Rollup of 5 pull requests #63671
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Rollup of 5 pull requests #63671
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Co-Authored-By: Mazdak Farrokhzad <[email protected]>
Co-Authored-By: Ralf Jung <[email protected]>
Co-Authored-By: Ralf Jung <[email protected]>
Add APIs for uninitialized Box, Rc, and Arc. (Plus get_mut_unchecked) Assigning `MaybeUninit::<Foo>::uninit()` to a local variable is usually free, even when `size_of::<Foo>()` is large. However, passing it for example to `Arc::new` [causes at least one copy](https://youtu.be/F1AquroPfcI?t=4116) (from the stack to the newly allocated heap memory) even though there is no meaningful data. It is theoretically possible that a Sufficiently Advanced Compiler could optimize this copy away, but this is [reportedly unlikely to happen soon in LLVM](https://youtu.be/F1AquroPfcI?t=5431). This PR proposes two sets of features: * Constructors for containers (`Box`, `Rc`, `Arc`) of `MaybeUninit<T>` or `[MaybeUninit<T>]` that do not initialized the data, and unsafe conversions to the known-initialized types (without `MaybeUninit`). The constructors are guaranteed not to make unnecessary copies. * On `Rc` and `Arc`, an unsafe `get_mut_unchecked` method that provides `&mut T` access without checking the reference count. `Arc::get_mut` involves multiple atomic operations whose cost can be non-trivial. `Rc::get_mut` is less costly, but we add `Rc::get_mut_unchecked` anyway for symmetry with `Arc`. These can be useful independently, but they will presumably be typical when the new constructors of `Rc` and `Arc` are used. An alternative with a safe API would be to introduce `UniqueRc` and `UniqueArc` types that have the same memory layout as `Rc` and `Arc` (and so zero-cost conversion to them) but are guaranteed to have only one reference. But introducing entire new types feels “heavier” than new constructors on existing types, and initialization of `MaybeUninit<T>` typically requires unsafe code anyway. Summary of new APIs (all unstable in this PR): ```rust impl<T> Box<T> { pub fn new_uninit() -> Box<MaybeUninit<T>> {…} } impl<T> Box<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Box<T> {…} } impl<T> Box<[T]> { pub fn new_uninit_slice(len: usize) -> Box<[MaybeUninit<T>]> {…} } impl<T> Box<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Box<[T]> {…} } impl<T> Rc<T> { pub fn new_uninit() -> Rc<MaybeUninit<T>> {…} } impl<T> Rc<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Rc<T> {…} } impl<T> Rc<[T]> { pub fn new_uninit_slice(len: usize) -> Rc<[MaybeUninit<T>]> {…} } impl<T> Rc<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Rc<[T]> {…} } impl<T> Arc<T> { pub fn new_uninit() -> Arc<MaybeUninit<T>> {…} } impl<T> Arc<MaybeUninit<T>> { pub unsafe fn assume_init(self) -> Arc<T> {…} } impl<T> Arc<[T]> { pub fn new_uninit_slice(len: usize) -> Arc<[MaybeUninit<T>]> {…} } impl<T> Arc<[MaybeUninit<T>]> { pub unsafe fn assume_init(self) -> Arc<[T]> {…} } impl<T: ?Sized> Rc<T> { pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {…} } impl<T: ?Sized> Arc<T> { pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {…} } ```
…s-in-src/test-2, r=Centril Remove meaningless comments in src/test Moved from rust-lang#63411
Crank up invalid value lint * Warn against uninit `bool` and `char`. * Warn against 0-init `NonNull` and friends * Detect transmute-from-0 as zero-initialization ([seen in the wild](glium/glium#1775 (comment)))
…ewjasper resolve: Properly integrate derives and `macro_rules` scopes So, ```rust #[derive(A, B)] struct S; m!(); ``` turns into something like ```rust struct S; A_placeholder!( struct S; ); B_placeholder!( struct S; ); m!(); ``` during expansion. And for `m!()` its "`macro_rules` scope" (aka "legacy scope") should point to the `B_placeholder` call rather than to the derive container `#[derive(A, B)]`. `fn build_reduced_graph` now makes sure the legacy scope points to the right thing. (It's still a mystery for me why this worked before rust-lang#63535.) Unfortunately, placeholders from derives are currently treated separately from placeholders from other macros and need to be passed as `extra_placeholders` rather than a part of the AST fragment. That's fixable, but I wanted to keep this PR more minimal to close the regression faster. Fixes rust-lang#63651 r? @matthewjasper
…vink fix typos in mir/interpret
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@bors r+ p=5 rollup=never |
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📌 Commit 4ec9703 has been approved by |
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Aug 17, 2019
Rollup of 5 pull requests Successful merges: - #62451 (Add APIs for uninitialized Box, Rc, and Arc. (Plus get_mut_unchecked)) - #63487 (Remove meaningless comments in src/test) - #63657 (Crank up invalid value lint) - #63667 (resolve: Properly integrate derives and `macro_rules` scopes) - #63669 (fix typos in mir/interpret) Failed merges: r? @ghost
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☀️ Test successful - checks-azure |
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Successful merges:
macro_rulesscopes #63667 (resolve: Properly integrate derives andmacro_rulesscopes)Failed merges:
r? @ghost