Rollup merge of rust-lang#96378 - compiler-errors:trait-upcast-error, r=nagisa

Mention traits and types involved in unstable trait upcasting

Fixes rust-lang#95972  by printing the traits being upcasted and the types being coerced that cause that upcasting...

---

the poor span mentioned in the original issue has nothing to do with trait upcasting diagnostic here...

>  The original example I had that made me run into this issue had an even longer expression there (multiple chained
iterator methods) which just got all highlighted as one big block saying "somewhere here trait coercion is used and it's not allowed".

I don't think I can solve that issue in general without fixing the ObligationCauseCode and span that gets passed into Coerce.

Author: Dylan-DPC

compiler/rustc_typeck/src/check/coercion.rs

@@ -616,7 +616,7 @@ 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
@@ -636,7 +636,7 @@ 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");
@@ -707,14 +707,19 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             .emit();
         }
 
-        if has_trait_upcasting_coercion && !self.tcx().features().trait_upcasting {
-            feature_err(
+        if let Some((sub, sup)) = has_trait_upcasting_coercion
+            && !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",
-            )
-            .emit();
+                &format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
+            );
+            err.note(&format!("required when coercing `{source}` into `{target}`"));
+            err.emit();
         }
 
         Ok(coercion)

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

@@ -1,11 +1,12 @@
-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;
    |                         ^^^
    |
    = 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

@@ -1,11 +1,12 @@
-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 };
    |                                 ^^^^^^^
    |
    = 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