From ca3cf29fe9f03867a5426afa58c9e968deccb61a Mon Sep 17 00:00:00 2001 From: Joshua Liebow-Feeser Date: Sat, 12 Aug 2023 00:19:23 -0700 Subject: [PATCH] [project] Initial commit Makes progress on #196 --- src/lib.rs | 6 + src/project.rs | 673 +++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 679 insertions(+) create mode 100644 src/project.rs diff --git a/src/lib.rs b/src/lib.rs index c462c15d7e..f0e78afe3e 100644 --- a/src/lib.rs +++ b/src/lib.rs @@ -162,6 +162,12 @@ pub mod byteorder; #[doc(hidden)] pub mod derive_util; +// TODO(#196): Once we figure out how to make the `project!` macro safe +// (specifically, by statically rejecting projection through `deref`), remove +// this `#[doc(hidden)]` and make `project` part of our public API. +#[doc(hidden)] +pub mod project; + #[cfg(feature = "byteorder")] pub use crate::byteorder::*; #[cfg(any(feature = "derive", test))] diff --git a/src/project.rs b/src/project.rs new file mode 100644 index 0000000000..2067ba056e --- /dev/null +++ b/src/project.rs @@ -0,0 +1,673 @@ +// Copyright 2023 The Fuchsia Authors. All rights reserved. +// Use of this source code is governed by a BSD-style license that can be +// found in the LICENSE file. + +//! Field projection inside of any container type. +//! +//! # How to use this module (for container users) +//! +//! Just call `project!(&a.b.c)`. It's that easy! +//! +//! Okay, maybe you want a bit more detail... +//! +//! ## The quick version +//! +//! Here's a quick, very dense explanation. If you want a friendlier +//! explanation, skip to the next section. +//! +//! Given a container type, `C`, an inner type, `I`, and a field in `I`, `f: F`, +//! if `C` implements the [`Projectable`] trait, then `project!` allows you +//! to project from a `&C` to a `&C` which points to the `f` field within +//! the original `C`. For `c: &C`, this is invoked as `project!(&c.f)`. +//! +//! ## The long version +//! +//! Let's say you're using a crate which provides a container type like the +//! following: +//! +//! ```rust +//! /// An unaligned `T`. +//! /// +//! /// `Unalign` has the same layout as `T`, except that its alignment +//! /// is always 1 regardless of `T`'s alignment. +//! #[repr(C, packed)] +//! pub struct Unalign{ +//! # t: T, +//! # } +//! ``` +//! +//! You're using it with a type from your crate: +//! +//! ```rust +//! #[repr(C)] +//! struct UdpHeader { +//! src_port: u16, +//! dst_port: u16, +//! length: u16, +//! checksum: u16, +//! } +//! ``` +//! +//! Let's say you're reading UDP packets off the network. `UdpHeader` contains +//! `u16`s, and so (on some platforms) it has an alignment of 2. But you don't +//! have any control of where your packets are stored in memory, so you can't +//! construct a `&UdpHeader` from the bytes you've received off the network. +//! Instead, you construct an `&Unalign`. +//! +//! That's all well and good, but what if you want to hand out a reference to a +//! packet's source port? It's a `u16` that might not be validly-aligned, so you +//! can't just create a `&u16` like you would be able to do if you just had a +//! `&UdpHeader`. What you'd really like to do is take your +//! `&Unalign` and take a reference to the `src_port`... and get back +//! an `&Unalign`. That's what `project!` lets you do: +//! +//! ```rust +//! # use zerocopy::project; +//! # struct UdpHeader { +//! # src_port: u16, +//! # dst_port: u16, +//! # length: u16, +//! # checksum: u16, +//! # } +//! # #[repr(C, packed)] +//! # struct Unalign{ +//! # t: T, +//! # } +//! # unsafe impl zerocopy::project::Projectable> for Unalign { +//! # type Inner = T; +//! # } +//! # fn read_udp_header() -> &'static Unalign { todo!() } +//! # fn __main() { +//! let packet = read_udp_header(); +//! // SAFETY: We don't perform projection through any references or other +//! // fields that implement `Deref` or `DerefMut`. +//! let src_port = unsafe { project!