Rollup merge of #60370 - Richard-W:const-layout-construction, r=sfackler

Mark core::alloc::Layout::from_size_align_unchecked const

Makes it possible (pending stabilization of #57563 (`const_fn`)) to rewrite code like

```rust
const BUFFER_SIZE: usize = 0x2000;
const BUFFER_ALIGN: usize = 0x1000;

fn foo() {
  let layout = std::alloc::Layout::from_size_align(BUFFER_SIZE, BUFFER_ALIGN)
    .unwrap();
  let buffer = std::alloc::alloc(layout);
}
```
to
```rust
const BUFFER_LAYOUT: std::alloc::Layout = unsafe {
  std::alloc::Layout::from_size_align_unchecked(0x2000, 0x1000)
};

fn foo() {
  let buffer = std::alloc::alloc(BUFFER_LAYOUT);
}
```

which (although `unsafe` is used) looks somewhat cleaner and is easier to read.

3 files changed, 13 insertions, 1 deletions

diff --git a/src/libcore/alloc.rs b/src/libcore/alloc.rs
index c124457118c..302a9d89e58 100644
--- a/src/libcore/alloc.rs
+++ b/src/libcore/alloc.rs
@@ -99,7 +99,7 @@ impl Layout {
     /// [`Layout::from_size_align`](#method.from_size_align).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[inline]
-    pub unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
+    pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
         Layout { size_: size, align_: NonZeroUsize::new_unchecked(align) }
     }
 
diff --git a/src/libcore/tests/alloc.rs b/src/libcore/tests/alloc.rs
new file mode 100644
index 00000000000..63537ba23d8
--- /dev/null
+++ b/src/libcore/tests/alloc.rs
@@ -0,0 +1,10 @@
+use core::alloc::Layout;
+
+#[test]
+fn const_unchecked_layout() {
+    const SIZE: usize = 0x2000;
+    const ALIGN: usize = 0x1000;
+    const LAYOUT: Layout = unsafe { Layout::from_size_align_unchecked(SIZE, ALIGN) };
+    assert_eq!(LAYOUT.size(), SIZE);
+    assert_eq!(LAYOUT.align(), ALIGN);
+}
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index b8075ef2942..c617596aba8 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -31,10 +31,12 @@
 #![feature(slice_partition_dedup)]
 #![feature(copy_within)]
 #![feature(int_error_matching)]
+#![feature(const_fn)]
 #![warn(rust_2018_idioms)]
 
 extern crate test;
 
+mod alloc;
 mod any;
 mod array;
 mod ascii;