Use safe transmute by default

Using the zerocopy crate, we can port all header conversions from pointer cast to safe transmutes. After this change, the code in `header.rs` does not use `unsafe` anymore when the `zerocopy` feature is activated.

Since this adds a derive crate without feature addition as dependency, zerocopy is an optional (but on-by-default) dependency.

Author: konstin

.github/workflows/ci.yml

@@ -34,6 +34,8 @@ jobs:
       - run: cargo hack check --all --ignore-private --each-feature --no-dev-deps
       - run: cargo check --all --all-targets --all-features
       - run: cargo test
+      # Test without zerocopy
+      - run: cargo test --no-default-features
       - run: cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
 
   msrv:

Cargo.lock

@@ -29,6 +29,7 @@ portable-atomic = "1"
 rustc-hash = "2.1.0"
 tokio = { version = "1", features = ["fs", "io-util", "rt"] }
 tokio-stream = "0.1"
+zerocopy = { version = "0.8.23", features = ["derive"], optional = true }
 
 [dev-dependencies]
 tempfile = "3"
@@ -40,7 +41,9 @@ rustix = { version = "0.38", features = ["fs"] }
 xattr = { version = "1.0", optional = true }
 
 [features]
-default = ["xattr"]
+default = ["xattr", "zerocopy"]
+# Use safe transmute for headers (otherwise, unsafe pointer casts are used)
+zerocopy = ["dep:zerocopy"]
 
 [lib]
 name = "tokio_tar"