Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::mixed_read_write_in_expression: unsequenced read of `x`

What it meansChecks for a read and a write to the same variable where whether the read occurs before or after the write depends on the evaluation order of sub-expressions. It is in the restriction group and off by default.

What you see

warning: unsequenced read of `x`
 --> main.rs:8:5
  |
8 | } + x;
  |     ^
  |
note: whether read occurs before this write depends on evaluation order
 --> main.rs:6:5
  |
6 |     x = 1;
  |     ^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#mixed_read_write_in_expression
  = note: requested on the command line with `-W clippy::mixed-read-write-in-expression`

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?

While [the evaluation order of sub-expressions] is fully specified in Rust, it still may be confusing to read an expression where the evaluation order affects its behavior.

Lints that forbid language features for specific codebases (no unwrap, no indexing...). Off by default; pick them one by one, never the whole group.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

let mut x = 0;

let a = {
    x = 1;
    1
} + x;
// Unclear whether a is 1 or 2.

Write this instead

let tmp = {
    x = 1;
    1
};
let a = tmp + 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]
mixed_read_write_in_expression = "warn"

In code

#![warn(clippy::mixed_read_write_in_expression)]

One run

cargo clippy -- -W clippy::mixed-read-write-in-expression

The [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.

Restrictiongroup

allowdefault level

nofixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for a read and a write to the same variable where whether the read occurs before or after the write depends on the evaluation order of sub-expressions.

Why restrict this?

While [the evaluation order of sub-expressions] is fully specified in Rust, it still may be confusing to read an expression where the evaluation order affects its behavior.

Known problems

Code which intentionally depends on the evaluation order, or which is correct for any evaluation order.

Example

let mut x = 0;

let a = {
    x = 1;
    1
} + x;
// Unclear whether a is 1 or 2.

Use instead:

let tmp = {
    x = 1;
    1
};
let a = tmp + x;

Past names

eval_order_dependence

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

Up next

Also worth your time

Did this answer your question?