[unsafe-fields] Add README.md

Makes progress on #1931

gherrit-pr-id: Icc9b6841e66c961989862ff6fb3b4f5140c54513

ci/check_readme.sh

@@ -16,4 +16,6 @@ set -eo pipefail
 cargo install -q cargo-readme --version 3.2.0
 
 diff <(cargo -q run --manifest-path tools/Cargo.toml -p generate-readme) README.md >&2
-exit $?
+
+cd unsafe-fields
+diff <(cargo -q run --manifest-path ../tools/Cargo.toml -p generate-readme) README.md >&2

unsafe-fields/Cargo.toml

@@ -8,7 +8,7 @@
 
 [package]
 name = "unsafe-fields"
-version = "0.2.0"
+version = "0.2.1"
 edition = "2021"
 description = "Make it unsafe to access or modify fields with safety invariants"
 license = "BSD-2-Clause OR Apache-2.0 OR MIT"

unsafe-fields/README.md

@@ -0,0 +1,112 @@
+<!-- Copyright 2024 The Fuchsia Authors
+
+Licensed under a BSD-style license <LICENSE-BSD>, Apache License, Version 2.0
+<LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
+license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
+This file may not be copied, modified, or distributed except according to
+those terms.
+
+WARNING: DO NOT EDIT THIS FILE. It is generated automatically. Edits should be
+made in the doc comment on `src/lib.rs` or in `tools/generate-readme`.
+-->
+
+# unsafe-fields
+
+Support for unsafe fields.
+
+This crate provides the `unsafe_fields!` macro, which can be used to mark
+fields as unsafe. Unsafe fields automatically have their types wrapped using
+the `Unsafe` wrapper type. An `Unsafe` is intended to be used to for
+struct, enum, or union fields which carry safety invariants. All accessors
+are `unsafe`, which requires any use of an `Unsafe` field to be inside an
+`unsafe` block. One exception is `Unsafe::as_ref`, which is available when
+the `zerocopy_0_8` feature is enabled. See its docs for more information.
+
+An unsafe field has the type `Unsafe<O, F, const NAME_HASH: u128>`. `O` is
+the enclosing type (struct, enum, or union), `F` is the type of the field,
+and `NAME_HASH` is the hash of the field's name. `O` prevents swapping
+unsafe fields of the same `F` type between different enclosing types, and
+`NAME_HASH` prevents swapping different fields of the same `F` type within
+the same enclosing type. Note that swapping the same field between instances
+of the same type [cannot be prevented](crate#limitations).
+
+[immutable]: zerocopy_0_8::Immutable
+
+## Examples
+
+```rust
+use unsafe_fields::{unsafe_fields, Unsafe};
+
+unsafe_fields! {
+    /// A `usize` which is guaranteed to be even.
+    pub struct EvenUsize {
+        // INVARIANT: `n` is even.
+        #[unsafe]
+        n: usize,
+    }
+}
+
+impl EvenUsize {
+    /// Constructs a new `EvenUsize`.
+    ///
+    /// Returns `None` if `n` is odd.
+    pub fn new(n: usize) -> Option<EvenUsize> {
+        if n % 2 != 0 {
+            return None;
+        }
+        // SAFETY: We just confirmed that `n` is even.
+        let n = unsafe { Unsafe::new(n) };
+        Some(EvenUsize { n })
+    }
+}
+```
+
+Attempting to swap unsafe fields of the same type is prevented:
+
+```compile_fail,E0308
+use unsafe_fields::{unsafe_fields, Unsafe};
+
+unsafe_fields! {
+    /// A range.
+    pub struct Range {
+        // INVARIANT: `lo <= hi`.
+        #[unsafe]
+        lo: usize,
+        #[unsafe]
+        hi: usize,
+    }
+}
+
+impl Range {
+    pub fn swap(&mut self) {
+        // ERROR: Mismatched types
+        core::mem::swap(&mut self.lo, &mut self.hi);
+    }
+}
+```
+
+## Limitations
+
+Note that we cannot prevent `Unsafe`s from being swapped between the same
+field in instances of the same type:
+
+```rust
+use unsafe_fields::{unsafe_fields, Unsafe};
+
+unsafe_fields! {
+    /// A `usize` which is guaranteed to be even.
+    pub struct EvenUsize {
+        // INVARIANT: `n` is even.
+        #[unsafe]
+        n: usize,
+    }
+}
+
+pub fn swap(a: &mut EvenUsize, b: &mut EvenUsize) {
+    core::mem::swap(&mut a.n, &mut b.n);
+}
+```
+
+## Disclaimer
+
+Disclaimer: Zerocopy is not an officially supported Google product.

unsafe-fields/src/lib.rs

@@ -6,6 +6,11 @@
 // This file may not be copied, modified, or distributed except according to
 // those terms.
 
+// After updating the following doc comment, make sure to run the following
+// command to update `README.md` based on its contents:
+//
+//   cargo -q run --manifest-path ../tools/Cargo.toml -p generate-readme > README.md
+
 //! Support for unsafe fields.
 //!
 //! This crate provides the [`unsafe_fields!`] macro, which can be used to mark