Sema: fix a few indexing bugs

* Indexing zero-bit types should not produce AIR indexing instructions
* Getting a runtime-known element pointer from a many-pointer should
  check that the many-pointer is not comptime-only

Resolves: #23405

src/Sema.zig

@@ -29037,6 +29037,14 @@ fn elemPtrOneLayerOnly(
             }
             const result_ty = try indexable_ty.elemPtrType(null, pt);
 
+            try sema.validateRuntimeElemAccess(block, elem_index_src, result_ty, indexable_ty, indexable_src);
+            try sema.validateRuntimeValue(block, indexable_src, indexable);
+
+            if (!try result_ty.childType(zcu).hasRuntimeBitsIgnoreComptimeSema(pt)) {
+                // zero-bit child type; just bitcast the pointer
+                return block.addBitCast(result_ty, indexable);
+            }
+
             return block.addPtrElemPtr(indexable, elem_index, result_ty);
         },
         .one => {
@@ -29497,6 +29505,11 @@ fn elemPtrSlice(
         const cmp_op: Air.Inst.Tag = if (slice_sent) .cmp_lte else .cmp_lt;
         try sema.addSafetyCheckIndexOob(block, src, elem_index, len_inst, cmp_op);
     }
+    if (!try slice_ty.childType(zcu).hasRuntimeBitsIgnoreComptimeSema(pt)) {
+        // zero-bit child type; just extract the pointer and bitcast it
+        const slice_ptr = try block.addTyOp(.slice_ptr, slice_ty.slicePtrFieldType(zcu), slice);
+        return block.addBitCast(elem_ptr_ty, slice_ptr);
+    }
     return block.addSliceElemPtr(slice, elem_index, elem_ptr_ty);
 }
 

test/behavior/pointers.zig

@@ -760,3 +760,27 @@ test "comptime pointer equality through distinct elements with well-defined layo
     comptime assert(buf[1] == 456);
     comptime assert(second_elem.* == 456);
 }
+
+test "pointers to elements of slice of zero-bit type" {
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
+    var slice: []const u0 = undefined;
+    slice = &.{ 0, 0 };
+
+    const a = &slice[0];
+    const b = &slice[1];
+
+    try expect(a == b);
+}
+
+test "pointers to elements of many-ptr to zero-bit type" {
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
+    var many_ptr: [*]const u0 = undefined;
+    many_ptr = &.{ 0, 0 };
+
+    const a = &many_ptr[0];
+    const b = &many_ptr[1];
+
+    try expect(a == b);
+}

test/cases/compile_errors/runtime_index_into_comptime_only_many_ptr.zig

@@ -0,0 +1,10 @@
+var rt: usize = 0;
+export fn foo() void {
+    const x: [*]const type = &.{ u8, u16 };
+    _ = &x[rt];
+}
+
+// error
+//
+// :4:12: error: values of type '[*]const type' must be comptime-known, but index value is runtime-known
+// :4:11: note: types are not available at runtime