Mention traits being upcasted, types being coerced

compiler/rustc_typeck/src/check/coercion.rs

@@ -615,7 +615,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         )];
 
         let mut has_unsized_tuple_coercion = false;
-        let mut has_trait_upcasting_coercion = false;
+        let mut has_trait_upcasting_coercion = None;
 
         // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
         // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
@@ -635,7 +635,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                             && data_a.principal_def_id() != data_b.principal_def_id()
                         {
                             debug!("coerce_unsized: found trait upcasting coercion");
-                            has_trait_upcasting_coercion = true;
+                            has_trait_upcasting_coercion = Some((self_ty, unsize_ty));
                         }
                         if let ty::Tuple(..) = unsize_ty.kind() {
                             debug!("coerce_unsized: found unsized tuple coercion");
@@ -706,14 +706,19 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             .emit();
         }
 
-        if has_trait_upcasting_coercion && !self.tcx().features().trait_upcasting {
+        if let Some((sub, sup)) = has_trait_upcasting_coercion
-            feature_err(
+            && !self.tcx().features().trait_upcasting
+        {
+            // Renders better when we erase regions, since they're not really the point here.
+            let (sub, sup) = self.tcx.erase_regions((sub, sup));
+            let mut err = feature_err(
                 &self.tcx.sess.parse_sess,
                 sym::trait_upcasting,
                 self.cause.span,
-                "trait upcasting coercion is experimental",
+                &format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
-            )
+            );
-            .emit();
+            err.note(&format!("required when coercing `{source}` into `{target}`"));
+            err.emit();
         }
 
         Ok(coercion)

src/test/ui/feature-gates/feature-gate-trait_upcasting.stderr

@@ -1,4 +1,4 @@
-error[E0658]: trait upcasting coercion is experimental
+error[E0658]: cannot cast `dyn Bar` to `dyn Foo`, trait upcasting coercion is experimental
   --> $DIR/feature-gate-trait_upcasting.rs:11:25
    |
 LL |     let foo: &dyn Foo = bar;
@@ -6,6 +6,7 @@ LL |     let foo: &dyn Foo = bar;
    |
    = note: see issue #65991 <https://github.com/rust-lang/rust/issues/65991> for more information
    = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable
+   = note: required when coercing `&dyn Bar` into `&dyn Foo`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-11515.stderr

@@ -1,4 +1,4 @@
-error[E0658]: trait upcasting coercion is experimental
+error[E0658]: cannot cast `dyn Fn()` to `dyn FnMut()`, trait upcasting coercion is experimental
   --> $DIR/issue-11515.rs:9:33
    |
 LL |     let test = box Test { func: closure };
@@ -6,6 +6,7 @@ LL |     let test = box Test { func: closure };
    |
    = note: see issue #65991 <https://github.com/rust-lang/rust/issues/65991> for more information
    = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable
+   = note: required when coercing `Box<(dyn Fn() + 'static)>` into `Box<(dyn FnMut() + 'static)>`
 
 error: aborting due to previous error