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`

src/common.rs

@@ -1,4 +1,5 @@
 use rustc_index::vec::IndexVec;
+use rustc_middle::ty::layout::{LayoutError, LayoutOfHelpers};
 use rustc_middle::ty::SymbolName;
 use rustc_target::abi::call::FnAbi;
 use rustc_target::abi::{Integer, Primitive};
@@ -256,12 +257,12 @@ pub(crate) struct FunctionCx<'m, 'clif, 'tcx: 'm> {
     pub(crate) inline_asm_index: u32,
 }
 
-impl<'tcx> LayoutOf<'tcx> for FunctionCx<'_, '_, 'tcx> {
-    type Ty = Ty<'tcx>;
-    type TyAndLayout = TyAndLayout<'tcx>;
+impl<'tcx> LayoutOfHelpers<'tcx> for FunctionCx<'_, '_, 'tcx> {
+    type LayoutOfResult = TyAndLayout<'tcx>;
 
-    fn layout_of(&self, ty: Ty<'tcx>) -> TyAndLayout<'tcx> {
-        RevealAllLayoutCx(self.tcx).layout_of(ty)
+    #[inline]
+    fn handle_layout_err(&self, err: LayoutError<'tcx>, span: Span, ty: Ty<'tcx>) -> ! {
+        RevealAllLayoutCx(self.tcx).handle_layout_err(err, span, ty)
     }
 }
 
@@ -364,19 +365,16 @@ pub(crate) fn anonymous_str(&mut self, msg: &str) -> Value {
 
 pub(crate) struct RevealAllLayoutCx<'tcx>(pub(crate) TyCtxt<'tcx>);
 
-impl<'tcx> LayoutOf<'tcx> for RevealAllLayoutCx<'tcx> {
-    type Ty = Ty<'tcx>;
-    type TyAndLayout = TyAndLayout<'tcx>;
+impl<'tcx> LayoutOfHelpers<'tcx> for RevealAllLayoutCx<'tcx> {
+    type LayoutOfResult = TyAndLayout<'tcx>;
 
-    fn layout_of(&self, ty: Ty<'tcx>) -> TyAndLayout<'tcx> {
-        assert!(!ty.still_further_specializable());
-        self.0.layout_of(ParamEnv::reveal_all().and(&ty)).unwrap_or_else(|e| {
-            if let layout::LayoutError::SizeOverflow(_) = e {
-                self.0.sess.fatal(&e.to_string())
-            } else {
-                bug!("failed to get layout for `{}`: {}", ty, e)
-            }
-        })
+    #[inline]
+    fn handle_layout_err(&self, err: LayoutError<'tcx>, span: Span, ty: Ty<'tcx>) -> ! {
+        if let layout::LayoutError::SizeOverflow(_) = err {
+            self.0.sess.span_fatal(span, &err.to_string())
+        } else {
+            span_bug!(span, "failed to get layout for `{}`: {}", ty, err)
+        }
     }
 }
 

src/lib.rs

@@ -79,12 +79,12 @@ mod prelude {
     pub(crate) use rustc_hir::def_id::{DefId, LOCAL_CRATE};
     pub(crate) use rustc_middle::bug;
     pub(crate) use rustc_middle::mir::{self, *};
-    pub(crate) use rustc_middle::ty::layout::{self, TyAndLayout};
+    pub(crate) use rustc_middle::ty::layout::{self, LayoutOf, TyAndLayout};
     pub(crate) use rustc_middle::ty::{
         self, FloatTy, Instance, InstanceDef, IntTy, ParamEnv, Ty, TyCtxt, TypeAndMut,
         TypeFoldable, UintTy,
     };
-    pub(crate) use rustc_target::abi::{Abi, LayoutOf, Scalar, Size, VariantIdx};
+    pub(crate) use rustc_target::abi::{Abi, Scalar, Size, VariantIdx};
 
     pub(crate) use rustc_data_structures::fx::FxHashMap;