Auto merge of #87515 - crlf0710:trait_upcasting_part2, r=bjorn3

Trait upcasting coercion (part2)

This is the second part of trait upcasting coercion implementation.

Currently this is blocked on #86264 .

The third part might be implemented using unsafety checking

r? `@bjorn3`

src/test/ui/traits/trait-upcasting/basic.rs

@@ -0,0 +1,87 @@
+// run-pass
+
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
+    fn a(&self) -> i32 {
+        10
+    }
+
+    fn z(&self) -> i32 {
+        11
+    }
+
+    fn y(&self) -> i32 {
+        12
+    }
+}
+
+trait Bar: Foo {
+    fn b(&self) -> i32 {
+        20
+    }
+
+    fn w(&self) -> i32 {
+        21
+    }
+}
+
+trait Baz: Bar {
+    fn c(&self) -> i32 {
+        30
+    }
+}
+
+impl Foo for i32 {
+    fn a(&self) -> i32 {
+        100
+    }
+}
+
+impl Bar for i32 {
+    fn b(&self) -> i32 {
+        200
+    }
+}
+
+impl Baz for i32 {
+    fn c(&self) -> i32 {
+        300
+    }
+}
+
+fn main() {
+    let baz: &dyn Baz = &1;
+    let _: &dyn std::fmt::Debug = baz;
+    assert_eq!(*baz, 1);
+    assert_eq!(baz.a(), 100);
+    assert_eq!(baz.b(), 200);
+    assert_eq!(baz.c(), 300);
+    assert_eq!(baz.z(), 11);
+    assert_eq!(baz.y(), 12);
+    assert_eq!(baz.w(), 21);
+
+    let bar: &dyn Bar = baz;
+    let _: &dyn std::fmt::Debug = bar;
+    assert_eq!(*bar, 1);
+    assert_eq!(bar.a(), 100);
+    assert_eq!(bar.b(), 200);
+    assert_eq!(bar.z(), 11);
+    assert_eq!(bar.y(), 12);
+    assert_eq!(bar.w(), 21);
+
+    let foo: &dyn Foo = baz;
+    let _: &dyn std::fmt::Debug = foo;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+    assert_eq!(foo.z(), 11);
+    assert_eq!(foo.y(), 12);
+
+    let foo: &dyn Foo = bar;
+    let _: &dyn std::fmt::Debug = foo;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+    assert_eq!(foo.z(), 11);
+    assert_eq!(foo.y(), 12);
+}

src/test/ui/traits/trait-upcasting/cyclic-trait-resolution.rs

@@ -0,0 +1,13 @@
+trait A: B + A {}
+//~^ ERROR cycle detected when computing the super predicates of `A` [E0391]
+
+trait B {}
+
+impl A for () {}
+
+impl B for () {}
+
+fn main() {
+    let a: Box<dyn A> = Box::new(());
+    let _b: Box<dyn B> = a;
+}

src/test/ui/traits/trait-upcasting/cyclic-trait-resolution.stderr

@@ -0,0 +1,21 @@
+error[E0391]: cycle detected when computing the super predicates of `A`
+  --> $DIR/cyclic-trait-resolution.rs:1:1
+   |
+LL | trait A: B + A {}
+   | ^^^^^^^^^^^^^^
+   |
+note: ...which requires computing the super traits of `A`...
+  --> $DIR/cyclic-trait-resolution.rs:1:14
+   |
+LL | trait A: B + A {}
+   |              ^
+   = note: ...which again requires computing the super predicates of `A`, completing the cycle
+note: cycle used when collecting item types in top-level module
+  --> $DIR/cyclic-trait-resolution.rs:1:1
+   |
+LL | trait A: B + A {}
+   | ^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.

