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.

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) }
     }
 

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);
+}

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;

src/test/ui/consts/std/alloc.rs

@@ -0,0 +1,10 @@
+use std::alloc::Layout;
+
+// ok
+const LAYOUT_VALID: Layout = unsafe { Layout::from_size_align_unchecked(0x1000, 0x08) };
+
+// not ok, since alignment needs to be non-zero.
+const LAYOUT_INVALID: Layout = unsafe { Layout::from_size_align_unchecked(0x1000, 0x00) };
+//~^ ERROR it is undefined behavior to use this value
+
+fn main() {}

src/test/ui/consts/std/alloc.stderr

@@ -0,0 +1,11 @@
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/alloc.rs:7:1
+   |
+LL | const LAYOUT_INVALID: Layout = unsafe { Layout::from_size_align_unchecked(0x1000, 0x00) };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0 at .align_, but expected something greater or equal to 1
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.