(&packet.src_port) }; +//! # } +//! ``` +//! +//! # How to use this module (for container authors) +//! +//! TODO(#196): Fill this section in before removing `#[doc(hidden)]` from this +//! module. + +/// A container which supports field projection of its contained type. +/// +/// # Example +/// +/// ```rust +/// # use zerocopy::project::Projectable; +/// #[repr(transparent)] +/// struct Wrapper(T); +/// +/// unsafe impl Projectable> for Wrapper { +/// type Inner = T; +/// } +/// ``` +/// +/// # Safety +/// +/// If `P: Projectable`, then the following must hold: +/// - Given `p: *const P` or `p: *mut P`, it is valid to perform `let i = p as +/// *const P::Inner` or `let i = p as *mut P::Inner`. The size of the +/// referents of `p` and `i` must be identical (e.g. as reported by +/// `size_of_val_raw`). +/// - If the following hold: +/// - `p: &P` or `p: &mut P`. +/// - Given an `i: P::Inner` of size `size_of_val(p)`, there exists an `F` at +/// byte range `f` within `i`. +/// +/// ...then it is sound to materialize a `&W` or `&mut W` which points to range +/// `f` within `p`. +/// +/// Note that this definition holds regardless of whether `P`, `P::Inner`, or +/// `F` are sized or unsized. +/// +// TODO(#196): Can we relax this safety requirement to support other types? +// E.g., what about `PhantomData`? If we can relax this requirement, it must be +// the case that, for DSTs, the prefix and tail-slice-element sizes must be the +// same (put another way, given a reference to the outer type with a certain +// number of tail elements, a reference to the inner type with the same number +// of tail elements will reference the same memory region). This is trivially +// true of the current safety requirement, but might need to be explicitly +// called out depending on how we relax that requirement. +pub unsafe trait Projectable { + /// The inner type. + type Inner: ?Sized; +} + +// TODO(#196): Once we expose this module in our public API (not +// `#[doc(hidden)]`), expose this for implementors to use? +macro_rules! unsafe_impl_projectable { + ($($c:ident)::* $(: ?$sized:ident)?) => { + unsafe impl Projectable> for $($c)::* { + type Inner = T; + } + }; +} + +safety_comment! { + /// SAFETY: + /// `MaybeUninit` and `Wrapping` are both documented to have the same + /// layout as `T`. + unsafe_impl_projectable!(core::mem::MaybeUninit); + unsafe_impl_projectable!(core::num::Wrapping); +} + +/// Performs field projection on `outer`, projecting into the field of type `F` +/// at the address provided by `inner_to_field`. +/// +/// # Safety +/// +/// `outer_to_inner(o)` must return a pointer, `i`, which has the same address +/// as `o`. It must be the case that `size_of_val(o) == size_of_val(i)`. +/// +/// `inner_to_field(i)` must return a pointer, `f`, with the following property: +/// If `i` points to a validly-initialized `P::Inner`, then `f` points to a +/// validly-initialized `F` which lives within the memory region addressed by +/// `i`. `inner_to_field` may NOT assume that `i` *does* actually point to a +/// validly-initialized `P::Inner`. `inner_to_field` may also NOT assume that +/// `i` is aligned. More specifically, `inner_to_field` may only assume that it +/// is sound to invoke `core::ptr::addr_of!(*i)`; it may not assume anything +/// that is not logically deducible from that assumption. +/// +/// TODO: Maybe this safety requirement is too weak. While the "lives in the +/// same memory region" constraint is meant to prevent dereferencing inside +/// `inner_to_field`, it would technically permit dereferencing a +/// self-referential pointer - after all, it's allowed to assume that `i` points +/// to a validly-initialized `P::Inner`. What we really want to say is that `i` +/// isn't necessarily initialized, but still has the correct field offsets, but +/// how do we articulate that? +/// +/// `field_to_wrapped_field(f)` must return a pointer, `w`, which has the same +/// address as `f`. It must be the case that `size_of_val(f) == size_of_val(w)`. +#[doc(hidden)] +#[inline(always)] +pub fn project( + _unsafe: unsafe_token::UnsafeToken, + outer: &P, + outer_to_inner: OuterToInner, + inner_to_field: InnerToField, + field_to_wrapped_field: FieldToWrappedField, +) -> &W +where + P: Projectable + ?Sized, + // TODO(#196), TODO(https://github.com/rust-lang/reference/pull/1387), + // TODO(https://github.com/rust-lang/rust/pull/114330): Remove this bound + // once we support unsized projection (see comment on `Unalign` for more + // details). + P::Inner: Sized, + F: ?Sized, + W: ?Sized, + OuterToInner: FnOnce(*const P) -> *const Unalign, + InnerToField: FnOnce(*const Unalign) -> *const F, + FieldToWrappedField: FnOnce(*const F) -> *const W, +{ + let outer: *const P = outer; + let inner = outer_to_inner(outer); + // SAFETY: We promise to only call `inner_to_field` with a value `inner` + // such that it is sound to call `addr_of!(*inner)`. Since this `inner` is + // derived from a vanilla reference, it is guaranteed to be non-zero. Since + // it is of type `*const Unalign<_>`, which is a `repr(packed)` type (and + // thus has alignment 1), it is trivially aligned. Those are the only two + // safety preconditions required in order to call `addr_of!(*inner)`. + let field = inner_to_field(inner); + let wrapped_field = field_to_wrapped_field(field); + // SAFETY: + // - Since `outer: *const P` is derived from the `outer` argument, which is + // a `&P`, `outer: *const P` points to a valid `P`. + // - Since `P: Projectable`, `P` has the same size and field offsets as + // `P::Inner`. + // - By safety precondition, `inner` points to the same memory region as + // `outer`. Based on the preceding premises, `inner` points to a valid + // `P::Inner`. + // - By safety precondition, since `inner_to_field` is invoked with a + // pointer to a valid `P::Inner`, it returns a pointer to a valid `F` + // which lives in the same memory region. Thus, `field` points to a valid + // `F`. TODO: Is this actually sufficient to prove that `field` points to + // a valid `F`? See the TODO in the doc comment on this function. + // - Since `P: Projectable`, `F` has the same size and field offsets + // as `W`. + // - By safety precondition, `wrapped_field` points to the same memory + // region as `field`, which we know points to a valid `F`. Since `F` has + // the same size and field offsets as `W`, `wrapped_field` also points to + // a valid `W`. Thus, it is sound to dereference `wrapped_field`. + unsafe { &*wrapped_field } +} + +/// Performs field projection on `outer`, projecting into the field of type `F` +/// at the address provided by `inner_to_field`. +/// +/// # Safety +/// +/// `project_mut` has the same safety requirements as `project`. +#[doc(hidden)] +#[inline(always)] +pub fn project_mut( + _unsafe: unsafe_token::UnsafeToken, + outer: &mut P, + outer_to_inner: OuterToInner, + inner_to_field: InnerToField, + field_to_wrapped_field: FieldToWrappedField, +) -> &mut W +where + P: Projectable + ?Sized, + // TODO(#196), TODO(https://github.com/rust-lang/reference/pull/1387), + // TODO(https://github.com/rust-lang/rust/pull/114330): Remove this bound + // once we support unsized projection (see comment on `Unalign` for more + // details). + P::Inner: Sized, + F: ?Sized, + W: ?Sized, + OuterToInner: FnOnce(*mut P) -> *mut Unalign, + InnerToField: FnOnce(*mut Unalign) -> *mut F, + FieldToWrappedField: FnOnce(*mut F) -> *mut W, +{ + let outer: *mut P = outer; + let inner = outer_to_inner(outer); + let field = inner_to_field(inner); + let wrapped_field = field_to_wrapped_field(field); + // SAFETY: See the safety comment in `project`. The same arguments apply + // here. + unsafe { &mut *wrapped_field } +} + +/// Performs field projection. +/// +/// Given a wrapper, `w: W`, and a field type in `T`, `f: F`, +/// `project!