src/test/ui/traits/trait-upcasting/diamond.rs

@@ -0,0 +1,115 @@
+// run-pass
+
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
+    fn a(&self) -> i32 {
+        10
+    }
+
+    fn z(&self) -> i32 {
+        11
+    }
+
+    fn y(&self) -> i32 {
+        12
+    }
+}
+
+trait Bar1: Foo {
+    fn b(&self) -> i32 {
+        20
+    }
+
+    fn w(&self) -> i32 {
+        21
+    }
+}
+
+trait Bar2: Foo {
+    fn c(&self) -> i32 {
+        30
+    }
+
+    fn v(&self) -> i32 {
+        31
+    }
+}
+
+trait Baz: Bar1 + Bar2 {
+    fn d(&self) -> i32 {
+        40
+    }
+}
+
+impl Foo for i32 {
+    fn a(&self) -> i32 {
+        100
+    }
+}
+
+impl Bar1 for i32 {
+    fn b(&self) -> i32 {
+        200
+    }
+}
+
+impl Bar2 for i32 {
+    fn c(&self) -> i32 {
+        300
+    }
+}
+
+impl Baz for i32 {
+    fn d(&self) -> i32 {
+        400
+    }
+}
+
+fn main() {
+    let baz: &dyn Baz = &1;
+    let _: &dyn std::fmt::Debug = baz;
+    assert_eq!(*baz, 1);
+    assert_eq!(baz.a(), 100);
+    assert_eq!(baz.b(), 200);
+    assert_eq!(baz.c(), 300);
+    assert_eq!(baz.d(), 400);
+    assert_eq!(baz.z(), 11);
+    assert_eq!(baz.y(), 12);
+    assert_eq!(baz.w(), 21);
+    assert_eq!(baz.v(), 31);
+
+    let bar1: &dyn Bar1 = baz;
+    let _: &dyn std::fmt::Debug = bar1;
+    assert_eq!(*bar1, 1);
+    assert_eq!(bar1.a(), 100);
+    assert_eq!(bar1.b(), 200);
+    assert_eq!(bar1.z(), 11);
+    assert_eq!(bar1.y(), 12);
+    assert_eq!(bar1.w(), 21);
+
+    let bar2: &dyn Bar2 = baz;
+    let _: &dyn std::fmt::Debug = bar2;
+    assert_eq!(*bar2, 1);
+    assert_eq!(bar2.a(), 100);
+    assert_eq!(bar2.c(), 300);
+    assert_eq!(bar2.z(), 11);
+    assert_eq!(bar2.y(), 12);
+    assert_eq!(bar2.v(), 31);
+
+    let foo: &dyn Foo = baz;
+    let _: &dyn std::fmt::Debug = foo;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+
+    let foo: &dyn Foo = bar1;
+    let _: &dyn std::fmt::Debug = foo;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+
+    let foo: &dyn Foo = bar2;
+    let _: &dyn std::fmt::Debug = foo;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+}

src/test/ui/traits/trait-upcasting/invalid-upcast.rs

@@ -0,0 +1,87 @@
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait Foo {
+    fn a(&self) -> i32 {
+        10
+    }
+
+    fn z(&self) -> i32 {
+        11
+    }
+
+    fn y(&self) -> i32 {
+        12
+    }
+}
+
+trait Bar {
+    fn b(&self) -> i32 {
+        20
+    }
+
+    fn w(&self) -> i32 {
+        21
+    }
+}
+
+trait Baz {
+    fn c(&self) -> i32 {
+        30
+    }
+}
+
+impl Foo for i32 {
+    fn a(&self) -> i32 {
+        100
+    }
+}
+
+impl Bar for i32 {
+    fn b(&self) -> i32 {
+        200
+    }
+}
+
+impl Baz for i32 {
+    fn c(&self) -> i32 {
+        300
+    }
+}
+
+fn main() {
+    let baz: &dyn Baz = &1;
+    let _: &dyn std::fmt::Debug = baz;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn Send = baz;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn Sync = baz;
+    //~^ ERROR mismatched types [E0308]
+
+    let bar: &dyn Bar = baz;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn std::fmt::Debug = bar;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn Send = bar;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn Sync = bar;
+    //~^ ERROR mismatched types [E0308]
+
+    let foo: &dyn Foo = baz;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn std::fmt::Debug = foo;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn Send = foo;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn Sync = foo;
+    //~^ ERROR mismatched types [E0308]
+
+    let foo: &dyn Foo = bar;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn std::fmt::Debug = foo;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn Send = foo;
+    //~^ ERROR mismatched types [E0308]
+    let _: &dyn Sync = foo;
+    //~^ ERROR mismatched types [E0308]
+}

