Rust offers different ways to initialize compile time-initialized variables. Recently, I had to create a runtime-initialized variable: existing approaches don't work in this case. I want to describe multiple ways to achieve it in this post. compile time-initialized Constants The Rust language allows you to create constants. Two keywords are available: const and static. const static Sometimes a certain value is used many times throughout a program, and it can become inconvenient to copy it over and over. What’s more, it’s not always possible or desirable to make it a variable that gets carried around to each function that needs it. In these cases, the const keyword provides a convenient alternative to code duplication:
const THING: u32 = 0xABAD1DEA;

let foo = 123 + THING;

-- Keyword const Sometimes a certain value is used many times throughout a program, and it can become inconvenient to copy it over and over. What’s more, it’s not always possible or desirable to make it a variable that gets carried around to each function that needs it. In these cases, the const keyword provides a convenient alternative to code duplication: const const THING: u32 = 0xABAD1DEA;

let foo = 123 + THING; const THING: u32 = 0xABAD1DEA;

let foo = 123 + THING; -- Keyword const Keyword const A static item is a value which is valid for the entire duration of your program (a 'static lifetime).
On the surface, static items seem very similar to const: both contain a value, both require type annotations and both can only be initialized with constant functions and values. However, static are notably different in that they represent a location in memory. That means that you can have references to static items and potentially even modify them, making them essentially global variables.Static items do not call drop at the end of the program.There are two types of static items: those declared in association with the mut keyword and those without.-- Keyword static A static item is a value which is valid for the entire duration of your program (a 'static lifetime). 'static On the surface, static items seem very similar to const: both contain a value, both require type annotations and both can only be initialized with constant functions and values. However, static are notably different in that they represent a location in memory. That means that you can have references to static items and potentially even modify them, making them essentially global variables. static const static static Static items do not call drop at the end of the program. drop There are two types of static items: those declared in association with the mut keyword and those without. static mut -- Keyword static Keyword static None of the above work with struct. struct I stand corrected: https://www.reddit.com/r/rust/comments/1l0uwzy/comment/mvgghb0/. https://www.reddit.com/r/rust/comments/1l0uwzy/comment/mvgghb0/ const REGEXP_WAREHOUSE_ENDPOINT: Regex = Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap(); const REGEXP_WAREHOUSE_ENDPOINT: Regex = Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap(); Trying to compile the previous line returns the following: error[E0015]: cannot call non-const fn `regex::Regex::new` in constants
  |
6 | const REGEXP_WAREHOUSE_ENDPOINT: Regex = Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap();
  |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: calls in constants are limited to constant functions, tuple structs and tuple variants error[E0015]: cannot call non-const fn `regex::Regex::new` in constants
  |
6 | const REGEXP_WAREHOUSE_ENDPOINT: Regex = Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap();
  |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: calls in constants are limited to constant functions, tuple structs and tuple variants The same happens with if we change const to static. const static error[E0015]: cannot call non-const fn `regex::Regex::new` in statics
 --> src/config.rs:6:43
  |
6 | static REGEXP_WAREHOUSE_ENDPOINT: Regex = Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap();
  |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: calls in statics are limited to constant functions, tuple structs and tuple variants error[E0015]: cannot call non-const fn `regex::Regex::new` in statics
 --> src/config.rs:6:43
  |
6 | static REGEXP_WAREHOUSE_ENDPOINT: Regex = Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap();
  |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: calls in statics are limited to constant functions, tuple structs and tuple variants We need a runtime-initialized variable for the struct. Neither const nor lazy work; time to search for something else. runtime-initialized struct const lazy The lazy_static crate lazy_static When you need something, there's a high chance somebody required it before. If lucky, they created a crate for it. Case in point, the lazy_static crate provides a solution. lazy_static A macro for declaring lazily evaluated statics in Rust.
Using this macro, it is possible to have statics that require code to be executed at runtime in order to be initialized. This includes anything requiring heap allocations, like vectors or hash maps, as well as anything that requires non-const function calls to be computed.-- lazy_static crate A macro for declaring lazily evaluated statics in Rust. Using this macro, it is possible to have statics that require code to be executed at runtime in order to be initialized. This includes anything requiring heap allocations, like vectors or hash maps, as well as anything that requires non-const function calls to be computed. -- lazy_static crate lazy_static crate The usage is straightforward: we wrap the static declaration in a macro. static lazy_static! {
    static ref REGEXP_WAREHOUSE_ENDPOINT: Regex = Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap();
} lazy_static! {
    static ref REGEXP_WAREHOUSE_ENDPOINT: Regex = Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap();
} We achieved what we wanted to. Yet, we can do better. LazyLock for the win LazyLock Rust 1.80 adds LazyLock. LazyLock is a built-in exact replacement for the lazy_static macro. LazyLock LazyLock lazy_static https://x.com/13erbse/status/1816491350235173228?embedable=true https://x.com/13erbse/status/1816491350235173228?embedable=true The documentation of LazyLock is straightforward: LazyLock pub struct LazyLock<T, F = fn() -> T> { /* private fields */ }
A value which is initialized on the first access.This type is a thread-safe LazyCell, and can be used in statics. Since initialization may be called from multiple threads, any dereferencing call will block the calling thread if another initialization routine is currently running.-- LazyLock pub struct LazyLock<T, F = fn() -> T> { /* private fields */ } pub struct LazyLock<T, F = fn() -> T> { /* private fields */ } A value which is initialized on the first access. This type is a thread-safe LazyCell, and can be used in statics. Since initialization may be called from multiple threads, any dereferencing call will block the calling thread if another initialization routine is currently running. -- LazyLock LazyLock At this point, we can use it directly: static REGEXP_WAREHOUSE_ENDPOINT: LazyLock<Regex> =
        LazyLock::new(|| Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap()); static REGEXP_WAREHOUSE_ENDPOINT: LazyLock<Regex> =
        LazyLock::new(|| Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap()); OnceLock OnceLock As an alternative, one can use OnceLock from Rust 1.70. OnceLock pub struct OnceLock<T> { /* private fields */ }
A synchronization primitive which can nominally be written to only once.This type is a thread-safe OnceCell, and can be used in statics. In many simple cases, you can use LazyLock<T, F> instead to get the benefits of this type with less effort: LazyLock<T, F> "looks like" &T because it initializes with F on deref! Where OnceLock shines is when LazyLock is too simple to support a given case, as LazyLock doesn’t allow additional inputs to its function after you call LazyLock::new(|| ...).-- OnceLock pub struct OnceLock<T> { /* private fields */ } pub struct OnceLock<T> { /* private fields */ } A synchronization primitive which can nominally be written to only once. This type is a thread-safe OnceCell, and can be used in statics. In many simple cases, you can use LazyLock<T, F> instead to get the benefits of this type with less effort: LazyLock<T, F> "looks like" &T because it initializes with F on deref! Where OnceLock shines is when LazyLock is too simple to support a given case, as LazyLock doesn’t allow additional inputs to its function after you call LazyLock::new(|| ...). OnceCell LazyLock<T, F> LazyLock<T, F> &T F LazyLock::new(|| ...) -- OnceLock OnceLock Usage is quite different: static REGEXP_WAREHOUSE_ENDPOINT: OnceLock<Regex> = OnceLock::new();       //1

