Add tcast crate with TransparentCast derive

Cargo.toml

@@ -70,6 +70,7 @@ members = [
     "utils/litemap",
     "utils/resb",
     "utils/tinystr",
+    "utils/tcast",
     "utils/tzif",
     "utils/potential_utf",
     "utils/writeable",
@@ -299,6 +300,7 @@ rand = "0.9"
 rand_distr = "0.5"
 rand_pcg = "0.9"
 rayon = "1.3.0"
+ref_cast = "1.0"
 regex = "1.12.2"
 rkyv = "0.7"
 rmp-serde = "1.2.0"

utils/tcast/Cargo.toml

@@ -0,0 +1,37 @@
+# This file is part of ICU4X. For terms of use, please see the file
+# called LICENSE at the top level of the ICU4X source tree
+# (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
+
+[package]
+name = "tcast"
+description = "Cast transparent wrappers between their inner and outer types"
+version = "0.1.0"
+authors = ["Shane F Carr <shane@unicode.org>"]
+categories = ["rust-patterns", "no-std", "no-std::no-alloc"]
+keywords = ["transparent", "cast", "ref", "box", "dst"]
+
+edition.workspace = true
+include.workspace = true
+license.workspace = true
+repository.workspace = true
+rust-version = "1.71.1"
+
+[lib]
+proc-macro = true
+path = "src/lib.rs"
+
+[package.metadata.workspaces]
+independent = true
+
+[package.metadata.docs.rs]
+all-features = true
+
+[dependencies]
+proc-macro2 = { workspace = true }
+quote = { workspace = true }
+syn = { workspace = true, features = ["fold"] }
+
+[dev-dependencies]
+
+[lints]
+workspace = true

utils/tcast/LICENSE

@@ -0,0 +1,46 @@
+UNICODE LICENSE V3
+
+COPYRIGHT AND PERMISSION NOTICE
+
+Copyright © 2020-2024 Unicode, Inc.
+
+NOTICE TO USER: Carefully read the following legal agreement. BY
+DOWNLOADING, INSTALLING, COPYING OR OTHERWISE USING DATA FILES, AND/OR
+SOFTWARE, YOU UNEQUIVOCALLY ACCEPT, AND AGREE TO BE BOUND BY, ALL OF THE
+TERMS AND CONDITIONS OF THIS AGREEMENT. IF YOU DO NOT AGREE, DO NOT
+DOWNLOAD, INSTALL, COPY, DISTRIBUTE OR USE THE DATA FILES OR SOFTWARE.
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of data files and any associated documentation (the "Data Files") or
+software and any associated documentation (the "Software") to deal in the
+Data Files or Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, and/or sell
+copies of the Data Files or Software, and to permit persons to whom the
+Data Files or Software are furnished to do so, provided that either (a)
+this copyright and permission notice appear with all copies of the Data
+Files or Software, or (b) this copyright and permission notice appear in
+associated Documentation.
+
+THE DATA FILES AND SOFTWARE ARE PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
+KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF
+THIRD PARTY RIGHTS.
+
+IN NO EVENT SHALL THE COPYRIGHT HOLDER OR HOLDERS INCLUDED IN THIS NOTICE
+BE LIABLE FOR ANY CLAIM, OR ANY SPECIAL INDIRECT OR CONSEQUENTIAL DAMAGES,
+OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
+WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
+ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THE DATA
+FILES OR SOFTWARE.
+
+Except as contained in this notice, the name of a copyright holder shall
+not be used in advertising or otherwise to promote the sale, use or other
+dealings in these Data Files or Software without prior written
+authorization of the copyright holder.
+
+SPDX-License-Identifier: Unicode-3.0
+
+—
+
+Portions of ICU4X may have been adapted from ICU4C and/or ICU4J.
+ICU 1.8.1 to ICU 57.1 © 1995-2016 International Business Machines Corporation and others.

utils/tcast/examples/transparent_cast.rs

@@ -0,0 +1,16 @@
+// This file is part of ICU4X. For terms of use, please see the file
+// called LICENSE at the top level of the ICU4X source tree
+// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
+
+use tcast::TransparentCast;
+
+#[derive(Debug, TransparentCast)]
+#[tcast(std)]
+#[repr(transparent)]
+struct Wrap<T: ?Sized>(T);
+
+fn main() {
+    let inner = "hello world";
+    let wrap = Wrap::<str>::tcast_ref(inner);
+    println!("{wrap:?}");
+}