From 51ef31a8331e92103253a37ddfdacbd263cee81d Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Mon, 4 Apr 2022 14:28:30 -0700
Subject: [PATCH] Sema: add empty tuple to mutable slice coercion

---
 src/Sema.zig           | 13 +++++++++++++
 test/behavior/cast.zig |  5 +++++
 2 files changed, 18 insertions(+)

diff --git a/src/Sema.zig b/src/Sema.zig
index 65550bec00dd944b6de26741a7362ac21a8cc24c..1430e80ac046c00d8e72fdfe61f6fd0d31ac7b83 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -18166,6 +18166,19 @@ fn coerce(
                     {
                         return sema.coerceTupleToSlicePtrs(block, dest_ty, dest_ty_src, inst, inst_src);
                     }
+
+                    // empty tuple to zero-length slice
+                    // note that this allows coercing to a mutable slice.
+                    if (inst_ty.isSinglePointer() and
+                        inst_ty.childType().tag() == .empty_struct_literal and
+                        dest_info.size == .Slice)
+                    {
+                        const slice_val = try Value.Tag.slice.create(sema.arena, .{
+                            .ptr = Value.undef,
+                            .len = Value.zero,
+                        });
+                        return sema.addConstant(dest_ty, slice_val);
+                    }
                 },
                 .Many => p: {
                     if (!inst_ty.isSlice()) break :p;
diff --git a/test/behavior/cast.zig b/test/behavior/cast.zig
index 4e952828c58d9fad598de500210fdd32a2d05318..0473a3603388b9e96b5d6184b2a00a8804b9dbf6 100644
--- a/test/behavior/cast.zig
+++ b/test/behavior/cast.zig
@@ -1386,3 +1386,8 @@ test "coerce undefined single-item pointer of array to error union of slice" {
     const s = try b;
     try expect(s.len == 0);
 }
+
+test "pointer to empty struct literal to mutable slice" {
+    var x: []i32 = &.{};
+    try expect(x.len == 0);
+}
-- 
2.54.0