Clippy lint · Performance · checked with Clippy 0.1.99

clippy::missing_const_for_thread_local: initializer for `thread_local` value can be made `const`

What it meansSuggests to use const in thread_local! macro if possible. It is in the performance group and warns by default.

What you see

warning: initializer for `thread_local` value can be made `const`
 --> main.rs:4:26
  |
4 |     static BUF: String = String::new();
  |                          ^^^^^^^^^^^^^ help: replace with: `const { String::new() }`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#missing_const_for_thread_local
  = note: `#[warn(clippy::missing_const_for_thread_local)]` on by default

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?

The thread_local! macro wraps static declarations and makes them thread-local. It supports using a const keyword that may be used for declarations that can be evaluated as a constant expression. This can enable a more efficient thread local implementation that can avoid lazy initialization. For types that do not need to be dropped, this can enable an even more efficient implementation that does not need to track any additional state. https://doc.rust-lang.org/std/macro.thread_local.html

Code that could be written to run faster. Warns by default.

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

thread_local! {
    static BUF: String = String::new();
}

Write this instead

thread_local! {
    static BUF: String = const { String::new() };
}

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::missing_const_for_thread_local)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
missing_const_for_thread_local = "allow"

One run

cargo clippy -- -A clippy::missing-const-for-thread-local

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.

Performancegroup

warndefault level

yesfixed by cargo clippy --fix

1.77.0added in Rust

Clippy's own documentation

What it does

Suggests to use const in thread_local! macro if possible.

Why is this bad?

The thread_local! macro wraps static declarations and makes them thread-local. It supports using a const keyword that may be used for declarations that can be evaluated as a constant expression. This can enable a more efficient thread local implementation that can avoid lazy initialization. For types that do not need to be dropped, this can enable an even more efficient implementation that does not need to track any additional state.

https://doc.rust-lang.org/std/macro.thread_local.html

Example

thread_local! {
    static BUF: String = String::new();
}

Use instead:

thread_local! {
    static BUF: String = const { String::new() };
}

Past names

thread_local_initializer_can_be_made_const

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

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