Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::manual_instant_elapsed: manual implementation of `Instant::elapsed`

What it meansLints subtraction between Instant::now() and another Instant. It is in the pedantic group and off by default.

What you see

warning: manual implementation of `Instant::elapsed`
 --> main.rs:5:16
  |
5 | let duration = Instant::now() - prev_instant;
  |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `prev_instant.elapsed()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#manual_instant_elapsed
  = note: `-W clippy::manual-instant-elapsed` 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 is easy to accidentally write prev_instant - Instant::now(), which will always be 0ns as Instant subtraction saturates. prev_instant.elapsed() also more clearly signals intention.

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

use std::time::Instant;
let prev_instant = Instant::now();
let duration = Instant::now() - prev_instant;

Write this instead

use std::time::Instant;
let prev_instant = Instant::now();
let duration = prev_instant.elapsed();

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]
manual_instant_elapsed = "warn"

In code

#![warn(clippy::manual_instant_elapsed)]

One run

cargo clippy -- -W clippy::manual-instant-elapsed

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.

Pedanticgroup

allowdefault level

yesfixed by cargo clippy --fix

1.65.0added in Rust

Clippy's own documentation

What it does

Lints subtraction between Instant::now() and another Instant.

Why is this bad?

It is easy to accidentally write prev_instant - Instant::now(), which will always be 0ns as Instant subtraction saturates.

prev_instant.elapsed() also more clearly signals intention.

Example

use std::time::Instant;
let prev_instant = Instant::now();
let duration = Instant::now() - prev_instant;

Use instead:

use std::time::Instant;
let prev_instant = Instant::now();
let duration = prev_instant.elapsed();

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

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