Stabilize Vec::leak as a method
Closes https://github.com/rust-lang/rust/issues/62195
The signature is changed to a method rather than an associated function:
```diff
-pub fn leak<'a>(vec: Vec<T>) -> &'a mut [T]
+pub fn leak<'a>(self) -> &'a mut [T]
```
The reason for `Box::leak` not to be a method (`Deref` to an arbitrary `T` which might have its own, different `leak` method) does not apply.
add `slice::array_chunks` to std
Now that #74113 has landed, these methods are suddenly usable. A rebirth of #72334
Tests are directly copied from `chunks_exact` and some additional tests for type inference.
r? @withoutboats as you are both part of t-libs and working on const generics. closes#60735
Previously `std::fs::File::metadata` on wasm32-wasi would call `fd_filestat_get`
to get metadata associated with fd, but that fd is opened without
RIGHTS_FD_FILESTAT_GET right, so it will failed on correctly implemented WASI
environment.
This change instead to add the missing rights when opening an fd.
Make `Option::unwrap` unstably const
This is lumped into the `const_option` feature gate (#67441), which enables a potpourri of `Option` methods.
cc @rust-lang/wg-const-eval
r? @oli-obk
Don't use "weak count" around Weak::from_raw_ptr
As `Rc/Arc::weak_count` returns 0 when having no strong counts, this
could be confusing and it's better to avoid using that completely.
Closes#73840.
`Result::unwrap` is not eligible becuase it formats the contents of the
`Err` variant. `unwrap_or`, `unwrap_or_else` and friends are not
eligible because they drop things or invoke closures.
Make `mem::size_of_val` and `mem::align_of_val` unstably const
Implements #46571 but does not stabilize it. I wanted this while working on something today.
The only reason not to immediately stabilize are concerns around [custom DSTs](https://github.com/rust-lang/rust/issues/46571#issuecomment-387669352). That proposal has made zero progress in the last two years and const eval is rich enough to support pretty much any user-defined `len` function as long as nightly features are allowed (`raw_ptr_deref`).
Currently, this raises a `const_err` lint when passed an `extern type`.
r? @oli-obk
cc @rust-lang/wg-const-eval
Remove links to rejected errata 4406 for RFC 4291
Fixes#74198.
For now I simply removed the links, the docs seems clear enough to me but I'm no expert in the domain so don't hesitate to correct me if more is needed.
cc @ghanan94.
@rustbot modify labels: T-doc, T-libs
Stabilize const_type_id feature
The tracking issue for `const_type_id` points to the ill-fated #41875. So I'm re-energizing `TypeId` shenanigans by opening this one up to see if there's anything blocking us from stabilizing the constification of type ids.
Will wait for CI before pinging teams/groups.
-----
This PR stabilizes the `const_type_id` feature, which allows `TypeId::of` (and the underlying unstable intrinsic) to be called in constant contexts.
There are some [sanity tests](https://github.com/rust-lang/rust/blob/master/src/test/ui/consts/const-typeid-of-rpass.rs) that demonstrate its usage, but I’ve included some more below.
As a simple example, you could create a constant item that contains some type ids:
```rust
use std::any::TypeId;
const TYPE_IDS: [TypeId; 2] = [
TypeId::of::<u32>(),
TypeId::of::<i32>(),
];
assert_eq!(TypeId::of::<u32>(), TYPE_IDS[0]);
```
Type ids can also now appear in associated constants. You could create a trait that associates each type with its constant type id:
```rust
trait Any where Self: 'static {
const TYPE_ID: TypeId = TypeId::of::<Self>();
}
impl<T: 'static> Any for T { }
assert_eq!(TypeId::of::<usize>(), usize::TYPE_ID);
```
`TypeId::of` is generic, which we saw above in the way the generic `Self` argument was used. This has some implications for const evaluation. It means we can make trait impls evaluate differently depending on information that wasn't directly passed through the trait system. This violates the _parametricity_ property, which requires all instances of a generic function to behave the same way with respect to its generic parameters. That's not unique to `TypeId::of`, other generic const functions based on compiler intrinsics like `mem::align_of` can also violate parametricity. In practice Rust doesn't really have type parametricity anyway since it monomorphizes generics into concrete functions, so violating it using type ids isn’t new.
As an example of how impls can behave differently, you could combine constant type ids with the `const_if_match` feature to dispatch calls based on the type id of the generic `Self`, rather than based on information about `Self` that was threaded through trait bounds. It's like a rough-and-ready form of specialization:
```rust
#![feature(const_if_match)]
trait Specialized where Self: 'static {
// An associated constant that determines the function to call
// at compile-time based on `TypeId::of::<Self>`.
const CALL: fn(&Self) = {
const USIZE: TypeId = TypeId::of::<usize>();
match TypeId::of::<Self>() {
// Use a closure for `usize` that transmutes the generic `Self` to
// a concrete `usize` and dispatches to `Self::usize`.
USIZE => |x| Self::usize(unsafe { &*(x as *const Self as *const usize) }),
// For other types, dispatch to the generic `Self::default`.
_ => Self::default,
}
};
fn call(&self) {
// Call the function we determined at compile-time
(Self::CALL)(self)
}
fn default(x: &Self);
fn usize(x: &usize);
}
// Implement our `Specialized` trait for any `Debug` type.
impl<T: fmt::Debug + 'static> Specialized for T {
fn default(x: &Self) {
println!("default: {:?}", x);
}
fn usize(x: &usize) {
println!("usize: {:?}", x);
}
}
// Will print "usize: 42"
Specialized::call(&42usize);
// Will print "default: ()"
Specialized::call(&());
```
Type ids have some edges that this stabilization exposes to more contexts. It's possible for type ids to collide (but this is a bug). Since they can change between compiler versions, it's never valid to cast a type id to its underlying value.
