Fix the size of niche enums with ZST alignment

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);

src/test/ui/layout/zero-sized-array-enum-niche.rs

@@ -0,0 +1,45 @@
+// normalize-stderr-test "pref: Align\([1-8] bytes\)" -> "pref: $$PREF_ALIGN"
+#![feature(rustc_attrs)]
+#![crate_type = "lib"]
+
+// Various tests around the behavior of zero-sized arrays and
+// enum niches, especially that they have coherent size and alignment.
+
+// The original problem in #99836 came from ndarray's `TryFrom` for
+// `SliceInfo<[SliceInfoElem; 0], Din, Dout>`, where that returns
+// `Result<Self, ShapeError>` ~= `Result<AlignedZST, TypeWithNiche>`.
+// This is a close enough approximation:
+#[rustc_layout(debug)]
+type AlignedResult = Result<[u32; 0], bool>; //~ ERROR: layout_of
+// The bug gave that size 1 with align 4, but the size should also be 4.
+// It was also using the bool niche for the enum tag, which is fine, but
+// after the fix, layout decides to use a direct tagged repr instead.
+
+// Here's another case with multiple ZST alignments, where we should
+// get the maximal alignment and matching size.
+#[rustc_layout(debug)]
+enum MultipleAlignments { //~ ERROR: layout_of
+    Align2([u16; 0]),
+    Align4([u32; 0]),
+    Niche(bool),
+}
+
+// Tagged repr is clever enough to grow tags to fill any padding, e.g.:
+// 1.   `T_FF` (one byte of Tag, one byte of padding, two bytes of align=2 Field)
+//   -> `TTFF` (Tag has expanded to two bytes, i.e. like `#[repr(u16)]`)
+// 2.    `TFF` (one byte of Tag, two bytes of align=1 Field)
+//   -> Tag has no room to expand!
+//   (this outcome can be forced onto 1. by wrapping Field in `Packed<...>`)
+#[repr(packed)]
+struct Packed<T>(T);
+
+#[rustc_layout(debug)]
+type NicheLosesToTagged = Result<[u32; 0], Packed<std::num::NonZeroU16>>; //~ ERROR: layout_of
+// Should get tag_encoding: Direct, size == align == 4.
+
+#[repr(u16)]
+enum U16IsZero { _Zero = 0 }
+
+#[rustc_layout(debug)]
+type NicheWinsOverTagged = Result<[u32; 0], Packed<U16IsZero>>; //~ ERROR: layout_of
+// Should get tag_encoding: Niche, size == align == 4.