From 6d0b8879728722d14561d747717f616cd4e7ae0c Mon Sep 17 00:00:00 2001
From: Justus Klausecker <justus@klausecker.de>
Date: Wed, 6 May 2026 09:42:40 +0200
Subject: [PATCH] Sema: disallow pointer cast from pointer to opaque type to
 slice

Fixes a regression where this conversion crashes. When the conversion was
still possible it would produce a slice with a length of zero, which doesn't
really make a lot of sense either. There's no way to determine the length
of the destination slice from a pointer to an opaque type, so it's a compile
error now. Users should just cast to a many-item pointer and slice to the
desired length manually instead.
---
 src/Sema.zig                                       | 10 ++++++++++
 .../compile_errors/ptrcast_anyopaque_to_slice.zig  | 14 ++++++++++++++
 2 files changed, 24 insertions(+)
 create mode 100644 test/cases/compile_errors/ptrcast_anyopaque_to_slice.zig

diff --git a/src/Sema.zig b/src/Sema.zig
index 645e639f141e15c245de6648935a35598b8bbbba..f691a972982c7b5e5d20d4b5860c5fbab5761672 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -21154,6 +21154,16 @@ fn ptrCastFull(
             break :len if (opt_src_len) |l| .{ .constant = l } else .equal_runtime_src_slice;
         }
         if (!src_elem_ty.comptimeOnly(zcu) and !dest_elem_ty.comptimeOnly(zcu)) {
+            if (src_elem_ty.zigTypeTag(zcu) == .@"opaque") {
+                return sema.failWithOwnedErrorMsg(block, msg: {
+                    const msg = try sema.errMsg(src, "cannot infer length of slice of '{f}' from pointer to opaque type '{f}' with unknown size", .{
+                        dest_elem_ty.fmt(pt), src_elem_ty.fmt(pt),
+                    });
+                    errdefer msg.destroy(gpa);
+                    try sema.addDeclaredHereNote(msg, src_elem_ty);
+                    break :msg msg;
+                });
+            }
             const src_elem_size = src_elem_ty.abiSize(zcu);
             const dest_elem_size = dest_elem_ty.abiSize(zcu);
             if (dest_elem_size == 0) {
diff --git a/test/cases/compile_errors/ptrcast_anyopaque_to_slice.zig b/test/cases/compile_errors/ptrcast_anyopaque_to_slice.zig
new file mode 100644
index 0000000000000000000000000000000000000000..12a9de61ee28192d87ffbb7e3ebd1091875e1659
--- /dev/null
+++ b/test/cases/compile_errors/ptrcast_anyopaque_to_slice.zig
@@ -0,0 +1,14 @@
+export fn entry1(x: *anyopaque) void {
+    _ = @as([]u8, @ptrCast(x));
+}
+
+const Opaque = opaque {};
+export fn entry2(x: *Opaque) void {
+    _ = @as([]u8, @ptrCast(x));
+}
+
+// error
+//
+// :2:19: error: cannot infer length of slice of 'u8' from pointer to opaque type 'anyopaque' with unknown size
+// :7:19: error: cannot infer length of slice of 'u8' from pointer to opaque type 'tmp.Opaque' with unknown size
+// :5:16: note: opaque declared here
-- 
2.54.0