Initial implementation of FnPtr trait

[carbotaniuman]

This commit is an initial implementation of the FnPtr trait as described in the fn_static tracking issue, which consists of moving the internally unstable core::marker::FnPtr to core::ops::FnPtr, as well as changing the API. Because NonNull is used in the new as_ptr signature, it was also turned into a proper lang item.

Part of fn_static: #148768

[rustbot]

The Clippy subtree was changed

cc @rust-lang/clippy

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @nia-e

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The reviewer was selected based on:

• Owners of files modified in this PR: @scottmcm, libs
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[nia-e]

Thanks for the PR! I'll take a proper look asap, but in the meantime could you add some tests for the feature & take a look at the build errors?

[rustbot]

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[nia-e]

What's holding us back from making this an extern type now?

View changes since the review

[carbotaniuman]

If I try I get:

error: foreign type has missing stability attribute
   --> library/core/src/ops/function.rs:318:5
    |
318 |     pub type Code;
    |     ^^^^^^^^^^^^^

And if I try to put a stability marker on it I get this:

extern "C" {
error: `#[unstable]` attribute cannot be used on foreign types
   --> library/core/src/ops/function.rs:318:5
    |
318 |     #[unstable(feature = "fn_static", issue = "148768")]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: `#[unstable]` can be applied to associated consts, associated types, constants, crates, data types, enum variants, extern crates, foreign statics, functions, impl blocks, macro defs, modules, statics, struct fields, trait aliases, traits, type aliases, type parameters, and use statements

[lcnr]

we should change our attribute handling to support stabiity attributes on foreign types. please either implement this or open a github issue for this

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[carbotaniuman]

@rustbot ready

[rust-bors]

☔ The latest upstream changes (presumably #157600) made this pull request unmergeable. Please resolve the merge conflicts.