(&w.f)` returns a reference to a `W` (this works for mutable +/// references too). +/// +/// # Safety +/// +/// It is unsound to project using a sequence of accesses that invoke +/// [`Deref::deref`] or [`DerefMut::deref_mut`]. +/// +/// [`Deref::deref`]: core::ops::Deref::deref +/// [`DerefMut::deref_mut`]: core::ops::DerefMut::deref_mut +// TODO(#196): Is there any way to teach Rust about when references are +// non-overlapping so you can borrow multiple fields mutably at a time? +#[doc(hidden)] // `#[macro_export]` bypasses this module's `#[doc(hidden)]`. +#[macro_export] +macro_rules! project { + // Note that it's very important that the `mut` branches comes first! If it + // came after the immutable branches, then a `mut` token could be matched by + // `$c:ident` in those branches. + (&mut $c:ident $($f:tt)*) => { + $crate::project!(&mut ($c) $($f)*) + }; + (&mut ($c:expr) $($f:tt)*) => { + $crate::project!( + @inner + BorrowMut, + borrow_mut, + project_mut, + *mut _, + inner, + &mut *inner, + addr_of_mut, + $c, + $($f)* + ) + }; + + (&$c:ident $($f:tt)*) => { + $crate::project!(&($c) $($f)*) + }; + (&($c:expr) $($f:tt)*) => { + $crate::project!( + @inner + Borrow, + borrow, + project, + *const _, + inner, + &*inner, + addr_of, + $c, + $($f)* + ) + }; + + ( + @inner + $borrow_trait:ident, + $borrow_method:ident, + $project_fn:ident, + $ptr_ty:ty, + $inner_name:ident, + $convert_inner_raw_to_ref:expr, + $addr_of:ident, + $c:expr, + $($f:tt)* + ) => {{ + // This function does nothing, but is unsafe to call, and so has the + // effect of requiring that the caller only invoke `project!` inside of + // an `unsafe` block. + $crate::project::promise_no_deref(); + // We generate an `UnsafeToken` so that `$project_fn` can itself be + // safe, and thus we don't need to put the entire call to `$project_fn` + // in an `unsafe` block. This, in turn, is done so that the + // meta-variables `$c` and `$($f)*` are not expanded inside of an + // `unsafe` block, which would allow safe Rust code to smuggle in unsafe + // code via a call to `project!` without needing to write the `unsafe` + // keyword. + // + // TODO: + // - Slicing seems to be bounds-checked at runtime if need be, but is + // this guaranteed by the reference/stdlib docs? + // + // SAFETY: + // - The arguments passed for `outer_to_inner` and + // `field_to_wrapped_field` are just `as` casts, so they return + // pointers to the same memory region as their arguments. + // - The argument passed for `inner_to_field` uses `addr_of!` (or + // `addr_of_mut!`) to take the address of a sequence of field + // accesses. By safety precondition of this macro, none of those field + // accesses can go through a dereference. The only other types of + // field accesses produce "places" which live entirely within the + // memory region of the argument to `inner_to_field` as required by + // the safety precondition of `project`/`project_mut`. + // + // Some older versions of Clippy have a bug in which they don't + // recognize the preceding safety comment. + #[allow(clippy::undocumented_unsafe_blocks)] + #[allow(unused_unsafe)] + let token = unsafe { $crate::project::unsafe_token::UnsafeToken::new() }; + use ::core::borrow::$borrow_trait as _; + $crate::project::$project_fn( + token, + // We call `borrow` or `borrow_mut` so that this macro works + // regardless of whether `$c` is owned or borrowed. + // + // TODO(#196): Is there any way to make sure this calls + // `Borrow::borrow` or `BorrowMut::borrow_mut` even if the type has + // an inherent method of the same name? + $c.$borrow_method(), + // Older versions of Clippy complain about this `as` cast; newer + // versions seem to know it's okay. + #[allow(clippy::as_conversions)] + |outer| outer as $ptr_ty, + |inner| if false { + // This branch is never executed, but allows us to ensure that + // `$($f)*` doesn't contain any unsafe code that isn't wrapped + // in an `unsafe` block. If it does, then wrapping it in + // `unsafe` - as we do in the `else` branch - would allow users + // to write unsafe code without needing to write `unsafe`. + // + // The way we accomplish this is to generate a reference from + // `inner` (which is a raw pointer). That allows us to extract + // the unsafe operation of converting to a reference and wrap it + // in an `unsafe` block