src/test/ui/traits/trait-upcasting/invalid-upcast.stderr

@@ -0,0 +1,168 @@
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:54:35
+   |
+LL |     let _: &dyn std::fmt::Debug = baz;
+   |            --------------------   ^^^ expected trait `Debug`, found trait `Baz`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Debug`
+              found reference `&dyn Baz`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:56:24
+   |
+LL |     let _: &dyn Send = baz;
+   |            ---------   ^^^ expected trait `Send`, found trait `Baz`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Send`
+              found reference `&dyn Baz`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:58:24
+   |
+LL |     let _: &dyn Sync = baz;
+   |            ---------   ^^^ expected trait `Sync`, found trait `Baz`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Sync`
+              found reference `&dyn Baz`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:61:25
+   |
+LL |     let bar: &dyn Bar = baz;
+   |              --------   ^^^ expected trait `Bar`, found trait `Baz`
+   |              |
+   |              expected due to this
+   |
+   = note: expected reference `&dyn Bar`
+              found reference `&dyn Baz`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:63:35
+   |
+LL |     let _: &dyn std::fmt::Debug = bar;
+   |            --------------------   ^^^ expected trait `Debug`, found trait `Bar`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Debug`
+              found reference `&dyn Bar`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:65:24
+   |
+LL |     let _: &dyn Send = bar;
+   |            ---------   ^^^ expected trait `Send`, found trait `Bar`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Send`
+              found reference `&dyn Bar`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:67:24
+   |
+LL |     let _: &dyn Sync = bar;
+   |            ---------   ^^^ expected trait `Sync`, found trait `Bar`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Sync`
+              found reference `&dyn Bar`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:70:25
+   |
+LL |     let foo: &dyn Foo = baz;
+   |              --------   ^^^ expected trait `Foo`, found trait `Baz`
+   |              |
+   |              expected due to this
+   |
+   = note: expected reference `&dyn Foo`
+              found reference `&dyn Baz`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:72:35
+   |
+LL |     let _: &dyn std::fmt::Debug = foo;
+   |            --------------------   ^^^ expected trait `Debug`, found trait `Foo`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Debug`
+              found reference `&dyn Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:74:24
+   |
+LL |     let _: &dyn Send = foo;
+   |            ---------   ^^^ expected trait `Send`, found trait `Foo`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Send`
+              found reference `&dyn Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:76:24
+   |
+LL |     let _: &dyn Sync = foo;
+   |            ---------   ^^^ expected trait `Sync`, found trait `Foo`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Sync`
+              found reference `&dyn Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:79:25
+   |
+LL |     let foo: &dyn Foo = bar;
+   |              --------   ^^^ expected trait `Foo`, found trait `Bar`
+   |              |
+   |              expected due to this
+   |
+   = note: expected reference `&dyn Foo`
+              found reference `&dyn Bar`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:81:35
+   |
+LL |     let _: &dyn std::fmt::Debug = foo;
+   |            --------------------   ^^^ expected trait `Debug`, found trait `Foo`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Debug`
+              found reference `&dyn Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:83:24
+   |
+LL |     let _: &dyn Send = foo;
+   |            ---------   ^^^ expected trait `Send`, found trait `Foo`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Send`
+              found reference `&dyn Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/invalid-upcast.rs:85:24
+   |
+LL |     let _: &dyn Sync = foo;
+   |            ---------   ^^^ expected trait `Sync`, found trait `Foo`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Sync`
+              found reference `&dyn Foo`
+
+error: aborting due to 15 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/traits/trait-upcasting/lifetime.rs

