Allow inline consts to reference generic params

[nbdd0121]

Tracking issue: #76001

The RFC says that inline consts cannot reference to generic parameters (for now), same as array length expressions. And expresses that it's desirable for it to reference in-scope generics, when array length expressions gain that feature as well.

However it is possible to implement this for inline consts before doing this for all anon consts, because inline consts are only used as values and they won't be used in the type system. So we can have:

fn foo<T>() {
    let x = [4i32; std::mem::size_of::<T>()];   // NOT ALLOWED (for now)
    let x = const { std::mem::size_of::<T>() }; // ALLOWED with this PR!
    let x = [4i32; const { std::mem::size_of::<T>() }];   // NOT ALLOWED (for now)
}

This would make inline consts super useful for compile-time checks and assertions:

fn assert_zst<T>() {
    const { assert!(std::mem::size_of::<T>() == 0) };
}

This would create an error during monomorphization when assert_zst is instantiated with non-ZST Ts. A error during mono might sound scary, but this is exactly what a "desugared" inline const would do:

fn assert_zst<T>() {
    struct F<T>(T);
    impl<T> F<T> {
        const V: () = assert!(std::mem::size_of::<T>() == 0);
    }
    let _ = F::<T>::V;
}

It should also be noted that the current inline const implementation can already reference the type params via type inference, so this resolver-level restriction is not any useful either:

fn foo<T>() -> usize {
    let (_, size): (PhantomData<T>, usize) = const {
        const fn my_size_of<T>() -> (PhantomData<T>, usize) {
            (PhantomData, std::mem::size_of::<T>())
        }
        my_size_of()
    };
    size
}

@rustbot label: F-inline_const

[rust-highfive]

r? @cjgillot

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[nbdd0121]

Marking as T-lang as well.

@rustbot label: +T-lang

[oli-obk]

The diagnostics of your tests changed after #96576 as we now display a note at the call site of functions that cause an error after monomorphization

You'll need to rebase and rebless.

[oli-obk]

r=me with that done. Thanks for doing this, it's been an inconsistency that I've been annoyed at for a year now 😅

[oli-obk]

r? @oli-obk

[oli-obk]

You may also want to wait for #96641 before rebasing, as that will just conflict with this PR again.

You PR caused me to spawn off two cleanup PRs 😄 (none of them your fault, just things that bugged me before and your PR reminded me of)

[nbdd0121]

Thanks for doing these PRs! I was annoyed by these as well and was originally tempted to fix them after this PR got merged, but I am glad that I now have less things to do :)

Will rebless and rebase later today.

[nbdd0121]

@rustbot ready

[oli-obk]

@bors r+

[bors]

📌 Commit 6baaa52 has been approved by oli-obk

[bors]

⌛ Testing commit 6baaa52 with merge 23638c523e111b5f70213691e3f9b58a7e7c2443...

[bors]

💔 Test failed - checks-actions

[oli-obk]

@bors r+

[bors]

📌 Commit 5b5ac28 has been approved by oli-obk

[leonardo-m]

const { assert!(std::mem::size_of::<T>() == 0) };

If I compile with "rustc --emit=metadata" it gives no compilation errors, this makes this improvement less useful than I expected.