Auto merge of #101832 - compiler-errors:dyn-star-plus, r=eholk

Make `dyn*` casts into a coercion, allow `dyn*` upcasting

I know that `dyn*` is likely not going to be a feature exposed to surface Rust, but this makes it slightly more ergonomic to write tests for these types anyways. ... and this was just fun to implement anyways.

1. Make `dyn*` into a coercion instead of a cast
2. Enable `dyn*` upcasting since we basically get it for free
3. Simplify some of the cast checking code since we're using the coercion path now

r? `@eholk` but feel free to reassign
cc `@nikomatsakis` and `@tmandry` who might care about making `dyn*` casts into a coercion

10 files changed, 81 insertions, 5 deletions

diff --git a/src/test/ui/dyn-star/auxiliary/dyn-star-foreign.rs b/src/test/ui/dyn-star/auxiliary/dyn-star-foreign.rs
new file mode 100644
index 00000000000..7673c793678
--- /dev/null
+++ b/src/test/ui/dyn-star/auxiliary/dyn-star-foreign.rs
@@ -0,0 +1,10 @@
+#![feature(dyn_star)]
+#![allow(incomplete_features)]
+
+use std::fmt::Display;
+
+pub fn require_dyn_star_display(_: dyn* Display) {}
+
+fn works_locally() {
+    require_dyn_star_display(1usize);
+}
diff --git a/src/test/ui/dyn-star/const.rs b/src/test/ui/dyn-star/const.rs
index e49caf649f8..67e3ab7ab35 100644
--- a/src/test/ui/dyn-star/const.rs
+++ b/src/test/ui/dyn-star/const.rs
@@ -6,7 +6,7 @@ use std::fmt::Debug;
 
 fn make_dyn_star() {
     let i = 42usize;
-    let dyn_i: dyn* Debug = i as dyn* Debug;
+    let dyn_i: dyn* Debug = i;
 }
 
 fn main() {
diff --git a/src/test/ui/dyn-star/drop.rs b/src/test/ui/dyn-star/drop.rs
index 46b232f3dd3..1478498c0a9 100644
--- a/src/test/ui/dyn-star/drop.rs
+++ b/src/test/ui/dyn-star/drop.rs
@@ -15,7 +15,7 @@ impl Drop for Foo {
 }
 
 fn make_dyn_star(i: Foo) {
-    let _dyn_i: dyn* Debug = i as dyn* Debug;
+    let _dyn_i: dyn* Debug = i;
 }
 
 fn main() {
diff --git a/src/test/ui/dyn-star/error.rs b/src/test/ui/dyn-star/error.rs
index 33eff80a5fe..d8261387efa 100644
--- a/src/test/ui/dyn-star/error.rs
+++ b/src/test/ui/dyn-star/error.rs
@@ -7,7 +7,7 @@ trait Foo {}
 
 fn make_dyn_star() {
     let i = 42;
-    let dyn_i: dyn* Foo = i as dyn* Foo; //~ ERROR trait bound `{integer}: Foo` is not satisfied
+    let dyn_i: dyn* Foo = i; //~ ERROR trait bound `{integer}: Foo` is not satisfied
 }
 
 fn main() {}
diff --git a/src/test/ui/dyn-star/error.stderr b/src/test/ui/dyn-star/error.stderr
index d612ccc630e..ae54b9ca707 100644
--- a/src/test/ui/dyn-star/error.stderr
+++ b/src/test/ui/dyn-star/error.stderr
@@ -1,7 +1,7 @@
 error[E0277]: the trait bound `{integer}: Foo` is not satisfied
   --> $DIR/error.rs:10:27
    |
-LL |     let dyn_i: dyn* Foo = i as dyn* Foo;
+LL |     let dyn_i: dyn* Foo = i;
    |                           ^ the trait `Foo` is not implemented for `{integer}`
 
 error: aborting due to previous error
diff --git a/src/test/ui/dyn-star/make-dyn-star.rs b/src/test/ui/dyn-star/make-dyn-star.rs
index 708ffa25d6f..e5255a64ba1 100644
--- a/src/test/ui/dyn-star/make-dyn-star.rs
+++ b/src/test/ui/dyn-star/make-dyn-star.rs
@@ -5,9 +5,14 @@
 use std::fmt::Debug;
 
 fn make_dyn_star(i: usize) {
+    let _dyn_i: dyn* Debug = i;
+}
+
+fn make_dyn_star_explicit(i: usize) {
     let _dyn_i: dyn* Debug = i as dyn* Debug;
 }
 
 fn main() {
     make_dyn_star(42);
+    make_dyn_star_explicit(42);
 }
diff --git a/src/test/ui/dyn-star/method.rs b/src/test/ui/dyn-star/method.rs
index d04958ca2aa..5a77640f0d9 100644
--- a/src/test/ui/dyn-star/method.rs
+++ b/src/test/ui/dyn-star/method.rs
@@ -1,4 +1,5 @@
 // run-pass
+
 #![feature(dyn_star)]
 #![allow(incomplete_features)]
 
@@ -17,7 +18,7 @@ fn invoke_dyn_star(i: dyn* Foo) -> usize {
 }
 
 fn make_and_invoke_dyn_star(i: usize) -> usize {
-    let dyn_i: dyn* Foo = i as dyn* Foo;
+    let dyn_i: dyn* Foo = i;
     invoke_dyn_star(dyn_i)
 }
 
diff --git a/src/test/ui/dyn-star/no-implicit-dyn-star.rs b/src/test/ui/dyn-star/no-implicit-dyn-star.rs
new file mode 100644
index 00000000000..d9470e28417
--- /dev/null
+++ b/src/test/ui/dyn-star/no-implicit-dyn-star.rs
@@ -0,0 +1,8 @@
+// aux-build:dyn-star-foreign.rs
+
+extern crate dyn_star_foreign;
+
+fn main() {
+    dyn_star_foreign::require_dyn_star_display(1usize);
+    //~^ ERROR mismatched types
+}
diff --git a/src/test/ui/dyn-star/no-implicit-dyn-star.stderr b/src/test/ui/dyn-star/no-implicit-dyn-star.stderr
new file mode 100644
index 00000000000..e7c5918629b
--- /dev/null
+++ b/src/test/ui/dyn-star/no-implicit-dyn-star.stderr
@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/no-implicit-dyn-star.rs:6:48
+   |
+LL |     dyn_star_foreign::require_dyn_star_display(1usize);
+   |     ------------------------------------------ ^^^^^^ expected trait object `dyn std::fmt::Display`, found `usize`
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected trait object `(dyn* std::fmt::Display + 'static)`
+                      found type `usize`
+note: function defined here
+  --> $DIR/auxiliary/dyn-star-foreign.rs:6:8
+   |
+LL | pub fn require_dyn_star_display(_: dyn* Display) {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/dyn-star/upcast.rs b/src/test/ui/dyn-star/upcast.rs
new file mode 100644
index 00000000000..cee76ada7df
--- /dev/null
+++ b/src/test/ui/dyn-star/upcast.rs
@@ -0,0 +1,33 @@
+// run-pass
+
+#![feature(dyn_star, trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait Foo: Bar {
+    fn hello(&self);
+}
+
+trait Bar {
+    fn world(&self);
+}
+
+struct W(usize);
+
+impl Foo for W {
+    fn hello(&self) {
+        println!("hello!");
+    }
+}
+
+impl Bar for W {
+    fn world(&self) {
+        println!("world!");
+    }
+}
+
+fn main() {
+    let w: dyn* Foo = W(0);
+    w.hello();
+    let w: dyn* Bar = w;
+    w.world();
+}