Add `Box<[T; N]>: TryFrom<Vec<T>>`
We have `[T; N]: TryFrom<Vec<T>>` (#76310) and `Box<[T; N]>: TryFrom<Box<[T]>>`, but not this combination.
`vec.into_boxed_slice().try_into()` isn't quite a replacement for this, as that'll reallocate unnecessarily in the error case.
**Insta-stable, so needs an FCP**
(I tried to make this work with `, A`, but that's disallowed because of `#[fundamental]` https://github.com/rust-lang/rust/issues/29635#issuecomment-1247598385)
We have `[T; N]: TryFrom<Vec<T>>` and `Box<[T; N]>: TryFrom<Box<[T]>>`, but not the combination.
`vec.into_boxed_slice().try_into()` isn't quite a replacement for this, as that'll reallocate unnecessarily in the error case.
**Insta-stable, so needs an FCP**
Box::from(slice): Clarify that contents are copied
A colleague mentioned that they interpreted the old text
as saying that only the pointer and the length are copied.
Add a clause so it is more clear that the pointed to contents
are also copied.
A colleague mentioned that they interpreted the old text
as saying that only the pointer and the length are copied.
Add a clause so it is more clear that the pointed to contents
are also copied.
Improve documentation for constructors of pinned `Box`es
Adds a cross-references between `Box::pin` and `Box::into_pin` (and other related methods, i.e. the equivalent `From` implementation, and the unstable `pin_in` method), in particular now that `into_pin` [was stabilized](https://github.com/rust-lang/rust/pull/97397). The main goal is to further improve visibility of the fact that `Box<T> -> Pin<Box<T>>` conversion exits in the first place, and that `Box::pin(x)` is – essentially – just a convenience function for `Box::into_pin(Box::new(x))`
The motivating context why I think this is important is even experienced Rust users overlooking the existence this kind of conversion, [e.g. in this thread on IRLO](https://internals.rust-lang.org/t/pre-rfc-function-variants/16732/7?u=steffahn); and also the fact that that discussion brought up that there would be a bunch of Box-construction methods "missing" such as e.g. methods with fallible allocation a la "`Box::try_pin`", and similar; while those are in fact *not* necessary, because you can use `Box::into_pin(Box::try_new(x)?)` instead.
I have *not* included explicit mention of methods (e.g. `try_new`) in the docs of stable methods (e.g. `into_pin`). (Referring to unstable API in stable API docs would be bad style IMO.) Stable examples I have in mind with the statement "constructing a (pinned) Box in a different way than with `Box::new`" are things like cloning a `Box`, or `Box::from_raw`. If/when `try_new` would get stabilized, it would become a very good concrete example use-case of `Box::into_pin` IMO.
Add #[rustc_box] and use it inside alloc
This commit adds an alternative content boxing syntax, and uses it inside alloc.
```Rust
#![feature(box_syntax)]
fn foo() {
let foo = box bar;
}
```
is equivalent to
```Rust
#![feature(rustc_attrs)]
fn foo() {
let foo = #[rustc_box] Box::new(bar);
}
```
The usage inside the very performance relevant code in
liballoc is the only remaining relevant usage of box syntax
in the compiler (outside of tests, which are comparatively easy to port).
box syntax was originally designed to be used by all Rust
developers. This introduces a replacement syntax more tailored
to only being used inside the Rust compiler, and with it,
lays the groundwork for eventually removing box syntax.
[Earlier work](https://github.com/rust-lang/rust/pull/87781#issuecomment-894714878) by `@nbdd0121` to lower `Box::new` to `box` during THIR -> MIR building ran into borrow checker problems, requiring the lowering to be adjusted in a way that led to [performance regressions](https://github.com/rust-lang/rust/pull/87781#issuecomment-894872367). The proposed change in this PR lowers `#[rustc_box] Box::new` -> `box` in the AST -> HIR lowering step, which is way earlier in the compiler, and thus should cause less issues both performance wise as well as regarding type inference/borrow checking/etc. Hopefully, future work can move the lowering further back in the compiler, as long as there are no performance regressions.
Currently, `Box<T>` gets `noalias`, meaning it has
the same rules as `&mut T`. This is
sparsely documented, even though it can have quite
a big impact on unsafe code using box. Therefore,
these rules are documented here, with a big warning
that they are not normative and subject to change,
since we have not yet committed to an aliasing model
and the state of `Box<T>` is especially uncertain.
This accomplishes two main goals:
- Make it clear who is responsible for prefix-freedom, including how they should do it
- Make it feasible for a `Hasher` that *doesn't* care about Hash-DoS resistance to get better performance by not hashing lengths
This does not change rustc-hash, since that's in an external crate, but that could potentially use it in future.
Relevant commit messages from squashed history in order:
Add initial version of ThinBox
update test to actually capture failure
swap to middle ptr impl based on matthieu-m's design
Fix stack overflow in debug impl
The previous version would take a `&ThinBox<T>` and deref it once, which
resulted in a no-op and the same type, which it would then print causing
an endless recursion. I've switched to calling `deref` by name to let
method resolution handle deref the correct number of times.
I've also updated the Drop impl for good measure since it seemed like it
could be falling prey to the same bug, and I'll be adding some tests to
verify that the drop is happening correctly.
add test to verify drop is behaving
add doc examples and remove unnecessary Pointee bounds
ThinBox: use NonNull
ThinBox: tests for size
Apply suggestions from code review
Co-authored-by: Alphyr <47725341+a1phyr@users.noreply.github.com>
use handle_alloc_error and fix drop signature
update niche and size tests
add cfg for allocating APIs
check null before calculating offset
add test for zst and trial usage
prevent optimizer induced ub in drop and cleanup metadata gathering
account for arbitrary size and alignment metadata
Thank you nika and thomcc!
Update library/alloc/src/boxed/thin.rs
Co-authored-by: Josh Triplett <josh@joshtriplett.org>
Update library/alloc/src/boxed/thin.rs
Co-authored-by: Josh Triplett <josh@joshtriplett.org>
Use modern formatting for format! macros
This updates the standard library's documentation to use the new format_args syntax.
The documentation is worthwhile to update as it should be more idiomatic
(particularly for features like this, which are nice for users to get acquainted
with). The general codebase is likely more hassle than benefit to update: it'll
hurt git blame, and generally updates can be done by folks updating the code if
(and when) that makes things more readable with the new format.
A few places in the compiler and library code are updated (mostly just due to
already having been done when this commit was first authored).
`eprintln!("{}", e)` becomes `eprintln!("{e}")`, but `eprintln!("{}", e.kind())` remains untouched.
This updates the standard library's documentation to use the new syntax. The
documentation is worthwhile to update as it should be more idiomatic
(particularly for features like this, which are nice for users to get acquainted
with). The general codebase is likely more hassle than benefit to update: it'll
hurt git blame, and generally updates can be done by folks updating the code if
(and when) that makes things more readable with the new format.
A few places in the compiler and library code are updated (mostly just due to
already having been done when this commit was first authored).
This adds method similar to `MaybeUninit::write` main difference being
it returns owned `Box`. This can be used to elide copy from stack
safely, however it's not currently tested that the optimization actually
occurs.
Analogous methods are not provided for `Rc` and `Arc` as those need to
handle the possibility of sharing. Some version of them may be added in
the future.
This was discussed in #63291 which this change extends.
