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, 0 insertions, 41 deletions

diff --git a/compiler/rustc_target/src/abi/mod.rs b/compiler/rustc_target/src/abi/mod.rs
index 820399943f0..b0ecd117dd2 100644
--- a/compiler/rustc_target/src/abi/mod.rs
+++ b/compiler/rustc_target/src/abi/mod.rs
@@ -13,7 +13,6 @@ use std::str::FromStr;
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_macros::HashStable_Generic;
 use rustc_serialize::json::{Json, ToJson};
-use rustc_span::Span;
 
 pub mod call;
 
@@ -1173,46 +1172,6 @@ impl<'a, Ty> Deref for TyAndLayout<'a, Ty> {
     }
 }
 
-/// Trait for context types that can compute layouts of things.
-pub trait LayoutOf<'a>: Sized {
-    type Ty: TyAbiInterface<'a, Self>;
-    type TyAndLayout: MaybeResult<TyAndLayout<'a, Self::Ty>>;
-
-    fn layout_of(&self, ty: Self::Ty) -> Self::TyAndLayout;
-    fn spanned_layout_of(&self, ty: Self::Ty, _span: Span) -> Self::TyAndLayout {
-        self.layout_of(ty)
-    }
-}
-
-pub trait MaybeResult<T> {
-    type Error;
-
-    fn from(x: Result<T, Self::Error>) -> Self;
-    fn to_result(self) -> Result<T, Self::Error>;
-}
-
-impl<T> MaybeResult<T> for T {
-    type Error = !;
-
-    fn from(Ok(x): Result<T, Self::Error>) -> Self {
-        x
-    }
-    fn to_result(self) -> Result<T, Self::Error> {
-        Ok(self)
-    }
-}
-
-impl<T, E> MaybeResult<T> for Result<T, E> {
-    type Error = E;
-
-    fn from(x: Result<T, Self::Error>) -> Self {
-        x
-    }
-    fn to_result(self) -> Result<T, Self::Error> {
-        self
-    }
-}
-
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub enum PointerKind {
     /// Most general case, we know no restrictions to tell LLVM.