error[E0119]: conflicting implementations of trait `MyTrait` for type `String`
  --> $DIR/ambiguity-cause.rs:10:1
   |
LL | impl<T: Copy> MyTrait for T { }
   | --------------------------- first implementation here
LL |
LL | impl MyTrait for String { }
   | ^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `String`
   |
   = note: upstream crates may add a new impl of trait `std::marker::Copy` for type `std::string::String` in future versions

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0119`.