Clippy lints / Pedantic
Clippy lint · Pedantic · checked with Clippy 0.1.99
clippy::cast_sign_loss: casting `i8` to `u64` may lose the sign of the value
What it meansChecks for casts from a signed to an unsigned numeric type. In this case, negative values wrap around to large positive values, which can be quite surprising in practice. However, since the cast works as defined, this lint is Allow by default. It is in the pedantic group and off by default.
What you see
warning: casting `i8` to `u64` may lose the sign of the value --> main.rs:4:1 | 4 | y as u64; // will return 18446744073709551615 | ^^^^^^^^ | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#cast_sign_loss = note: `-W clippy::cast-sign-loss` 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?
Possibly surprising results. You can activate this lint as a one-time check to see where numeric wrapping can arise.
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 (its suggestion may change behaviour, so it only shows it). Change the code by hand:
Flagged
let y: i8 = -1;
y as u64; // will return 18446744073709551615
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_sign_loss = "warn"In code
#![warn(clippy::cast_sign_loss)]One run
cargo clippy -- -W clippy::cast-sign-lossThe [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 from a signed to an unsigned numeric
type. In this case, negative values wrap around to large positive values,
which can be quite surprising in practice. However, since the cast works as
defined, this lint is Allow by default.
Why is this bad?
Possibly surprising results. You can activate this lint as a one-time check to see where numeric wrapping can arise.
Example
let y: i8 = -1;
y as u64; // will return 18446744073709551615
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_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)
- checked_conversionsChecks for explicit bounds checking when casting
- cloned_instead_of_copiedused `cloned` where `copied` could be used instead
- collapsible_else_ifChecks for collapsible else { if
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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