Add BTF relocation builtins

[vadorovsky]

Add experimental Rust support for Compile Once, Run Everywhere (CO-RE) relocations based on the BPF Type Format (BTF). The entire feature is specific to the BPF architecture and cannot be used elsewhere. The feature introduces a #[btf_relocatable] attribute for structs and unions whose field layout must be queried through BTF-aware operations, and adds BTF-aware bultins:

• btf_field_exists
• btf_field_byte_offset
• btf_field_byte_size

This PR does not yet expose these builtins to the users, the core_arch API will be added as a follow-up change.

The user-facing feature is gated by #![feature(btf_relocations)].

See individual commits for details.

Created with the help of an LLM, initially used to analyze the existing MIR, codegen SSA and LLVM backend code, to help identifying which parts of code need modification. Afterwards, the code and commit messages were written by hand, then reviewed by an LLM.

Tracking issue: #160616
RFC: rust-lang/rfcs#3966

r? @traviscross @nagisa

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to constck

cc @fee1-dead

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

cc @Amanieu, @folkertdev, @sayantn

This PR changes rustc_public

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

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

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

cc @bjorn3

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_ir

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in rustc_ty_utils::consts.rs

cc @BoxyUwU

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

[vadorovsky]

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

cc @Amanieu, @folkertdev, @sayantn

[...]

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

cc @bjorn3

Sorry, I will trim this PR only to the builtins. They can be still tested with minicore and a macro embedded in test. And I'm going to leave the core_arch change for later.

[vadorovsky]

Now CI fails on rustc_codegen_cranelift. https://github.com/rust-lang/rust/actions/runs/31870909600/job/94979319914 How do we usually fix such chicken-and-egg problems?

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    Checking cranelift-object v0.134.0
[RUSTC-TIMING] cranelift_frontend test:false 0.209
[RUSTC-TIMING] cranelift_object test:false 0.097
    Checking rustc_codegen_cranelift v0.1.0 (/checkout/compiler/rustc_codegen_cranelift)
error[E0004]: non-exhaustive patterns: `rustc_middle::mir::Rvalue::BtfFieldInfo { .. }` not covered
    --> compiler/rustc_codegen_cranelift/src/base.rs:632:19
     |
 632 |             match to_place_and_rval.1 {
     |                   ^^^^^^^^^^^^^^^^^^^ pattern `rustc_middle::mir::Rvalue::BtfFieldInfo { .. }` not covered
     |
note: `rustc_middle::mir::Rvalue<'_>` defined here
    --> compiler/rustc_middle/src/mir/syntax.rs:1342:1
     |
1342 | pub enum Rvalue<'tcx> {
     | ^^^^^^^^^^^^^^^^^^^^^
...
1469 |     BtfFieldInfo { base_ty: Ty<'tcx>, path: Box<[BtfFieldStep<'tcx>]>, kind: BtfFieldInfoKind },
     |     ------------ not covered
     = note: the matched value is of type `rustc_middle::mir::Rvalue<'_>`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
     |
 924 ~                 Rvalue::CopyForDeref(_) => bug!("`CopyForDeref` in codegen"),
 925 ~                 rustc_middle::mir::Rvalue::BtfFieldInfo { .. } => todo!(),
     |

For more information about this error, try `rustc --explain E0004`.
[RUSTC-TIMING] rustc_codegen_cranelift test:false 0.944
error: could not compile `rustc_codegen_cranelift` (lib) due to 1 previous error