Opt-in built-in traits allowed one to explicitly implement both `Drop`
and `Copy` for a type. This can theoretically make some sense, but the
current implementation means it is codegened totally incorrectly which
can lead to memory unsafety, so this feature is disabled for now.
Fixes#20126.
- the self type includes some local type; and,
- type parameters in the self type must be constrained by a local type.
A type parameter is called *constrained* if it appears in some type-parameter of a local type.
Here are some examples that are accepted. In all of these examples, I
assume that `Foo` is a trait defined in another crate. If `Foo` were
defined in the local crate, then all the examples would be legal.
- `impl Foo for LocalType`
- `impl<T> Foo<T> for LocalType` -- T does not appear in Self, so it is OK
- `impl<T> Foo<T> for LocalType<T>` -- T here is constrained by LocalType
- `impl<T> Foo<T> for (LocalType<T>, T)` -- T here is constrained by LocalType
Here are some illegal examples (again, these examples assume that
`Foo` is not local to the current crate):
- `impl Foo for int` -- the Self type is not local
- `impl<T> Foo for T` -- T appears in Self unconstrained by a local type
- `impl<T> Foo for (LocalType, T)` -- T appears in Self unconstrained by a local type
This is a [breaking-change]. For the time being, you can opt out of
the new rules by placing `#[old_orphan_check]` on the trait (and
enabling the feature gate where the trait is defined). Longer term,
you should restructure your traits to avoid the problem. Usually this
means changing the order of parameters so that the "central" type
parameter is in the `Self` position.
As an example of that refactoring, consider the `BorrowFrom` trait:
```rust
pub trait BorrowFrom<Sized? Owned> for Sized? {
fn borrow_from(owned: &Owned) -> &Self;
}
```
As defined, this trait is commonly implemented for custom pointer
types, such as `Arc`. Those impls follow the pattern:
```rust
impl<T> BorrowFrom<Arc<T>> for T {...}
```
Unfortunately, this impl is illegal because the self type `T` is not
local to the current crate. Therefore, we are going to change the order of the parameters,
so that `BorrowFrom` becomes `Borrow`:
```rust
pub trait Borrow<Sized? Borrowed> for Sized? {
fn borrow_from(owned: &Self) -> &Borrowed;
}
```
Now the `Arc` impl is written:
```rust
impl<T> Borrow<T> for Arc<T> { ... }
```
This impl is legal because the self type (`Arc<T>`) is local.
The `print!` and `println!` macros are now the preferred method of printing, and so there is no reason to export the `stdio` functions in the prelude. The functions have also been replaced by their macro counterparts in the tutorial and other documentation so that newcomers don't get confused about what they should be using.
"Dual impls" are impls that are both type implementations and trait
implementations. They can lead to ambiguity and so this patch removes them
from the language.
This also enforces coherence rules. Without this patch, records can implement
traits not defined in the current crate. This patch fixes this, and updates
all of rustc to adhere to the new enforcement. Most of this patch is fixing
rustc to obey the coherence rules, which involves converting a bunch of records
to structs.
