type.rs:32:34
    |
 LL | trait Foo<T, U>: Super<ActuallySuper, Assoc = T>
    |       --- this trait is not dyn compatible...
@@ -32,24 +32,6 @@ LL |     fn transmute(&self, t: T) -> <Self as Super<NotActuallySuper>>::Assoc;
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...because method `transmute` references the `Self` type in its return type
    = help: consider moving `transmute` to another trait
 
-error[E0038]: the trait `Foo` is not dyn compatible
-  --> $DIR/almost-supertrait-associated-type.rs:7:6
-   |
-LL |     (&PhantomData::<T> as &dyn Foo<T, U>).transmute(t)
-   |      ^^^^^^^^^^^^^^^^^ `Foo` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/almost-supertrait-associated-type.rs:33:34
-   |
-LL | trait Foo<T, U>: Super<ActuallySuper, Assoc = T>
-   |       --- this trait is not dyn compatible...
-...
-LL |     fn transmute(&self, t: T) -> <Self as Super<NotActuallySuper>>::Assoc;
-   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...because method `transmute` references the `Self` type in its return type
-   = help: consider moving `transmute` to another trait
-   = note: required for the cast from `&PhantomData<T>` to `&dyn Foo<T, U>`
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0038`.
diff --git a/tests/ui/dyn-compatibility/associated-consts.rs b/tests/ui/dyn-compatibility/associated-consts.rs
@@ -8,7 +8,6 @@ trait Bar {
 fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
     //~^ ERROR E0038
     t
-    //~^ ERROR E0038
 }
 
 fn main() {

diff --git a/tests/ui/dyn-compatibility/associated-consts.stderr b/tests/ui/dyn-compatibility/associated-consts.stderr
@@ -14,23 +14,6 @@ LL |     const X: usize;
    |           ^ ...because it contains this associated `const`
    = help: consider moving `X` to another trait
 
-error[E0038]: the trait `Bar` is not dyn compatible
-  --> $DIR/associated-consts.rs:10:5
-   |
-LL |     t
-   |     ^ `Bar` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/associated-consts.rs:5:11
-   |
-LL | trait Bar {
-   |       --- this trait is not dyn compatible...
-LL |     const X: usize;
-   |           ^ ...because it contains this associated `const`
-   = help: consider moving `X` to another trait
-   = note: required for the cast from `&T` to `&dyn Bar`
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0038`.
diff --git a/tests/ui/dyn-compatibility/generics.rs b/tests/ui/dyn-compatibility/generics.rs
@@ -15,14 +15,12 @@ trait Quux {
 fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
     //~^ ERROR E0038
     t
-    //~^ ERROR E0038
 }
 
 fn make_bar_explicit<T:Bar>(t: &T) -> &dyn Bar {
     //~^ ERROR E0038
     t as &dyn Bar
     //~^ ERROR E0038
-    //~| ERROR E0038
 }
 
 fn make_quux<T:Quux>(t: &T) -> &dyn Quux {

diff --git a/tests/ui/dyn-compatibility/generics.stderr b/tests/ui/dyn-compatibility/generics.stderr
@@ -15,7 +15,7 @@ LL |     fn bar<T>(&self, t: T);
    = help: consider moving `bar` to another trait
 
 error[E0038]: the trait `Bar` is not dyn compatible
-  --> $DIR/generics.rs:21:40
+  --> $DIR/generics.rs:20:40
    |
 LL | fn make_bar_explicit<T:Bar>(t: &T) -> &dyn Bar {
    |                                        ^^^^^^^ `Bar` is not dyn compatible
@@ -31,24 +31,7 @@ LL |     fn bar<T>(&self, t: T);
    = help: consider moving `bar` to another trait
 
 error[E0038]: the trait `Bar` is not dyn compatible
-  --> $DIR/generics.rs:17:5
-   |
-LL |     t
-   |     ^ `Bar` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/generics.rs:7:8
-   |
-LL | trait Bar {
-   |       --- this trait is not dyn compatible...
-LL |     fn bar<T>(&self, t: T);
-   |        ^^^ ...because method `bar` has generic type parameters
-   = help: consider moving `bar` to another trait
-   = note: required for the cast from `&T` to `&dyn Bar`
-
-error[E0038]: the trait `Bar` is not dyn compatible
-  --> $DIR/generics.rs:23:10
+  --> $DIR/generics.rs:22:10
    |
 LL |     t as &dyn Bar
    |          ^^^^^^^^ `Bar` is not dyn compatible
@@ -63,23 +46,6 @@ LL |     fn bar<T>(&self, t: T);
    |        ^^^ ...because method `bar` has generic type parameters
    = help: consider moving `bar` to another trait
 
