Auto merge of #88499 - eddyb:layout-off, r=nagisa

Provide `layout_of` automatically (given tcx + param_env + error handling).

After #88337, there's no longer any uses of `LayoutOf` within `rustc_target` itself, so I realized I could move the trait to `rustc_middle::ty::layout` and redesign it a bit.

This is similar to #88338 (and supersedes it), but at no ergonomic loss, since there's no funky `C: LayoutOf<Ty = Ty>` -> `Ty: TyAbiInterface<C>` generic `impl` chain, and each `LayoutOf` still corresponds to one `impl` (of `LayoutOfHelpers`) for the specific context.

After this PR, this is what's needed to get `trait LayoutOf` (with the `layout_of` method) implemented on some context type:
* `TyCtxt`, via `HasTyCtxt`
* `ParamEnv`, via `HasParamEnv`
* a way to transform `LayoutError`s into the desired error type
  * an error type of `!` can be paired with having `cx.layout_of(...)` return `TyAndLayout` *without* `Result<...>` around it, such as used by codegen
  * this is done through a new `LayoutOfHelpers` trait (and so is specifying the type of `cx.layout_of(...)`)

When going through this path (and not bypassing it with a manual `impl` of `LayoutOf`), the end result is that only the error case can be customized, the query itself and the success paths are guaranteed to be uniform.

(**EDIT**: just noticed that because of the supertrait relationship, you cannot actually implement `LayoutOf` yourself, the blanket `impl` fully covers all possible context types that could ever implement it)

Part of the motivation for this shape of API is that I've been working on querifying `FnAbi::of_*`, and what I want/need to introduce for that looks a lot like the setup in this PR - in particular, it's harder to express the `FnAbi` methods in `rustc_target`, since they're much more tied to `rustc` concepts.

r? `@nagisa` cc `@oli-obk` `@bjorn3`

1 files changed, 4 insertions, 10 deletions

diff --git a/compiler/rustc_codegen_ssa/src/traits/backend.rs b/compiler/rustc_codegen_ssa/src/traits/backend.rs
index 96e53f51dac..dbb7e1ee8b1 100644
--- a/compiler/rustc_codegen_ssa/src/traits/backend.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/backend.rs
@@ -8,15 +8,14 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::ErrorReported;
 use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
 use rustc_middle::middle::cstore::{EncodedMetadata, MetadataLoaderDyn};
-use rustc_middle::ty::layout::{HasTyCtxt, TyAndLayout};
+use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf, TyAndLayout};
 use rustc_middle::ty::query::Providers;
-use rustc_middle::ty::{Ty, TyCtxt};
+use rustc_middle::ty::TyCtxt;
 use rustc_session::{
     config::{self, OutputFilenames, PrintRequest},
     Session,
 };
 use rustc_span::symbol::Symbol;
-use rustc_target::abi::LayoutOf;
 use rustc_target::spec::Target;
 
 pub use rustc_data_structures::sync::MetadataRef;
@@ -39,17 +38,12 @@ pub trait BackendTypes {
 }
 
 pub trait Backend<'tcx>:
-    Sized
-    + BackendTypes
-    + HasTyCtxt<'tcx>
-    + LayoutOf<'tcx, Ty = Ty<'tcx>, TyAndLayout = TyAndLayout<'tcx>>
+    Sized + BackendTypes + HasTyCtxt<'tcx> + LayoutOf<'tcx, LayoutOfResult = TyAndLayout<'tcx>>
 {
 }
 
 impl<'tcx, T> Backend<'tcx> for T where
-    Self: BackendTypes
-        + HasTyCtxt<'tcx>
-        + LayoutOf<'tcx, Ty = Ty<'tcx>, TyAndLayout = TyAndLayout<'tcx>>
+    Self: BackendTypes + HasTyCtxt<'tcx> + LayoutOf<'tcx, LayoutOfResult = TyAndLayout<'tcx>>
 {
 }