Clippy lints / Pedantic
Clippy lint · Pedantic · checked with Clippy 0.1.99
clippy::cast_ptr_alignment: casting from `*const u8` to a more-strictly-aligned pointer (`*const u16`) (1 < 2 bytes)
What it meansChecks for casts, using as or pointer::cast, from a less strictly aligned pointer to a more strictly aligned pointer. It is in the pedantic group and off by default.
What you see
warning: casting from `*const u8` to a more-strictly-aligned pointer (`*const u16`) (1 < 2 bytes) --> main.rs:3:9 | 3 | let _ = (&1u8 as *const u8) as *const u16; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#cast_ptr_alignment = note: `-W clippy::cast-ptr-alignment` implied by `-W clippy::pedantic`
Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.
Does it matter?
Dereferencing the resulting pointer may be undefined behavior.
Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.
How to fix it
Clippy does not fix this one automatically. Change the code by hand:
Flagged
let _ = (&1u8 as *const u8) as *const u16;
let _ = (&mut 1u8 as *mut u8) as *mut u16;
(&1u8 as *const u8).cast::<u16>();
(&mut 1u8 as *mut u8).cast::<u16>();
Turn it off, or on
It is off by default. To use it, set it to warn or deny:
Whole crate (Cargo.toml)
[lints.clippy]
cast_ptr_alignment = "warn"In code
#![warn(clippy::cast_ptr_alignment)]One run
cargo clippy -- -W clippy::cast-ptr-alignmentThe [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.
Clippy's own documentation
What it does
Checks for casts, using as or pointer::cast, from a
less strictly aligned pointer to a more strictly aligned pointer.
Why is this bad?
Dereferencing the resulting pointer may be undefined behavior.
Known problems
Using std::ptr::read_unaligned and std::ptr::write_unaligned or
similar on the resulting pointer is fine. Is over-zealous: casts with
manual alignment checks or casts like u64 -> u8 -> u16 can be
fine. Miri is able to do a more in-depth analysis.
Example
let _ = (&1u8 as *const u8) as *const u16;
let _ = (&mut 1u8 as *mut u8) as *mut u16;
(&1u8 as *const u8).cast::<u16>();
(&mut 1u8 as *mut u8).cast::<u16>();
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
Related lints
- too_many_linesChecks for functions with a large amount of lines
- missing_errors_docChecks the doc comments of publicly visible functions that return a Result type and warns if there is no # Errors section
- must_use_candidateChecks for public functions that have no #[must_use] attribute, but return something not already marked must-use, have no mutable arg and mutate no statics
- cast_possible_truncationcasting `u64` to `u8` may truncate the value
- cast_possible_wrapcasting `u32` to `i32` may wrap around the value
- cast_precision_losscasting `u64` to `f64` may cause a loss of precision (`u64` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
- cast_sign_losscasting `i8` to `u64` may lose the sign of the value
- checked_conversionsChecks for explicit bounds checking when casting
- cloned_instead_of_copiedused `cloned` where `copied` could be used instead
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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