-error[E0038]: the trait `Bar` is not dyn compatible
-  --> $DIR/generics.rs:23:5
-   |
-LL |     t as &dyn Bar
-   |     ^ `Bar` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/generics.rs:7:8
-   |
-LL | trait Bar {
-   |       --- this trait is not dyn compatible...
-LL |     fn bar<T>(&self, t: T);
-   |        ^^^ ...because method `bar` has generic type parameters
-   = help: consider moving `bar` to another trait
-   = note: required for the cast from `&T` to `&dyn Bar`
-
-error: aborting due to 5 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0038`.
diff --git a/tests/ui/dyn-compatibility/mention-correct-dyn-incompatible-trait.rs b/tests/ui/dyn-compatibility/mention-correct-dyn-incompatible-trait.rs
@@ -18,5 +18,4 @@ fn main() {
     let mut thing = Thing;
     let test: &mut dyn Bar = &mut thing;
     //~^ ERROR E0038
-    //~| ERROR E0038
 }
diff --git a/tests/ui/dyn-compatibility/mention-correct-dyn-incompatible-trait.stderr b/tests/ui/dyn-compatibility/mention-correct-dyn-incompatible-trait.stderr
@@ -1,22 +1,3 @@
-error[E0038]: the trait `Bar` is not dyn compatible
-  --> $DIR/mention-correct-dyn-incompatible-trait.rs:19:30
-   |
-LL |     let test: &mut dyn Bar = &mut thing;
-   |                              ^^^^^^^^^^ `Bar` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/mention-correct-dyn-incompatible-trait.rs:4:8
-   |
-LL |     fn foo<T>(&self, val: T);
-   |        ^^^ ...because method `foo` has generic type parameters
-...
-LL | trait Bar: Foo { }
-   |       --- this trait is not dyn compatible...
-   = help: consider moving `foo` to another trait
-   = help: only type `Thing` implements `Bar`; consider using it directly instead.
-   = note: required for the cast from `&mut Thing` to `&mut dyn Bar`
-
 error[E0038]: the trait `Bar` is not dyn compatible
   --> $DIR/mention-correct-dyn-incompatible-trait.rs:19:15
    |
@@ -35,6 +16,6 @@ LL | trait Bar: Foo { }
    = help: consider moving `foo` to another trait
    = help: only type `Thing` implements `Bar`; consider using it directly instead.
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0038`.
diff --git a/tests/ui/dyn-compatibility/mentions-Self.rs b/tests/ui/dyn-compatibility/mentions-Self.rs
@@ -18,13 +18,11 @@ trait Quux {
 fn make_bar<T:Bar>(t: &T) -> &dyn Bar {
     //~^ ERROR E0038
     t
-    //~^ ERROR E0038
 }
 
 fn make_baz<T:Baz>(t: &T) -> &dyn Baz {
     //~^ ERROR E0038
     t
-    //~^ ERROR E0038
 }
 
 fn make_quux<T:Quux>(t: &T) -> &dyn Quux {

diff --git a/tests/ui/dyn-compatibility/mentions-Self.stderr b/tests/ui/dyn-compatibility/mentions-Self.stderr
@@ -15,7 +15,7 @@ LL |     fn bar(&self, x: &Self);
    = help: consider moving `bar` to another trait
 
 error[E0038]: the trait `Baz` is not dyn compatible
-  --> $DIR/mentions-Self.rs:24:31
+  --> $DIR/mentions-Self.rs:23:31
    |
 LL | fn make_baz<T:Baz>(t: &T) -> &dyn Baz {
    |                               ^^^^^^^ `Baz` is not dyn compatible
@@ -30,40 +30,6 @@ LL |     fn baz(&self) -> Self;
    |                      ^^^^ ...because method `baz` references the `Self` type in its return type
    = help: consider moving `baz` to another trait
 
-error[E0038]: the trait `Bar` is not dyn compatible
-  --> $DIR/mentions-Self.rs:20:5
-   |
-LL |     t
-   |     ^ `Bar` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/mentions-Self.rs:7:22
-   |
-LL | trait Bar {
-   |       --- this trait is not dyn compatible...
-LL |     fn bar(&self, x: &Self);
-   |                      ^^^^^ ...because method `bar` references the `Self` type in this parameter
-   = help: consider moving `bar` to another trait
-   = note: required for the cast from `&T` to `&dyn Bar`
-
-error[E0038]: the trait `Baz` is not dyn compatible
-  --> $DIR/mentions-Self.rs:26:5
-   |
-LL |     t
-   |     ^ `Baz` is not dyn compatible
-   |
-note: for a trait to be dyn compatible it needs to allow building a vtable
-      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
-  --> $DIR/mentions-Self.rs:11:22
-   |
-LL | trait Baz {
-   |       --- this trait is not dyn compatible...
-LL |     fn baz(&self) -> Self;
-   |                      ^^^^ ...because method `baz` references the `Self` type in its return type
-   = help: consider moving `baz` to another trait
-   = note: required for the cast from `&T` to `&dyn Baz`
-
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0038`.
diff --git a/tests/ui/dyn-compatibility/no-static.rs b/tests/ui/dyn-compatibility/no-static.rs
@@ -17,5 +17,4 @@ impl Foo for Bar {}
 fn main() {
     let b: Box<dyn Foo> = Box::new(Bar);
     //~^ ERROR E0038
-    //~| ERROR E0038
 }