Clippy lints / Pedantic
Clippy lint · Pedantic · checked with Clippy 0.1.99
clippy::cast_lossless: casts from `u8` to `u64` can be expressed infallibly using `From`
What it meansChecks for casts between numeric types that can be replaced by safe conversion functions. It is in the pedantic group and off by default.
What you see
warning: casts from `u8` to `u64` can be expressed infallibly using `From` --> main.rs:4:5 | 4 | x as u64 | ^^^^^^^^ | = help: an `as` cast can become silently lossy if the types change in the future = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#cast_lossless = note: `-W clippy::cast-lossless` implied by `-W clippy::pedantic` help: use `u64::from` instead | 4 - x as u64 4 + u64::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?
Rust’s as keyword will perform many kinds of conversions, including silently lossy conversions. Conversion functions such as i32::from will only perform lossless conversions. Using the conversion functions prevents conversions from becoming silently lossy if the input types ever change, and makes it clear for people reading the code that the conversion is lossless.
Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.
How to fix it
Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.
Flagged
fn as_u64(x: u8) -> u64 {
x as u64
}
Write this instead
fn as_u64(x: u8) -> u64 {
u64::from(x)
}
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_lossless = "warn"In code
#![warn(clippy::cast_lossless)]One run
cargo clippy -- -W clippy::cast-losslessThe [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 can be replaced by safe conversion functions.
Why is this bad?
Rust’s as keyword will perform many kinds of conversions, including
silently lossy conversions. Conversion functions such as i32::from
will only perform lossless conversions. Using the conversion functions
prevents conversions from becoming silently lossy if the input types
ever change, and makes it clear for people reading the code that the
conversion is lossless.
Example
fn as_u64(x: u8) -> u64 {
x as u64
}
Using ::from would look like this:
fn as_u64(x: u8) -> u64 {
u64::from(x)
}
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
- bool_to_int_with_ifInstead of using an if statement to convert a bool to an int, this lint suggests using a from() function or an as coercion
- borrow_as_ptrborrow as raw pointer
- case_sensitive_file_extension_comparisonscase-sensitive file extension comparison
- 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)
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
Up next
Related lintclippy::too_many_linesChecks for functions with a large amount of linesRead it next →Did this answer your question?