Clippy lints / Pedantic
Clippy lint · Pedantic · checked with Clippy 0.1.99
clippy::cast_precision_loss: casting `u64` to `f64` may cause a loss of precision (`u64` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide)
What it meansChecks for casts from any numeric type to a float type where the receiving type cannot store all values from the original type without rounding errors. This possible rounding is to be expected, so this lint is Allow by default. Basically, this warns on casting any integer with 32 or more bits to f32 or any 64-bit integer to f64. It is in the pedantic group and off by default.
What you see
warning: casting `u64` to `f64` may cause a loss of precision (`u64` is 64 bits wide, but `f64`'s mantissa is only 52 bits wide) --> main.rs:4:1 | 4 | x as f64; | ^^^^^^^^ | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#cast_precision_loss = note: `-W clippy::cast-precision-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?
It’s not bad at all. But in some applications it can be helpful to know where precision loss can take place. This lint can help find those places in the code.
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 x = u64::MAX;
x as f64;
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_precision_loss = "warn"In code
#![warn(clippy::cast_precision_loss)]One run
cargo clippy -- -W clippy::cast-precision-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 any numeric type to a float type where
the receiving type cannot store all values from the original type without
rounding errors. This possible rounding is to be expected, so this lint is
Allow by default.
Basically, this warns on casting any integer with 32 or more bits to f32
or any 64-bit integer to f64.
Why is this bad?
It’s not bad at all. But in some applications it can be helpful to know where precision loss can take place. This lint can help find those places in the code.
Example
let x = u64::MAX;
x as f64;
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_losslesscasts from `u8` to `u64` can be expressed infallibly using `From`
- cast_possible_truncationcasting `u64` to `u8` may truncate the value
- cast_possible_wrapcasting `u32` to `i32` may wrap around the value
- cast_ptr_alignmentcasting from `*const u8` to a more-strictly-aligned pointer (`*const u16`) (1 < 2 bytes)
- cast_sign_losscasting `i8` to `u64` may lose the sign of the value
- checked_conversionsChecks for explicit bounds checking when casting
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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