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Rollup merge of #67494 - lukaslueg:const_alloc, r=oli-obk

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Constify more of alloc::Layout

Making more methods of `alloc::Layout` const would allow us to compute alignment/size information for arbitrary (sized) types at compile-time, including placing the information in associated constants. While `mem::size_of` and `mem::align_of` are already const and `Layout` is solely based on those, there is no guarantee in the implementation that a const derived from these functions will be exactly the same as what `Layout` uses and is subsequently used in a call to `alloc::alloc`. Constifying `Layout` makes this possible.

First contribution to core, excuse my ignorance.
This commit is contained in:
Mazdak Farrokhzad 2020-01-12 03:27:54 +01:00 committed by GitHub
commit 019790c72a
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2 changed files with 9 additions and 4 deletions
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12
src/libcore/alloc.rs
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@ -64,8 +64,9 @@ impl Layout {
/// must not overflow (i.e., the rounded value must be less than
/// `usize::MAX`).
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
#[inline]
pub fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
if !align.is_power_of_two() {
return Err(LayoutErr { private: () });
}
@ -106,15 +107,17 @@ impl Layout {
/// The minimum size in bytes for a memory block of this layout.
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
#[inline]
pub fn size(&self) -> usize {
pub const fn size(&self) -> usize {
self.size_
}
/// The minimum byte alignment for a memory block of this layout.
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
#[inline]
pub fn align(&self) -> usize {
pub const fn align(&self) -> usize {
self.align_.get()
}
@ -181,8 +184,9 @@ impl Layout {
/// address for the whole allocated block of memory. One way to
/// satisfy this constraint is to ensure `align <= self.align()`.
#[unstable(feature = "alloc_layout_extra", issue = "55724")]
#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
#[inline]
pub fn padding_needed_for(&self, align: usize) -> usize {
pub const fn padding_needed_for(&self, align: usize) -> usize {
let len = self.size();
// Rounded up value is:

1
src/libcore/lib.rs
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@ -70,6 +70,7 @@
#![feature(bound_cloned)]
#![feature(cfg_target_has_atomic)]
#![feature(concat_idents)]
#![feature(const_alloc_layout)]
#![feature(const_if_match)]
#![feature(const_panic)]
#![feature(const_fn_union)]