#env-var #macro #utility


Lightweight yet useful macro for capturing the environment context at startup

7 releases

0.1.6 Apr 24, 2024
0.1.5 Apr 24, 2024

#5 in Accessibility

Download history 382/week @ 2024-04-19 75/week @ 2024-04-26

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MIT license

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mkenv is a lightweight Rust crate that provides the make_env! macro used to generate a struct containing all the necessary environment context. This allows to remove runtime errors when retrieving an environment variable that doesn't exist, by capturing them all at the beginning of the program. It is designed to raise an error with a clear message about all the variables the application uses when the environment initialization fails.

Example usage

For each environment variable declaration, you need to provide at least these fields:

  • id: The identifier of the associated struct. This is generally the same identifier of the struct field but in CamelCase.
  • kind: How to retrieve the environment variable.
  • var: The name of the environment variable as a string literal.
  • desc: A short description of it as a string literal.

See the example below:

mkenv::make_env! {AppEnv:
  db_url: {
    id: DbUrl(String),
    kind: normal,
    var: "DB_URL",
    desc: "The URL to the database",

The above example will generate a struct defined as below:

struct AppEnv {
  db_url: String,

This struct implements the Env trait which allows it to be instanciated via the get method. This method will fill the fields of this struct according to the configuration made in the macro call.

You also have other methods if you wish to trace the captured variables during the construction of the struct.

The idea is to use the output instance of this struct to initialize a static variable, and use the latter to get the necessary variables from anywhere in your code. A basic example would be to do (using the once_cell crate):

static ENV: Lazy<AppEnv> = Lazy::new(AppEnv::get);

fn env() -> &'static AppEnv {

This way, you can access the db_url field from anywhere in your code with crate::env().db_url.

If the construction failed, meaning it couldn't retrieve the required environment variables, then this error is shown:

Cannot initialize environment:
Got 0 valid variable
Got 1 incorrect variable
- Missing `DB_URL`
Note: full required environment description:
- `DB_URL`: The URL to the database


Many ways to get an environment variable


The macro obviously supports retrieving environment variable normally, meaning as a String containing the value of the variable. This is done via the normal kind.

By parsing

The macro supports parsing using the FromStr trait implementation of the target type. For this, you need to provide the parse kind to the declaration of the environment variable.

By reading the content of a file

You can provide the file kind to the declaration, which will read the file at the path given by the environment variable. For now, it is not possible to mix the file and parse kinds. The output of the file kind is a String, like the normal kind.

Wrapping types

The macro supports wrapping types, or types that can be constructed from a string or a single value that can be parsed from a string. For this, you need to provide an argument to the kind field, which is the method used to construct the wrapping type. See the example below:

mkenv::make_env! {AppEnv:
  timeout: {
    id: Timeout(std::time::Duration),
    kind: parse(from_secs),
    var: "TIMEOUT",
    desc: "The duration of the timeout (in seconds)",

Default values

The macro supports optional variables, as long as you provide a default value. This is done with the default field:

const DEFAULT_PORT: u16 = 3000;

mkenv::make_env! {AppEnv:
  port: {
    id: Port(u16),
    kind: parse,
    var: "PORT",
    desc: "The port used by the application",
    default: DEFAULT_PORT,

Note that the default value must be an identifier, meaning it should be defined as a constant in advance. The type must implement Debug, because it is printed in case of errors.

Also note that if the environment variable is present but the parsing failed, it will raise an error even if it has a default value.

Conditional compilation

The macro supports conditional compilation attributes for the declaration of the environment variables. It does not yet support any other attribute declared on it however.

For example:

mkenv::make_env! {DbEnv:
  db_url: {
    // ... in debug mode
  db_url: {
    // ... in release mode

Composable declarations

The macro supports composable declarations, meaning including the declaration of other environment types into another. See the example:

mkenv::make_env! {DbEnv:
  db_url: {
    // ...

mkenv::make_env! {AppEnv includes [DbEnv as db_env]:
  port: {
    // ...

The struct declarations will roughly be like so:

struct DbEnv {
  db_url: ...,

struct AppEnv {
  db_env: DbEnv,
  port: ...,


For security reasons, you most likely don't want to keep the value of an environment variable in memory. In most of the cases, you want it to be dropped after it is used once, for initialization purposes.

To achieve this, you need to do the above composable pattern, by declaring the environment variables you need to drop in a separate macro call:

mkenv::make_env! {UsedOnce:
  sess_key: {
    // ...

mkenv::make_env! {AppEnv includes [UsedOnce as used_once]:
  // ...

Then, when constructing the struct, you can call the split method from the EnvSplitIncluded trait. This will give you a tuple containing:

  • A struct with the fields included in the includes [...] clause
  • A struct with all the other fields, that can safely be kept in memory at runtime

By splitting the struct in 2 pieces, you split the ownership in 2: you can drop the first piece after doing your initialization process, and you can initialize a static variable with the second piece:

fn init_my_env() {
  let env = AppEnv::get();
  let (used_once, rest) = env.split();
  // do things with `used_once`, will be dropped at the end
  MY_STATIC_VAR.set(rest).unwrap_or_else(|_| panic!("wtf?"));

But what is the type of the static variable?

You can declare your static variable like so (using the once_cell crate as an example):

static MY_STATIC_VAR: OnceCell<mkenv::init_env!(AppEnv)> = OnceCell::new();


The library is very light, it has 0 dependency!

No runtime deps