Auto merge of #56862 - arielb1:fundamentally-clean, r=nikomatsakis

stop treating trait objects from #[fundamental] traits as fundamental

This is a [breaking-change] to code that exploits this functionality (which should be limited to code using `#![feature(fundamental)]`.

Fixes #56503.

r? @nikomatsakis

src/test/ui/coherence/auxiliary/coherence_fundamental_trait_lib.rs

@@ -0,0 +1,7 @@
+#![crate_type = "rlib"]
+#![feature(fundamental)]
+
+pub trait Misc {}
+
+#[fundamental]
+pub trait Fundamental<T> {}

src/test/ui/coherence/coherence-fundamental-trait-objects.rs

@@ -0,0 +1,15 @@
+// Check that trait objects from #[fundamental] traits are not
+// treated as #[fundamental] types - the 2 meanings of #[fundamental]
+// are distinct.
+
+// aux-build:coherence_fundamental_trait_lib.rs
+
+extern crate coherence_fundamental_trait_lib;
+
+use coherence_fundamental_trait_lib::{Fundamental, Misc};
+
+pub struct Local;
+impl Misc for dyn Fundamental<Local> {}
+//~^ ERROR E0117
+
+fn main() {}

src/test/ui/coherence/coherence-fundamental-trait-objects.stderr

@@ -0,0 +1,12 @@
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/coherence-fundamental-trait-objects.rs:12:1
+   |
+LL | impl Misc for dyn Fundamental<Local> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl doesn't use types inside crate
+   |
+   = note: the impl does not reference any types defined in this crate
+   = note: define and implement a trait or new type instead
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0117`.