Use a delayed bug for this layout ICE

Fixes 144501

compiler/rustc_errors/src/lib.rs

@@ -1366,7 +1366,7 @@ impl<'a> DiagCtxtHandle<'a> {
         self.create_err(err).emit()
     }
 
-    /// Ensures that an error is printed. See `Level::DelayedBug`.
+    /// Ensures that an error is printed. See [`Level::DelayedBug`].
     //
     // No `#[rustc_lint_diagnostics]` and no `impl Into<DiagMessage>` because bug messages aren't
     // user-facing.

compiler/rustc_ty_utils/src/layout/invariant.rs

@@ -281,10 +281,16 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
                     }
 
                     // Ensure that for niche encoded tags the discriminant coincides with the variant index.
-                    assert_eq!(
-                        layout.ty.discriminant_for_variant(tcx, idx).unwrap().val,
-                        u128::from(idx.as_u32()),
-                    );
+                    let val = layout.ty.discriminant_for_variant(tcx, idx).unwrap().val;
+                    if val != u128::from(idx.as_u32()) {
+                        let adt_def = layout.ty.ty_adt_def().unwrap();
+                        cx.tcx().dcx().span_delayed_bug(
+                            cx.tcx().def_span(adt_def.did()),
+                            format!(
+                                "variant {idx:?} has discriminant {val:?} in niche-encoded type"
+                            ),
+                        );
+                    }
                 }
             }
             for variant in variants.iter() {

tests/ui/enum-discriminant/invalid-niche-discriminant.normal.stderr

@@ -0,0 +1,35 @@
+error[E0732]: `#[repr(inttype)]` must be specified for enums with explicit discriminants and non-unit variants
+  --> $DIR/invalid-niche-discriminant.rs:11:1
+   |
+LL |   enum E {
+   |   ^^^^^^
+...
+LL |       S0 {
+   |       -- non-unit discriminant declared here
+...
+LL |       Bar = {
+   |  ___________-
+LL | |         let x = 1;
+LL | |         3
+LL | |     },
+   | |_____- explicit discriminant specified here
+
+error[E0599]: no variant named `S1` found for enum `E`
+  --> $DIR/invalid-niche-discriminant.rs:23:18
+   |
+LL | enum E {
+   | ------ variant `S1` not found here
+...
+LL | static C: E = E::S1 { u: 23 };
+   |                  ^^
+   |
+help: there is a variant with a similar name
+   |
+LL - static C: E = E::S1 { u: 23 };
+LL + static C: E = E::S0 { u: 23 };
+   |
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0599, E0732.
+For more information about an error, try `rustc --explain E0599`.

tests/ui/enum-discriminant/invalid-niche-discriminant.rs

@@ -0,0 +1,25 @@
+//@ needs-rustc-debug-assertions
+//@ revisions: normal with_delayed
+//@ [with_delayed] compile-flags: -Z eagerly-emit-delayed-bugs
+
+#![crate_type = "lib"]
+
+// Repro for <https://github.com/rust-lang/rust/issues/144501>
+// which ICEd because the calculated layout is invalid
+// but which we needn't care about as the discriminant already was.
+
+enum E {
+//~^ ERROR must be specified
+//[with_delayed]~| ERROR variant 1 has discriminant 3
+    S0 {
+        s: String,
+    },
+    Bar = {
+        let x = 1;
+        3
+    },
+}
+
+static C: E = E::S1 { u: 23 };
+//~^ ERROR no variant named
+//[with_delayed]~| ERROR but no error emitted

tests/ui/enum-discriminant/invalid-niche-discriminant.with_delayed.stderr

@@ -0,0 +1,47 @@
+error[E0732]: `#[repr(inttype)]` must be specified for enums with explicit discriminants and non-unit variants
+  --> $DIR/invalid-niche-discriminant.rs:11:1
+   |
+LL |   enum E {
+   |   ^^^^^^
+...
+LL |       S0 {
+   |       -- non-unit discriminant declared here
+...
+LL |       Bar = {
+   |  ___________-
+LL | |         let x = 1;
+LL | |         3
+LL | |     },
+   | |_____- explicit discriminant specified here
+
+error: variant 1 has discriminant 3 in niche-encoded type
+  --> $DIR/invalid-niche-discriminant.rs:11:1
+   |
+LL | enum E {
+   | ^^^^^^
+
+error[E0599]: no variant named `S1` found for enum `E`
+  --> $DIR/invalid-niche-discriminant.rs:23:18
+   |
+LL | enum E {
+   | ------ variant `S1` not found here
+...
+LL | static C: E = E::S1 { u: 23 };
+   |                  ^^
+   |
+help: there is a variant with a similar name
+   |
+LL - static C: E = E::S1 { u: 23 };
+LL + static C: E = E::S0 { u: 23 };
+   |
+
+error: `Res::Err` but no error emitted
+  --> $DIR/invalid-niche-discriminant.rs:23:15
+   |
+LL | static C: E = E::S1 { u: 23 };
+   |               ^^^^^
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0599, E0732.
+For more information about an error, try `rustc --explain E0599`.