Rollup merge of #124504 - gurry:123710-union-ICE, r=oli-obk

Mark unions non-const-propagatable in `KnownPanicsLint` without calling layout

Fixes #123710

The ICE occurs during the layout calculation of the union `InvalidTag` in #123710 because the following assert fails:https://github.com/rust-lang/rust/blob/5fe8b697e729b6eb64841a3905e57da1b47f4ca3/compiler/rustc_abi/src/layout.rs#L289-L292

The layout calculation is invoked by `KnownPanicsLint` when it is trying to figure out which locals it can const prop. Since `KnownPanicsLint` is never actually going to const props unions thanks to PR https://github.com/rust-lang/rust/pull/121628 there's no point calling layout to check if it can. So in this fix I skip the call to layout and just mark the local non-const propagatable if it is a union.

1 files changed, 13 insertions, 7 deletions

diff --git a/compiler/rustc_mir_transform/src/known_panics_lint.rs b/compiler/rustc_mir_transform/src/known_panics_lint.rs
index b8dbf8a759f..b2030efca8f 100644
--- a/compiler/rustc_mir_transform/src/known_panics_lint.rs
+++ b/compiler/rustc_mir_transform/src/known_panics_lint.rs
@@ -896,13 +896,19 @@ impl CanConstProp {
         };
         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
             let ty = body.local_decls[local].ty;
-            match tcx.layout_of(param_env.and(ty)) {
-                Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
-                // Either the layout fails to compute, then we can't use this local anyway
-                // or the local is too large, then we don't want to.
-                _ => {
-                    *val = ConstPropMode::NoPropagation;
-                    continue;
+            if ty.is_union() {
+                // Do not const prop unions as they can
+                // ICE during layout calc
+                *val = ConstPropMode::NoPropagation;
+            } else {
+                match tcx.layout_of(param_env.and(ty)) {
+                    Ok(layout) if layout.size < Size::from_bytes(MAX_ALLOC_LIMIT) => {}
+                    // Either the layout fails to compute, then we can't use this local anyway
+                    // or the local is too large, then we don't want to.
+                    _ => {
+                        *val = ConstPropMode::NoPropagation;
+                        continue;
+                    }
                 }
             }
         }