Clippy lints / Pedantic
Clippy lint · Pedantic · checked with Clippy 0.1.99
clippy::cast_possible_truncation: casting `u64` to `u8` may truncate the value
What it meansChecks for casts between numeric types that may truncate large values. This is expected behavior, so the cast is Allow by default. It suggests user either explicitly ignore the lint, or use try_from() and handle the truncation, default, or panic explicitly. It is in the pedantic group and off by default.
What you see
warning: casting `u64` to `u8` may truncate the value --> main.rs:4:5 | 4 | x as u8 | ^^^^^^^ | = help: if this is intentional allow the lint with `#[allow(clippy::cast_possible_truncation)]` ... = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#cast_possible_truncation = note: `-W clippy::cast-possible-truncation` implied by `-W clippy::pedantic` help: ... or use `try_from` and handle the error accordingly | 4 - x as u8 4 + u8::try_from(x) |
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?
In some problem domains, it is good practice to avoid truncation. This lint can be activated to help assess where additional checks could be beneficial.
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
fn as_u8(x: u64) -> u8 {
x as u8
}
Write this instead
fn as_u8(x: u64) -> u8 {
if let Ok(x) = u8::try_from(x) {
x
} else {
todo!();
}
}
// Or
#[allow(clippy::cast_possible_truncation)]
fn as_u16(x: u64) -> u16 {
x as 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_possible_truncation = "warn"In code
#![warn(clippy::cast_possible_truncation)]One run
cargo clippy -- -W clippy::cast-possible-truncationThe [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 between numeric types that may
truncate large values. This is expected behavior, so the cast is Allow by
default. It suggests user either explicitly ignore the lint,
or use try_from() and handle the truncation, default, or panic explicitly.
Why is this bad?
In some problem domains, it is good practice to avoid truncation. This lint can be activated to help assess where additional checks could be beneficial.
Example
fn as_u8(x: u64) -> u8 {
x as u8
}
Use instead:
fn as_u8(x: u64) -> u8 {
if let Ok(x) = u8::try_from(x) {
x
} else {
todo!();
}
}
// Or
#[allow(clippy::cast_possible_truncation)]
fn as_u16(x: u64) -> u16 {
x as 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
- borrow_as_ptrborrow as raw pointer
- case_sensitive_file_extension_comparisonscase-sensitive file extension comparison
- cast_losslesscasts from `u8` to `u64` can be expressed infallibly using `From`
- 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_ptr_alignmentcasting from `*const u8` to a more-strictly-aligned pointer (`*const u16`) (1 < 2 bytes)
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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