@@ -0,0 +1,96 @@
+// run-pass
+// ignore-compare-mode-nll
+
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
+    fn a(&self) -> i32 {
+        10
+    }
+
+    fn z(&self) -> i32 {
+        11
+    }
+
+    fn y(&self) -> i32 {
+        12
+    }
+}
+
+trait Bar: Foo {
+    fn b(&self) -> i32 {
+        20
+    }
+
+    fn w(&self) -> i32 {
+        21
+    }
+}
+
+trait Baz: Bar {
+    fn c(&self) -> i32 {
+        30
+    }
+}
+
+impl Foo for i32 {
+    fn a(&self) -> i32 {
+        100
+    }
+}
+
+impl Bar for i32 {
+    fn b(&self) -> i32 {
+        200
+    }
+}
+
+impl Baz for i32 {
+    fn c(&self) -> i32 {
+        300
+    }
+}
+
+// Note: upcast lifetime means a shorter lifetime.
+fn upcast_baz<'a: 'b, 'b, T>(v: Box<dyn Baz + 'a>, _l: &'b T) -> Box<dyn Baz + 'b> {
+    v
+}
+fn upcast_bar<'a: 'b, 'b, T>(v: Box<dyn Bar + 'a>, _l: &'b T) -> Box<dyn Bar + 'b> {
+    v
+}
+fn upcast_foo<'a: 'b, 'b, T>(v: Box<dyn Foo + 'a>, _l: &'b T) -> Box<dyn Foo + 'b> {
+    v
+}
+
+fn main() {
+    let v = Box::new(1);
+    let l = &(); // dummy lifetime (shorter than `baz`)
+
+    let baz: Box<dyn Baz> = v.clone();
+    let u = upcast_baz(baz, &l);
+    assert_eq!(*u, 1);
+    assert_eq!(u.a(), 100);
+    assert_eq!(u.b(), 200);
+    assert_eq!(u.c(), 300);
+
+    let baz: Box<dyn Baz> = v.clone();
+    let bar: Box<dyn Bar> = baz;
+    let u = upcast_bar(bar, &l);
+    assert_eq!(*u, 1);
+    assert_eq!(u.a(), 100);
+    assert_eq!(u.b(), 200);
+
+    let baz: Box<dyn Baz> = v.clone();
+    let foo: Box<dyn Foo> = baz;
+    let u = upcast_foo(foo, &l);
+    assert_eq!(*u, 1);
+    assert_eq!(u.a(), 100);
+
+    let baz: Box<dyn Baz> = v.clone();
+    let bar: Box<dyn Bar> = baz;
+    let foo: Box<dyn Foo> = bar;
+    let u = upcast_foo(foo, &l);
+    assert_eq!(*u, 1);
+    assert_eq!(u.a(), 100);
+}

src/test/ui/traits/trait-upcasting/replace-vptr.rs

@@ -0,0 +1,49 @@
+// run-pass
+
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait A {
+    fn foo_a(&self);
+}
+
+trait B {
+    fn foo_b(&self);
+}
+
+trait C: A + B {
+    fn foo_c(&self);
+}
+
+struct S(i32);
+
+impl A for S {
+    fn foo_a(&self) {
+        unreachable!();
+    }
+}
+
+impl B for S {
+    fn foo_b(&self) {
+        assert_eq!(42, self.0);
+    }
+}
+
+impl C for S {
+    fn foo_c(&self) {
+        unreachable!();
+    }
+}
+
+fn invoke_inner(b: &dyn B) {
+    b.foo_b();
+}
+
+fn invoke_outer(c: &dyn C) {
+    invoke_inner(c);
+}
+
+fn main() {
+    let s = S(42);
+    invoke_outer(&s);
+}

