feat: Add env var placeholders support

Cargo.toml

@@ -35,6 +35,7 @@ quick-xml = "0.38.4"
 quote = "1.0.42"
 rand = "0.9.2"
 rayon = "1.11.0"
+regex = "1.12.2"
 reqwest = { version = "0.12.26", default-features = false, features = ["json", "rustls-tls-native-roots", "http2"] }
 serde = { version = "1.0.228", features = ["derive"] }
 serde_json = "1.0.145"

pico_limbo/Cargo.toml

@@ -17,6 +17,7 @@ clap = { workspace = true }
 futures = { workspace = true }
 hmac = { workspace = true }
 rand = { workspace = true }
+regex = { workspace = true }
 reqwest = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
@@ -26,4 +27,4 @@ thiserror = { workspace = true }
 tokio = { workspace = true }
 toml = { workspace = true }
 tracing = { workspace = true }
-tracing-subscriber = { workspace = true }
+tracing-subscriber = { workspace = true }

pico_limbo/src/configuration/config.rs

@@ -1,5 +1,6 @@
 use crate::configuration::boss_bar::BossBarConfig;
 use crate::configuration::compression::CompressionConfig;
+use crate::configuration::env_placeholders::expand_env_placeholders;
 use crate::configuration::forwarding::ForwardingConfig;
 use crate::configuration::game_mode_config::GameModeConfig;
 use crate::configuration::server_list::ServerListConfig;
@@ -21,6 +22,9 @@ pub enum ConfigError {
 
     #[error("TOML deserialization error: {0}")]
     TomlDeserialize(#[from] toml::de::Error),
+
+    #[error("Missing environment variable: {0}")]
+    MissingEnvVar(String),
 }
 
 /// Application configuration, serializable to/from TOML.
@@ -100,12 +104,13 @@ pub fn load_or_create<P: AsRef<Path>>(path: P) -> Result<Config, ConfigError> {
     let path = path.as_ref();
 
     if path.exists() {
-        let toml_str = fs::read_to_string(path)?;
+        let raw_toml_str = fs::read_to_string(path)?;
 
-        if toml_str.trim().is_empty() {
+        if raw_toml_str.trim().is_empty() {
             create_default_config(path)
         } else {
-            let cfg: Config = toml::from_str(&toml_str)?;
+            let expanded_toml_str = expand_env_placeholders(&raw_toml_str)?;
+            let cfg: Config = toml::from_str(expanded_toml_str.as_ref())?;
             Ok(cfg)
         }
     } else {

pico_limbo/src/configuration/env_placeholders.rs

@@ -0,0 +1,39 @@
+use regex::{Captures, Regex};
+
+use crate::configuration::config::ConfigError;
+use std::{borrow::Cow, sync::LazyLock};
+
+/// Expands environment placeholders in the given text.
+///
+/// Replaces occurrences of `${ENV_VAR}` with the corresponding value from the
+/// process environment (via `std::env`). If a referenced variable is not set,
+/// returns `ConfigError::MissingEnvVar`.
+///
+/// The sequence `\${` is treated as an escape and is converted to a literal `${`
+/// without performing substitution.
+pub fn expand_env_placeholders<'a>(input: &'a str) -> Result<Cow<'a, str>, ConfigError> {
+    static RE: LazyLock<Regex> =
+        LazyLock::new(|| Regex::new(r#"(\\)?\$\{([A-Za-z_][A-Za-z0-9_]*)\}"#).unwrap());
+
+    for caps in RE.captures_iter(input) {
+        let is_escaped = caps.get(1).is_some();
+        let name = &caps[2];
+        if is_escaped {
+            continue;
+        }
+        if std::env::var(name).is_err() {
+            return Err(ConfigError::MissingEnvVar(name.to_string()));
+        }
+    }
+
+    let out = RE.replace_all(input, |caps: &Captures| {
+        let is_escaped = caps.get(1).is_some();
+        let name = &caps[2];
+        if is_escaped {
+            format!("${{{}}}", name)
+        } else {
+            std::env::var(name).unwrap()
+        }
+    });
+    Ok(out)
+}

pico_limbo/src/configuration/mod.rs

@@ -1,6 +1,7 @@
 pub mod boss_bar;
 mod compression;
 pub mod config;
+mod env_placeholders;
 mod forwarding;
 mod game_mode_config;
 mod require_boolean;

pico_limbo/src/server/start_server.rs

@@ -38,6 +38,12 @@ fn load_configuration(config_path: &PathBuf) -> Option<Config> {
         Err(ConfigError::Io(message, ..)) => {
             error!("Failed to load configuration: {}", message);
         }
+        Err(ConfigError::MissingEnvVar(var)) => {
+            error!(
+                "Failed to load configuration: missing environment variable {}",
+                var
+            );
+        }
         Err(ConfigError::TomlSerialize(message, ..)) => {
             error!("Failed to save default configuration file: {}", message);
         }