Auto merge of rust-lang#141039 - lqd:expensive-sanity, r=compiler-errors

move expensive layout sanity check to debug assertions

It is [hard to fix](rust-lang#141006 (comment)) the slowness in the uninhabitedness computation for very big types but we can fix the very specific case of them being called during the layout sanity checks, as described in rust-lang#140944.

So this PR moves this uninhabitedness check to the other expensive layout sanity checks that are ran under `debug_assertions`.

It makes building the `lemmy_api_routes` crate's self-profile `layout_of` query go from

```
+--------------------------------------------------------+-----------+-----------------+----------+------------+---------------------------------+
| Item                                                   | Self time | % of total time | Time     | Item count | Incremental result hashing time |
+--------------------------------------------------------+-----------+-----------------+----------+------------+---------------------------------+
| layout_of                                              | 63.02s    | 41.895          | 244.26s  | 123703     | 50.30ms                         |
+--------------------------------------------------------+-----------+-----------------+----------+------------+---------------------------------+
```

on master (2m17s total), to

```
| layout_of                                              | 330.21ms  | 0.372           | 26.90s   | 123703     | 53.19ms                         |
```

with this PR (1m15s total).

(Note that the [perf run results](rust-lang#141039 (comment)) below look a bit better than [an earlier run](https://perf.rust-lang.org/compare.html?start=4eca99a18eab3d4e28ed1ce3ee620d442955a470&end=c4a00993f8ee02c7565e7be652608817ea2fb97d&stat=instructions:u) I did in another PR. There may be some positive noise there, or post-merge results could differ a bit)

Since we discussed this today, r? `@compiler-errors` — and cc `@lcnr` and `@RalfJung.`

Author: bors

compiler/rustc_ty_utils/src/layout/invariant.rs

@@ -8,15 +8,6 @@ use rustc_middle::ty::layout::{HasTyCtxt, LayoutCx, TyAndLayout};
 pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayout<'tcx>) {
     let tcx = cx.tcx();
 
-    // Type-level uninhabitedness should always imply ABI uninhabitedness.
-    if layout.ty.is_privately_uninhabited(tcx, cx.typing_env) {
-        assert!(
-            layout.is_uninhabited(),
-            "{:?} is type-level uninhabited but not ABI-uninhabited?",
-            layout.ty
-        );
-    }
-
     if layout.size.bytes() % layout.align.abi.bytes() != 0 {
         bug!("size is not a multiple of align, in the following layout:\n{layout:#?}");
     }
@@ -29,6 +20,19 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
         return;
     }
 
+    // Type-level uninhabitedness should always imply ABI uninhabitedness. This can be expensive on
+    // big non-exhaustive types, and is [hard to
+    // fix](https://github.com/rust-lang/rust/issues/141006#issuecomment-2883415000) in general.
+    // Only doing this sanity check when debug assertions are turned on avoids the issue for the
+    // very specific case of #140944.
+    if layout.ty.is_privately_uninhabited(tcx, cx.typing_env) {
+        assert!(
+            layout.is_uninhabited(),
+            "{:?} is type-level uninhabited but not ABI-uninhabited?",
+            layout.ty
+        );
+    }
+
     /// Yields non-ZST fields of the type
     fn non_zst_fields<'tcx, 'a>(
         cx: &'a LayoutCx<'tcx>,