Auto merge of #88337 - eddyb:field-failure-is-not-an-option, r=nagisa

rustc_target: `TyAndLayout::field` should never error.

This refactor (making `TyAndLayout::field` return `TyAndLayout` without any `Result` around it) is based on a simple observation, regarding `TyAndLayout::field`:

If `cx.layout_of(ty)` succeeds (for some `cx` and `ty`), then `.field(cx, i)` on the resulting `TyAndLayout` should *always* succeed in computing `cx.layout_of(field_ty)` (where `field_ty` is the type of the `i`th field of `ty`).

The reason for this is that no matter which field is chosen, `cx.layout_of(field_ty)` *will have already been computed*, as part of computing `cx.layout_of(ty)`, as we cannot determine the layout of *any* type without considering the layouts of *all* of its fields.

And so it should be fine to turn any errors into ICEs, since they likely indicate a `cx` mismatch, or some other edge case that is due to a compiler bug (as opposed to ever being an user-facing error).

<hr/>

Each commit should probably be reviewed separately, though note that there's some `where` clauses (in `rustc_target::abi::call::*`) that change in most commits.

cc `@nagisa` `@oli-obk`

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -472,10 +472,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             let layout = bx.layout_of(ty);
             let do_panic = match intrinsic {
                 Inhabited => layout.abi.is_uninhabited(),
-                // We unwrap as the error type is `!`.
-                ZeroValid => !layout.might_permit_raw_init(bx, /*zero:*/ true).unwrap(),
-                // We unwrap as the error type is `!`.
-                UninitValid => !layout.might_permit_raw_init(bx, /*zero:*/ false).unwrap(),
+                ZeroValid => !layout.might_permit_raw_init(bx, /*zero:*/ true),
+                UninitValid => !layout.might_permit_raw_init(bx, /*zero:*/ false),
             };
             if do_panic {
                 let msg_str = with_no_trimmed_paths(|| {

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -39,12 +39,17 @@ pub trait BackendTypes {
 }
 
 pub trait Backend<'tcx>:
-    Sized + BackendTypes + HasTyCtxt<'tcx> + LayoutOf<Ty = Ty<'tcx>, TyAndLayout = TyAndLayout<'tcx>>
+    Sized
+    + BackendTypes
+    + HasTyCtxt<'tcx>
+    + LayoutOf<'tcx, Ty = Ty<'tcx>, TyAndLayout = TyAndLayout<'tcx>>
 {
 }
 
 impl<'tcx, T> Backend<'tcx> for T where
-    Self: BackendTypes + HasTyCtxt<'tcx> + LayoutOf<Ty = Ty<'tcx>, TyAndLayout = TyAndLayout<'tcx>>
+    Self: BackendTypes
+        + HasTyCtxt<'tcx>
+        + LayoutOf<'tcx, Ty = Ty<'tcx>, TyAndLayout = TyAndLayout<'tcx>>
 {
 }