fix(rustc_typeck): produce better errors for dyn auto trait

Fixes #85026

compiler/rustc_error_codes/src/error_codes.rs

@@ -480,6 +480,7 @@ E0781: include_str!("./error_codes/E0781.md"),
 E0782: include_str!("./error_codes/E0782.md"),
 E0783: include_str!("./error_codes/E0783.md"),
 E0784: include_str!("./error_codes/E0784.md"),
+E0785: include_str!("./error_codes/E0785.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

compiler/rustc_error_codes/src/error_codes/E0785.md

@@ -0,0 +1,30 @@
+An inherent `impl` was written on a dyn auto trait.
+
+Erroneous code example:
+
+```compile_fail,E0785
+#![feature(auto_traits)]
+
+auto trait AutoTrait {}
+
+impl dyn AutoTrait {}
+```
+
+Dyn objects allow any number of auto traits, plus at most one non-auto trait.
+The non-auto trait becomes the "principal trait".
+
+When checking if an impl on a dyn trait is coherent, the principal trait is
+normally the only one considered. Since the erroneous code has no principal
+trait, it cannot be implemented at all.
+
+Working example:
+
+```
+#![feature(auto_traits)]
+
+trait PrincipalTrait {}
+
+auto trait AutoTrait {}
+
+impl dyn (PrincipalTrait + AutoTrait + Send) {}
+```

compiler/rustc_typeck/src/coherence/inherent_impls.rs

@@ -60,6 +60,17 @@ impl ItemLikeVisitor<'v> for InherentCollect<'tcx> {
             ty::Dynamic(ref data, ..) if data.principal_def_id().is_some() => {
                 self.check_def_id(item, data.principal_def_id().unwrap());
             }
+            ty::Dynamic(..) => {
+                struct_span_err!(
+                    self.tcx.sess,
+                    ty.span,
+                    E0785,
+                    "cannot define inherent `impl` for a dyn auto trait"
+                )
+                .span_label(ty.span, "impl requires a principal trait")
+                .note("define and implement a new trait or type instead")
+                .emit();
+            }
             ty::Bool => {
                 self.check_primitive_impl(
                     item.def_id,

src/test/ui/coherence/issue-85026.rs

@@ -0,0 +1,10 @@
+#![feature(auto_traits)]
+auto trait AutoTrait {}
+
+// You cannot impl your own `dyn AutoTrait`.
+impl dyn AutoTrait {} //~ERROR E0785
+
+// You cannot impl someone else's `dyn AutoTrait`
+impl dyn Unpin {} //~ERROR E0785
+
+fn main() {}

src/test/ui/coherence/issue-85026.stderr

@@ -0,0 +1,19 @@
+error[E0785]: cannot define inherent `impl` for a dyn auto trait
+  --> $DIR/issue-85026.rs:5:6
+   |
+LL | impl dyn AutoTrait {}
+   |      ^^^^^^^^^^^^^ impl requires a principal trait
+   |
+   = note: define and implement a new trait or type instead
+
+error[E0785]: cannot define inherent `impl` for a dyn auto trait
+  --> $DIR/issue-85026.rs:8:6
+   |
+LL | impl dyn Unpin {}
+   |      ^^^^^^^^^ impl requires a principal trait
+   |
+   = note: define and implement a new trait or type instead
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0785`.