Update Box::from_raw example to generalize better
I know very little about rust, so I saw the example here
```
use std::alloc::{alloc, Layout};
unsafe {
let ptr = alloc(Layout::new::<i32>()) as *mut i32;
*ptr = 5;
let x = Box::from_raw(ptr);
}
```
and tried to generalize it by writing,
```
let layout = Layout::new::<T>();
let new_obj = unsafe {
let ptr = alloc(layout) as *mut T;
*ptr = obj;
Box::from_raw(ptr)
};
```
for some more complicated `T`, which ended up crashing with SIGSEGV,
because it tried to `drop_in_place` the previous object in `ptr` which is
of course garbage. I think that changing this example to use `.write` instead
would be a good idea to suggest the correct generalization. It is also more
consistent with other documentation items in this file, which use `.write`.
I also added a comment to explain it, but I'm not too attached to that,
and can see it being too verbose in this place.
This commit adds a new lint - `improper_ctypes_definitions` - which
functions identically to `improper_ctypes`, but on `extern "C" fn`
definitions (as opposed to `improper_ctypes`'s `extern "C" {}`
declarations).
Signed-off-by: David Wood <david@davidtw.co>
I know very little about rust, so I saw this example and tried to generalize it by writing,
```
let layout = Layout::new::<T>();
let new_obj = unsafe {
let ptr = alloc(layout) as *mut T;
*ptr = obj;
Box::from_raw(ptr)
};
```
for some more complicated `T`, which ended up crashing with SIGSEGV,
because it tried to `drop_in_place` the previous object in `ptr` which is
of course garbage. I also added a comment that explains why `.write`
is used, but I think adding that comment is optional and may be too verbose
here. I do however think that changing this example is a good idea to
suggest the correct generalization. `.write` is also used in most of the rest
of the documentation here, even if the example is `i32`, so it would additionally
be more consistent.
`#[deny(unsafe_op_in_unsafe_fn)]` in liballoc
This PR proposes to make use of the new `unsafe_op_in_unsafe_fn` lint, i.e. no longer consider the body of an unsafe function as an unsafe block and require explicit unsafe block to perform unsafe operations.
This has been first (partly) suggested by @Mark-Simulacrum in https://github.com/rust-lang/rust/pull/69245#issuecomment-587817065
Tracking issue for the feature: #71668.
~~Blocked on #71862.~~
r? @Mark-Simulacrum cc @nikomatsakis can you confirm that those changes are desirable? Should I restrict it to only BTree for the moment?
Override Box::<[T]>::clone_from
Avoid dropping and reallocating when cloning to an existing box if the lengths are the same.
It would be nice if this could also be specialized for `Copy` but I don't know how that works since it's not on stable. Will gladly look into it if it's deemed as a good idea.
This is my first PR with code, hope I did everything right 😄
impl From<Cow> for Box, Rc, and Arc
These forward `Borrowed`/`Owned` values to existing `From` impls.
- `Box<T>` is a fundamental type, so it would be a breaking change to add a blanket impl. Therefore, `From<Cow>` is only implemented for `[T]`, `str`, `CStr`, `OsStr`, and `Path`.
- For `Rc<T>` and `Arc<T>`, `From<Cow>` is implemented for everything that implements `From` the borrowed and owned types separately.
Add a function to turn Box<T> into Box<[T]>
Hi,
I think this is very useful, as currently it's not possible in safe rust to do this without re-allocating.
an alternative implementation of the same function can be:
```rust
pub fn into_boxed_slice<T>(boxed: Box<T>) -> Box<[T]> {
unsafe {
let slice = slice::from_raw_parts_mut(Box::into_raw(boxed), 1);
Box::from_raw(slice)
}
}
```
The only thing that makes me a little uncomfortable is this line :
> The alignment of array types is greater or equal to the alignment of its element type
from https://rust-lang.github.io/unsafe-code-guidelines/layout/arrays-and-slices.html
But then I see:
> The alignment of &T, &mut T, *const T and *mut T are the same, and are at least the word size.
> The alignment of &[T] is the word size.
from https://rust-lang.github.io/unsafe-code-guidelines/layout/pointers.html#representation
So I do believe this is valid(FWIW it also passes in miri https://play.rust-lang.org/?gist=c002b99364ee6b29862aeb3565a91c19)
These forward `Borrowed`/`Owned` values to existing `Box::from` impls.
- `From<Cow<'_, [T]>> for Box<[T]>`
- `From<Cow<'_, str>> for Box<str>`
- `From<Cow<'_, CStr>> for Box<CStr>`
- `From<Cow<'_, OsStr>> for Box<OsStr>`
- `From<Cow<'_, Path>> for Box<Path>`
This commit changes some usage of mem::forget into mem::ManuallyDrop
in some Vec, VecDeque, BTreeMap and Box methods.
Before the commit, the generated IR for some of the methods was
longer, and even after optimization, some unwinding artifacts were
still present.
Remove `usable_size` APIs
This removes the usable size APIs:
- remove `usable_size` (obv)
- change return type of allocating methods to include the allocated size
- remove `_excess` API
r? @Amanieu
closesrust-lang/wg-allocators#17
The bespoke `BoxBuilder` was basically a very simple `Vec`. Instead,
let's clone to a real `Vec`, with all of its specialization for the
task, then convert back to `Box<[T]>`.
Manish Goregaokar