Auto merge of #99959 - cuviper:niche-size, r=eddyb

Fix the size of niche enums with ZST alignment

For enums with an aligned ZST variant, like `[T; 0]`, the niche layout
was not computing a sufficient size to be consistent with alignment. Now
we pad that size up to the alignment, and also make sure to only use the
niche variant's ABI when the size and alignment still match.

Fixes #99836
r? `@eddyb`

1 files changed, 8 insertions, 2 deletions

diff --git a/compiler/rustc_middle/src/ty/layout.rs b/compiler/rustc_middle/src/ty/layout.rs
index dde55dd9655..833edd22805 100644
--- a/compiler/rustc_middle/src/ty/layout.rs
+++ b/compiler/rustc_middle/src/ty/layout.rs
@@ -1231,11 +1231,15 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                                 .collect::<Result<IndexVec<VariantIdx, _>, _>>()?;
 
                             let offset = st[i].fields().offset(field_index) + niche.offset;
-                            let size = st[i].size();
+
+                            // Align the total size to the largest alignment.
+                            let size = st[i].size().align_to(align.abi);
 
                             let abi = if st.iter().all(|v| v.abi().is_uninhabited()) {
                                 Abi::Uninhabited
-                            } else {
+                            } else if align == st[i].align() && size == st[i].size() {
+                                // When the total alignment and size match, we can use the
+                                // same ABI as the scalar variant with the reserved niche.
                                 match st[i].abi() {
                                     Abi::Scalar(_) => Abi::Scalar(niche_scalar),
                                     Abi::ScalarPair(first, second) => {
@@ -1249,6 +1253,8 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                                     }
                                     _ => Abi::Aggregate { sized: true },
                                 }
+                            } else {
+                                Abi::Aggregate { sized: true }
                             };
 
                             let largest_niche = Niche::from_scalar(dl, offset, niche_scalar);