on its own, while leaving the `$($f)*` + // not wrapped in an `unsafe` block. Note that this is NOT sound + // to execute in the general case, but that's okay because we're + // in an `if false` branch. For example, if the wrapper type is + // `#[repr(packed)]`, then `inner_ref` may not be validly + // aligned, which is unsound. + let $inner_name = inner; + // SAFETY: This code is never executed. + // + // Some older versions of Clippy have a bug in which they don't + // recognize the preceding safety comment. + #[allow(clippy::undocumented_unsafe_blocks)] + #[allow(unused_unsafe)] + let inner_ref = unsafe { $convert_inner_raw_to_ref }; + ::core::ptr::$addr_of!(inner_ref .0 $($f)*) + } else { + // SAFETY: By safety invariant of `project`/project_mut`, it is + // sound to call `addr_of!`/`addr_of_mut!` on `*inner`. By + // safety precondition of this macro, none of the field accesses + // go through a dereference, and so no loads are generated. + // Thus, even if `inner` does not point to a validly-initialized + // value, this call is still sound. + // + // Some older versions of Clippy have a bug in which they don't + // recognize the preceding safety comment. + #[allow(clippy::undocumented_unsafe_blocks)] + #[allow(unused_unsafe)] + unsafe { ::core::ptr::$addr_of!((*inner) .0 $($f)* ) } + }, + // Older versions of Clippy complain about this `as` cast; newer + // versions seem to know it's okay. + #[allow(clippy::as_conversions)] + |field| field as $ptr_ty, + ) + }}; +} + +#[doc(hidden)] +#[inline(always)] +pub const unsafe fn promise_no_deref() {} + +// TODO(#196), TODO(https://github.com/rust-lang/reference/pull/1387), +// TODO(https://github.com/rust-lang/rust/pull/114330): Remove this once it is +// no longer UB to use `addr_of!` with an unaligned pointer, and once Miri knows +// that this isn't UB. Note that this struct is the only reason that `project!` +// doesn't support unsized types, so removing this will also address that +// limitation. +#[allow(missing_debug_implementations)] +#[doc(hidden)] +#[repr(packed)] +pub struct Unalign(pub T); + +#[doc(hidden)] +pub mod unsafe_token { + /// A token used to prove that the `unsafe` keyword has been written + /// somewhere. + #[allow(missing_copy_implementations, missing_debug_implementations)] + pub struct UnsafeToken(()); + + impl UnsafeToken { + /// Constructs a new `UnsafeToken`. + /// + /// # Safety + /// + /// The caller is responsible for ensuring that they uphold the safety + /// invariants of any APIs which consume this token. + pub unsafe fn new() -> UnsafeToken { + UnsafeToken(()) + } + } +} + +#[cfg(test)] +mod tests { + use super::*; + + #[derive(Copy)] + #[repr(C, packed)] + struct Unalign(T); + + safety_comment! { + /// SAFETY: + /// `Unalign` has the same layout as `T`. + unsafe_impl_projectable!(Unalign); + } + + impl Clone for Unalign { + fn clone(&self) -> Unalign { + *self + } + } + + #[derive(Eq, PartialEq, Debug)] + #[repr(transparent)] + struct Wrapper(T); + safety_comment! { + /// SAFETY: + /// `Wrapper` has the same layout as `T`. + unsafe_impl_projectable!(Wrapper: ?Sized); + } + + #[derive(Copy, Clone, Debug, Eq, PartialEq)] + struct Foo { + a: u8, + b: u16, + c: T, + } + + macro_rules! test_project { + (($c:ident $($f:tt)*) => $expect:expr) => {{ + // SAFETY: None of the tests in this module use fields which + // implement `Deref` or `DerefMut`. + // + // Some older versions of Clippy have a bug in which they don't + // recognize the preceding safety comment. + #[allow(clippy::undocumented_unsafe_blocks)] + #[allow(unused_unsafe)] + unsafe { + // Test with an immutable reference. + let f = project!(&$c $($f)*); + assert_eq!({ f.0 }, $expect); + // Test with a mutable reference. + let f = project!(&mut $c $($f)*); + assert_eq!({ f.0 }, $expect); + + // Run the same tests with `$c` in parentheses. + test_project!