src/test/ui/traits/trait-upcasting/struct.rs

@@ -0,0 +1,174 @@
+// run-pass
+
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+use std::rc::Rc;
+use std::sync::Arc;
+
+trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
+    fn a(&self) -> i32 {
+        10
+    }
+
+    fn z(&self) -> i32 {
+        11
+    }
+
+    fn y(&self) -> i32 {
+        12
+    }
+}
+
+trait Bar: Foo {
+    fn b(&self) -> i32 {
+        20
+    }
+
+    fn w(&self) -> i32 {
+        21
+    }
+}
+
+trait Baz: Bar {
+    fn c(&self) -> i32 {
+        30
+    }
+}
+
+impl Foo for i32 {
+    fn a(&self) -> i32 {
+        100
+    }
+}
+
+impl Bar for i32 {
+    fn b(&self) -> i32 {
+        200
+    }
+}
+
+impl Baz for i32 {
+    fn c(&self) -> i32 {
+        300
+    }
+}
+
+fn test_box() {
+    let v = Box::new(1);
+
+    let baz: Box<dyn Baz> = v.clone();
+    assert_eq!(*baz, 1);
+    assert_eq!(baz.a(), 100);
+    assert_eq!(baz.b(), 200);
+    assert_eq!(baz.c(), 300);
+    assert_eq!(baz.z(), 11);
+    assert_eq!(baz.y(), 12);
+    assert_eq!(baz.w(), 21);
+
+    let baz: Box<dyn Baz> = v.clone();
+    let bar: Box<dyn Bar> = baz;
+    assert_eq!(*bar, 1);
+    assert_eq!(bar.a(), 100);
+    assert_eq!(bar.b(), 200);
+    assert_eq!(bar.z(), 11);
+    assert_eq!(bar.y(), 12);
+    assert_eq!(bar.w(), 21);
+
+    let baz: Box<dyn Baz> = v.clone();
+    let foo: Box<dyn Foo> = baz;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+    assert_eq!(foo.z(), 11);
+    assert_eq!(foo.y(), 12);
+
+    let baz: Box<dyn Baz> = v.clone();
+    let bar: Box<dyn Bar> = baz;
+    let foo: Box<dyn Foo> = bar;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+    assert_eq!(foo.z(), 11);
+    assert_eq!(foo.y(), 12);
+}
+
+fn test_rc() {
+    let v = Rc::new(1);
+
+    let baz: Rc<dyn Baz> = v.clone();
+    assert_eq!(*baz, 1);
+    assert_eq!(baz.a(), 100);
+    assert_eq!(baz.b(), 200);
+    assert_eq!(baz.c(), 300);
+    assert_eq!(baz.z(), 11);
+    assert_eq!(baz.y(), 12);
+    assert_eq!(baz.w(), 21);
+
+    let baz: Rc<dyn Baz> = v.clone();
+    let bar: Rc<dyn Bar> = baz;
+    assert_eq!(*bar, 1);
+    assert_eq!(bar.a(), 100);
+    assert_eq!(bar.b(), 200);
+    assert_eq!(bar.z(), 11);
+    assert_eq!(bar.y(), 12);
+    assert_eq!(bar.w(), 21);
+
+    let baz: Rc<dyn Baz> = v.clone();
+    let foo: Rc<dyn Foo> = baz;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+    assert_eq!(foo.z(), 11);
+    assert_eq!(foo.y(), 12);
+
+    let baz: Rc<dyn Baz> = v.clone();
+    let bar: Rc<dyn Bar> = baz;
+    let foo: Rc<dyn Foo> = bar;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+    assert_eq!(foo.z(), 11);
+    assert_eq!(foo.y(), 12);
+    assert_eq!(foo.z(), 11);
+    assert_eq!(foo.y(), 12);
+}
+
+fn test_arc() {
+    let v = Arc::new(1);
+
+    let baz: Arc<dyn Baz> = v.clone();
+    assert_eq!(*baz, 1);
+    assert_eq!(baz.a(), 100);
+    assert_eq!(baz.b(), 200);
+    assert_eq!(baz.c(), 300);
+    assert_eq!(baz.z(), 11);
+    assert_eq!(baz.y(), 12);
+    assert_eq!(baz.w(), 21);
+
+    let baz: Arc<dyn Baz> = v.clone();
+    let bar: Arc<dyn Bar> = baz;
+    assert_eq!(*bar, 1);
+    assert_eq!(bar.a(), 100);
+    assert_eq!(bar.b(), 200);
+    assert_eq!(bar.z(), 11);
+    assert_eq!(bar.y(), 12);
+    assert_eq!(bar.w(), 21);
+
+    let baz: Arc<dyn Baz> = v.clone();
+    let foo: Arc<dyn Foo> = baz;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+    assert_eq!(foo.z(), 11);
+    assert_eq!(foo.y(), 12);
+
+    let baz: Arc<dyn Baz> = v.clone();
+    let bar: Arc<dyn Bar> = baz;
+    let foo: Arc<dyn Foo> = bar;
+    assert_eq!(*foo, 1);
+    assert_eq!(foo.a(), 100);
+    assert_eq!(foo.z(), 11);
+    assert_eq!(foo.y(), 12);
+}
+
+fn main() {
+    test_box();
+    test_rc();
+    test_arc();
+}