let endpoint = REGEXP_WAREHOUSE_ENDPOINT.get_or_init(
        || Regex::new(r"^WAREHOUSE__(?P<index>\d+)__ENDPOINT$").unwrap()); //2

endpoint.find("whatever").is_some()                                        //3 static REGEXP_WAREHOUSE_ENDPOINT: OnceLock<Regex> = OnceLock::new();       //1

let endpoint = REGEXP_WAREHOUSE_ENDPOINT.get_or_init(
        || Regex::new(r"^WAREHOUSE__(?P<index>\d+)__ENDPOINT$").unwrap()); //2

endpoint.find("whatever").is_some()                                        //3 Create the OnceLock
Get or initialize the variable
Use the variable Create the OnceLock OnceLock Get or initialize the variable Use the variable Summary In this post, we described several options to create lazy-initialized variables. Before, we had to rely on the lazy_static crate. With Rust 1.80, we can replace it with LazyLock. If you need additional input after the LazyLock::new(), OnceLock is your friend. lazy_static LazyLock LazyLock::new() OnceLock To go further: To go further: Keyword const
lazy_static crate
Struct LazyLock
Struct OnceLock Keyword const Keyword const lazy_static crate lazy_static crate Struct LazyLock Struct LazyLock Struct OnceLock Struct OnceLock Originally published at A Java Geek on June 1st, 2025 Originally published at A Java Geek on June 1st, 2025 A Java Geek

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