disallow slicing many-item pointer with different sentinel (rev. 2)

src/Sema.zig

@@ -33409,6 +33409,43 @@ fn analyzeSlice(
                 break :e try sema.coerce(block, Type.usize, uncasted_end, end_src);
             } else break :e try sema.coerce(block, Type.usize, uncasted_end_opt, end_src);
         }
+
+        // when slicing a many-item pointer, if a sentinel `S` is provided as in `ptr[a.. :S]`, it
+        // must match the sentinel of `@TypeOf(ptr)`.
+        if (sentinel_opt != .none) sentinel_check: {
+            const provided = provided: {
+                const casted = try sema.coerce(block, elem_ty, sentinel_opt, sentinel_src);
+                try checkSentinelType(sema, block, sentinel_src, elem_ty);
+                break :provided try sema.resolveConstDefinedValue(block, sentinel_src, casted, .{
+                    .needed_comptime_reason = "slice sentinel must be comptime-known",
+                });
+            };
+
+            const msg = msg: {
+                const parent = if (ptr_sentinel) |expected| parent: {
+                    if (expected.eql(provided, elem_ty, zcu)) break :sentinel_check;
+                    break :parent try sema.errMsg(
+                        sentinel_src,
+                        "sentinel-terminated slicing of many-item pointer must preserve sentinel; expected sentinel {}, found {}",
+                        .{
+                            expected.fmtValue(pt),
+                            provided.fmtValue(pt),
+                        },
+                    );
+                } else parent: {
+                    break :parent try sema.errMsg(
+                        sentinel_src,
+                        "sentinel-terminated slicing is not permitted on many-item pointer without sentinel",
+                        .{},
+                    );
+                };
+                errdefer parent.destroy(sema.gpa);
+
+                try sema.errNote(src, parent, "use @ptrCast to cast pointer sentinel", .{});
+                break :msg parent;
+            };
+            return sema.failWithOwnedErrorMsg(block, msg);
+        }
         return sema.analyzePtrArithmetic(block, src, ptr, start, .ptr_add, ptr_src, start_src);
     };
 

test/behavior/slice.zig

@@ -617,9 +617,6 @@ test "slice syntax resulting in pointer-to-array" {
             comptime assert(@TypeOf(ptr[1..][0..2]) == *[2]u8);
             comptime assert(@TypeOf(ptr[1..][0..4]) == *[4]u8);
             comptime assert(@TypeOf(ptr[1..][0..2 :4]) == *[2:4]u8);
-            comptime assert(@TypeOf(ptr[1.. :0][0..2]) == *[2]u8);
-            comptime assert(@TypeOf(ptr[1.. :0][0..4]) == *[4]u8);
-            comptime assert(@TypeOf(ptr[1.. :0][0..2 :4]) == *[2:4]u8);
 
             var ptr_z: [*:0]u8 = &array;
             comptime assert(@TypeOf(ptr_z[1..][0..2]) == *[2]u8);

test/cases/compile_errors/slice_of_many-item_pointer_preserves_sentinel.zig

@@ -0,0 +1,16 @@
+comptime {
+    var ptr: [*]const u8 = undefined;
+    _ = ptr[0.. :0];
+}
+
+comptime {
+    var ptrz: [*:0]const u8 = undefined;
+    _ = ptrz[0.. :1];
+}
+
+// error
+//
+// :3:18: error: sentinel-terminated slicing is not permitted on many-item pointer without sentinel
+// :3:12: note: use @ptrCast to cast pointer sentinel
+// :8:19: error: sentinel-terminated slicing of many-item pointer must preserve sentinel; expected sentinel 0, found 1
+// :8:13: note: use @ptrCast to cast pointer sentinel