Add field representing types

[BennoLossin]

Add Field Representing Types (FRTs)

This PR implements the first step of the field projection lang experiment (Tracking Issue: #145383). Field representing types (FRTs) are a new kind of type. They can be named through the use of the field_of! macro with the first argument being the type and the second the name of the field (or variant and field in the case of an enum). No nested fields are supported.

FRTs natively implement the Field trait that's also added in this PR. It exposes information about the field such as the type of the field, the type of the base (i.e. the type that contains the field) and the offset within that base type. Only fields of non-packed structs are supported, fields of enums an unions have unique types for each field, but those do not implement the Field trait.

This PR was created in collaboration with @dingxiangfei2009, it wouldn't have been possible without him, so huge thanks for mentoring me!

I updated my library solution for field projections to use the FRTs from core instead of creating my own using the hash of the name of the field. See the Rust-for-Linux/field-projection lang-experiment branch.

API added to core::field

pub unsafe trait Field {
    type Base;
    
    type Type;

    const OFFSET: usize;
}

pub macro field_of($Container:ty, $($fields:expr)+ $(,)?);

Explanation of Field Representing Types (FRTs)

FRTs are used for compile-time & trait-level reflection for fields of structs & tuples. Each struct & tuple has a unique compiler-generated type nameable through the field_of! macro. This type natively contains information about the field such as the outermost container, type of the field and its offset. Users may implement additional traits on these types in order to record custom information (for example a crate may define a PinnableField trait that records whether the field is structurally pinned).

They are the foundation of field projections, a general operation that's generic over the fields of a struct. This genericism needs to be expressible in the trait system. FRTs make this possible, since an operation generic over fields can just be a function with a generic parameter F: Field.

Note

The approach of field projections has changed considerably since this PR was opened. In the end we might not need FRTs, so this API is highly experimental.

FRTs should act as though they were defined as struct MyStruct_my_field<StructGenerics>; next to the struct. So it should be local to the crate defining the struct so that one can implement any trait for the FRT from that crate. The Field traits should be implemented by the compiler & populated with correct information (unsafe code needs to be able to rely on them being correct).

Design Decisions

During the development several desirable properties of FRTs were discovered. Either due to simplifying the implementation or because of language design reasons. These decisions are:

• FRTs are inhabited ZSTs, they have the same layout as ()

  • They only contain type information via the Field trait. Their only use of the values is a future ergonomics change that allows skipping turbofish syntax.
  • This simplifies const-eval.
  • They are thus also Freeze

• The coherence check of FRTs amounts to checking coherence of the base type.

  So field_of!(Foo, bar) is local if Foo is local (Bar is allowed to be foreign). Fundamental types are not handled specially, so field_of!(MaybeUninit<Struct>, value) still is local to core (aside from the fact that non-visible fields cannot be accessed in the first place).

• Traits may be implemented for FRTs in the usual way (so if the FRT or the trait is local). There is an exception:

  • Drop: since FRTs are uninhabited, having a Drop impl doesn't really make sense. It also would have required duplicating a lot of logic from ADTs and as such it simplified the implementation.

• The Field trait is not user-implementable

  • this also can be lifted in the future, but for now only FRTs will implement it.

• FRTs implement all auto traits as well as Copy and Clone.

TODOs

There are several FIXME(FRTs) scattered around the code:

• rustdoc doesn't yet support them, this can be implemented later
  • src/librustdoc/json/conversions.rs
• Debuginfo and symbol mangling is not implemented, for this PR they aren't important, but later we should implement it
  • compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs
  • compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/encode.rs
  • compiler/rustc_symbol_mangling/src/v0.rs
• Diagnostics for field_of! can be improved
  • tests/ui/field_representing_types/nonexistent.rs
  • tests/ui/field_representing_types/non-struct.rs
  • tests/ui/field_representing_types/offset.rs
  • tests/ui/field_representing_types/not-field-if-packed.rs
  • tests/ui/field_representing_types/invalid.rs
• Simple type alias already seem to work, but might need some extra work in compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

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[rustbot]

changes to the core type system

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Some changes occurred in src/tools/clippy

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Some changes occurred in diagnostic error codes

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Some changes occurred to the CTFE machinery

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Some changes occurred in compiler/rustc_sanitizers

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HIR ty lowering was modified

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Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred in exhaustiveness checking

cc @Nadrieril

This PR changes MIR

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

Some changes occurred in compiler/rustc_codegen_ssa

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This PR changes rustc_public

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

Some changes occurred in src/tools/rustfmt

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Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[BennoLossin]

There are some changes to symbol mangling & constant names that I'm not sure how they resulted from my changes. Anybody has any idea?

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[rustbot]

Some changes occurred to constck

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Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

[RalfJung]

Could you ensure you have a test that is generic over the field and that queries the offset? That would hit all these points I just mentioned here (or at least some of them), and anyway seems like a very non-trivial case to test.

Or... wait the intrinsic is only used to define that one associated const? I don't know exactly when and how we normalize those but it may be that we indeed never see that MIR in its generic form then...

The test may have to invoke the intrinsic directly to trigger the case I am thinking of.

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[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.