((($c) $($f)*) => $expect); + } + }}; + ((($c:expr) $($f:tt)*) => $expect:expr) => {{ + // SAFETY: None of the tests in this module use fields which + // implement `Deref` or `DerefMut`. + // + // Some older versions of Clippy have a bug in which they don't + // recognize the preceding safety comment. + #[allow(clippy::undocumented_unsafe_blocks)] + #[allow(unused_unsafe)] + unsafe { + // Test with an immutable reference. + let f = project!(&($c) $($f)*); + assert_eq!({ f.0 }, $expect); + // Test with a mutable reference. + let f = project!(&mut ($c) $($f)*); + assert_eq!({ f.0 }, $expect); + } + }}; + } + + #[test] + fn test_project() { + let mut u = Unalign(Foo:: { a: 1, b: 2, c: 3 }); + + test_project!((u.a) => 1); + test_project!((u.b) => 2); + test_project!((u.c) => 3); + } + + #[test] + fn test_project_complex() { + // Test projection using a complex expression rather than just the + // identifier of a local variable. + + let mut u = Unalign(Foo:: { a: 1, b: 2, c: 3 }); + + fn ident(t: T) -> T { + t + } + + // SAFETY: None of these projections go through a field which implements + // `Deref` or `DerefMut`. + unsafe { + let ua = project!(&(ident(&u)).a); + let ub = project!(&(ident(&u)).b); + assert_eq!({ ua.0 }, 1); + assert_eq!({ ub.0 }, 2); + + let uc = project!(&mut (ident(&mut u)).c); + assert_eq!({ uc.0 }, 3); + } + } + + #[test] + fn test_project_complex_access() { + let mut u = Unalign(Foo::> { a: 1, b: 2, c: Foo { a: 3, b: 4, c: 5 } }); + test_project!((u.c) => Foo { a: 3, b: 4, c: 5 }); + test_project!((u.c.a) => 3); + + let mut u = Unalign(Foo::<[u32; 3]> { a: 1, b: 2, c: [3, 4, 5] }); + test_project!((u.c) => [3, 4, 5]); + test_project!((u.c[0]) => 3); + + let mut u = Unalign([0u8, 1, 2]); + test_project!((u[0]) => 0); + test_project!((u[1]) => 1); + test_project!((u[2]) => 2); + + // Test that indexing works using variables rather than literals. + for i in 0u8..3 { + test_project!((u[usize::from(i)]) => i); + } + + let mut u = Unalign([[0u8, 1, 2], [3, 4, 5], [6, 7, 8]]); + test_project!((u[0][0]) => 0); + test_project!((u[1][1]) => 4); + test_project!((u[2][2]) => 8); + + // Test that indexing works using variables rather than literals. + for (i, elem) in [(0usize, 0u8), (1, 4), (2, 8)] { + test_project!((u[i][i]) => elem); + } + } + + // TODO(#196), TODO(https://github.com/rust-lang/reference/pull/1387), + // TODO(https://github.com/rust-lang/rust/pull/114330): Uncomment this once + // unsized projection is supported. + // + // #[test] + // fn test_project_unsized() { + // let inner = [0u8, 1, 2]; + // let inner_ref: &[u8] = &inner[..]; + // let wrapper_ref: &Wrapper<([u8],)> = unsafe { &*(inner_ref as *const _ as *const _) }; + + // let first = project!(&wrapper_ref.0[1]); + // assert_eq!(first, &Wrapper(1u8)); + // let first_two = project!(&wrapper_ref.0[0..2]); + // assert_eq!(&first_two.0, &[0, 1]); + // } + + #[test] + #[should_panic(expected = "index out of bounds: the len is 3 but the index is 3")] + fn test_project_out_of_bounds() { + let u = Unalign([0u8, 1, 2]); + + // SAFETY: None of these projections go through a field which implements + // `Deref` or `DerefMut`. + unsafe { + let u0 = project!(&u[0]); + assert_eq!({ u0.0 }, 0); + let _ = project!(&u[3]); + } + } + + #[test] + #[should_panic(expected = "index out of bounds: the len is 3 but the index is 3")] + fn test_project_out_of_bounds_variable() { + let u = Unalign([0u8, 1, 2]); + + // SAFETY: None of these projections go through a field which implements + // `Deref` or `DerefMut`. + unsafe { + let u0 = project!(&u[0]); + assert_eq!({ u0.0 }, 0); + let i = 3; + let _ = project!(&u[i]); + } + } + + #[test] + #[should_panic(expected = "range end index 4 out of range for slice of length 3")] + fn test_project_out_of_bounds_range() { + let u = Wrapper([0u8, 1, 2]); + + // SAFETY: None of these projections go through a field which implements + // `Deref` or `DerefMut`. + unsafe { + let u0 = project!(&u[0]); + assert_eq!({ u0.0 }, 0); + // Good catch, Clippy! But we do need to be able to test this, so + // shhh.... + #[allow(clippy::out_of_bounds_indexing)] + let _ = project!(&u[0..4]); + } + } +}