src/test/ui/traits/trait-upcasting/subtrait-method.rs

@@ -0,0 +1,70 @@
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
+    fn a(&self) -> i32 {
+        10
+    }
+
+    fn z(&self) -> i32 {
+        11
+    }
+
+    fn y(&self) -> i32 {
+        12
+    }
+}
+
+trait Bar: Foo {
+    fn b(&self) -> i32 {
+        20
+    }
+
+    fn w(&self) -> i32 {
+        21
+    }
+}
+
+trait Baz: Bar {
+    fn c(&self) -> i32 {
+        30
+    }
+}
+
+impl Foo for i32 {
+    fn a(&self) -> i32 {
+        100
+    }
+}
+
+impl Bar for i32 {
+    fn b(&self) -> i32 {
+        200
+    }
+}
+
+impl Baz for i32 {
+    fn c(&self) -> i32 {
+        300
+    }
+}
+
+fn main() {
+    let baz: &dyn Baz = &1;
+
+    let bar: &dyn Bar = baz;
+    bar.c();
+    //~^ ERROR no method named `c` found for reference `&dyn Bar` in the current scope [E0599]
+
+    let foo: &dyn Foo = baz;
+    foo.b();
+    //~^ ERROR no method named `b` found for reference `&dyn Foo` in the current scope [E0599]
+    foo.c();
+    //~^ ERROR no method named `c` found for reference `&dyn Foo` in the current scope [E0599]
+
+    let foo: &dyn Foo = bar;
+    foo.b();
+    //~^ ERROR no method named `b` found for reference `&dyn Foo` in the current scope [E0599]
+    foo.c();
+    //~^ ERROR no method named `c` found for reference `&dyn Foo` in the current scope [E0599]
+}

src/test/ui/traits/trait-upcasting/subtrait-method.stderr

@@ -0,0 +1,68 @@
+error[E0599]: no method named `c` found for reference `&dyn Bar` in the current scope
+  --> $DIR/subtrait-method.rs:56:9
+   |
+LL |     bar.c();
+   |         ^ help: there is an associated function with a similar name: `a`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+note: `Baz` defines an item `c`, perhaps you need to implement it
+  --> $DIR/subtrait-method.rs:28:1
+   |
+LL | trait Baz: Bar {
+   | ^^^^^^^^^^^^^^
+
+error[E0599]: no method named `b` found for reference `&dyn Foo` in the current scope
+  --> $DIR/subtrait-method.rs:60:9
+   |
+LL |     foo.b();
+   |         ^ help: there is an associated function with a similar name: `a`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+note: `Bar` defines an item `b`, perhaps you need to implement it
+  --> $DIR/subtrait-method.rs:18:1
+   |
+LL | trait Bar: Foo {
+   | ^^^^^^^^^^^^^^
+
+error[E0599]: no method named `c` found for reference `&dyn Foo` in the current scope
+  --> $DIR/subtrait-method.rs:62:9
+   |
+LL |     foo.c();
+   |         ^ help: there is an associated function with a similar name: `a`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+note: `Baz` defines an item `c`, perhaps you need to implement it
+  --> $DIR/subtrait-method.rs:28:1
+   |
+LL | trait Baz: Bar {
+   | ^^^^^^^^^^^^^^
+
+error[E0599]: no method named `b` found for reference `&dyn Foo` in the current scope
+  --> $DIR/subtrait-method.rs:66:9
+   |
+LL |     foo.b();
+   |         ^ help: there is an associated function with a similar name: `a`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+note: `Bar` defines an item `b`, perhaps you need to implement it
+  --> $DIR/subtrait-method.rs:18:1
+   |
+LL | trait Bar: Foo {
+   | ^^^^^^^^^^^^^^
+
+error[E0599]: no method named `c` found for reference `&dyn Foo` in the current scope
+  --> $DIR/subtrait-method.rs:68:9
+   |
+LL |     foo.c();
+   |         ^ help: there is an associated function with a similar name: `a`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+note: `Baz` defines an item `c`, perhaps you need to implement it
+  --> $DIR/subtrait-method.rs:28:1
+   |
+LL | trait Baz: Bar {
+   | ^^^^^^^^^^^^^^
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0599`.