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.

2 files changed, 21 insertions, 0 deletions

diff --git a/src/test/ui/consts/std/alloc.rs b/src/test/ui/consts/std/alloc.rs
new file mode 100644
index 00000000000..65ac7e44926
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/consts/std/alloc.stderr b/src/test/ui/consts/std/alloc.stderr
new file mode 100644
index 00000000000..74a8f3daf6a
--- /dev/null
+++ b/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`.