make TrivialClone a #[marker]-trait to keep it from appearing in error messages

Author: joboet

library/core/src/clone.rs

@@ -275,6 +275,10 @@ pub const trait Clone: Sized {
 // its invariant holds whenever `Clone` is implemented, even if the actual
 // `TrivialClone` bound would not be satisfied because of lifetime bounds.
 #[rustc_unsafe_specialization_marker]
+// If `#[derive(Clone, Clone, Copy)]` is written, there will be multiple
+// implementations of `TrivialClone`. To keep it from appearing in error
+// messages, make it a `#[marker]` trait.
+#[marker]
 pub const unsafe trait TrivialClone: [const] Clone {}
 
 /// Derive macro generating an impl of the trait `Clone`.

tests/ui/derives/duplicate-derive-copy-clone-diagnostics.rs

@@ -6,7 +6,6 @@
 #[derive(Copy, Clone)]
 //~^ ERROR conflicting implementations of trait `Clone` for type `E`
 //~| ERROR conflicting implementations of trait `Copy` for type `E`
-//~| ERROR conflicting implementations of trait `TrivialClone` for type `E`
 enum E {}
 
 fn main() {}

tests/ui/derives/duplicate-derive-copy-clone-diagnostics.stderr

@@ -6,16 +6,6 @@ LL | #[derive(Copy, Clone)]
 LL | #[derive(Copy, Clone)]
    |          ^^^^ conflicting implementation for `E`
 
-error[E0119]: conflicting implementations of trait `TrivialClone` for type `E`
-  --> $DIR/duplicate-derive-copy-clone-diagnostics.rs:6:16
-   |
-LL | #[derive(Copy, Clone)]
-   |                ----- first implementation here
-LL | #[derive(Copy, Clone)]
-   |                ^^^^^ conflicting implementation for `E`
-   |
-   = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)
-
 error[E0119]: conflicting implementations of trait `Clone` for type `E`
   --> $DIR/duplicate-derive-copy-clone-diagnostics.rs:6:16
    |
@@ -24,6 +14,6 @@ LL | #[derive(Copy, Clone)]
 LL | #[derive(Copy, Clone)]
    |                ^^^^^ conflicting implementation for `E`
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0119`.