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<!-- Copyright 2025 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. -->
# Unsafe Code Guidelines
`unsafe` code is extremely dangerous and should be avoided unless absolutely
necessary. When it is absolutely necessary to write `unsafe` code, it should be
done extremely carefully.
This document covers guidelines for writing unsafe code, including pointer casts
and safety comments.
## Pointer Casts
- **Avoid `&slice[0] as *const T`**: Use `slice.as_ptr()`. Accessing subsequent
elements via pointer arithmetic on a single-element pointer is UB.
```rust
let slice = &[1, 2];
// BAD: Derived from reference to single element.
let ptr = &slice[0] as *const i32;
// SAFETY: UB! `ptr` has provenance only for the first element.
// Accessing `ptr.add(1)` is out of bounds for this provenance.
let val = unsafe { *ptr.add(1) };
// GOOD: Derived from the slice itself.
let ptr = slice.as_ptr();
// SAFETY: Safe because `ptr` has provenance for the entire slice.
let val = unsafe { *ptr.add(1) };
```
- **Avoid converting `&mut T` to `*const T`**: This reborrows as a shared
reference, restricting permissions. Cast `&mut T` to `*mut T` first.
```rust
let mut val = 42;
let r = &mut val;
// BAD: `r as *const i32` creates a shared reborrow.
// The resulting pointer loses write provenance.
let ptr = r as *const i32 as *mut i32;
// SAFETY: UB! Writing to a pointer derived from a shared reborrow.
unsafe { *ptr = 0 };
// GOOD: `r as *mut i32` preserves write provenance.
let ptr = r as *mut i32;
// SAFETY: Safe because `ptr` retains mutable provenance.
unsafe { *ptr = 0 };
```
## Safety Comments
Every `unsafe` block must be documented with a `// SAFETY:` comment.
- **Requirement:** The comment must prove soundness using *only* text from the
stable [Rust Reference](https://doc.rust-lang.org/reference/) or [standard
library documentation](https://doc.rust-lang.org/std/).
- **Citation:** You must cite and quote the relevant text from the
documentation. Citations must cite a specific version of the documentation
(e.g. https://doc.rust-lang.org/1.91.0/reference/ or
https://doc.rust-lang.org/1.91.0/std/).
- **Prohibition:** Do not rely on "common sense" or behavior not guaranteed by
the docs.
```rust
// BAD: Missing justification for "obvious" properties.
// SAFETY: `ptr` and `field` are from the same object.
let offset = unsafe { field.cast::<u8>().offset_from(ptr.cast::<u8>()) };
// GOOD: Explicitly justifies every requirement, even trivial ones.
// SAFETY:
// - `ptr` and `field` are derived from the same allocated object [1].
// - The distance between them is trivially a multiple of `u8`'s size (1) [2],
// satisfying `offset_from`'s alignment requirement [1].
//
// [1] Per https://doc.rust-lang.org/1.91.0/std/primitive.pointer.html#method.offset_from:
//
// Both pointers must be derived from the same allocated object, and the
// distance between them must be a multiple of the element size.
//
// [2] https://doc.rust-lang.org/1.91.0/reference/type-layout.html#primitive-data-layout
let offset = unsafe { field.cast::<u8>().offset_from(ptr.cast::<u8>()) };
```
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