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

[Richard-W]

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

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

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.

[rust-highfive]

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[oli-obk]

Please add the example as a test and also add a test that passing 0 as the alignment will cause a const eval error (since it's a type-layout violation)

[Richard-W]

@oli-obk, I added a test similar to the example, but I was not able to figure out how I'd go about the eval error test.

I looked for a similar test for nonzero integers as an example (grep -Hrn "new_unchecked(0)") but I was not able to find one.

[rasendubi]

Check rustc-guide chapter on testing (especially, adding new tests). You need to add a ui test.

This boils down to adding a single file to src/test/ui directory (something like src/test/ui/consts/std/alloc.rs should be fine). You could use src/test/ui/consts/std/cell.rs as an inspiration.

[Richard-W]

Thanks for your help @rasendubi. I added the test.

[bors]

☔ The latest upstream changes (presumably #60794) made this pull request unmergeable. Please resolve the merge conflicts.

[Centril]

r? @sfackler

[sfackler]

@bors r+

[bors]

📌 Commit c0b6d3c has been approved by sfackler

[bors]

⌛ Testing commit c0b6d3c with merge 26ab324...