feat: Add env var placeholders support

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 +22,12 @@ pub enum ConfigError {
 
     #[error("TOML deserialization error: {0}")]
     TomlDeserialize(#[from] toml::de::Error),
+
+    #[error("Missing environment variable: {0}")]
+    MissingEnvVar(String),
+
+    #[error("Invalid environment placeholder at {line}:{char}")]
+    InvalidEnvPlaceholder { line: usize, char: usize },
 }
 
 /// Application configuration, serializable to/from TOML.
@@ -100,12 +107,13 @@ 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 +1,134 @@
+use crate::configuration::config::ConfigError;
+use std::borrow::Cow;
+
+/// 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.
+///
+/// On malformed placeholders (e.g. missing closing }, empty/invalid variable
+/// name, or a newline inside the placeholder), returns
+/// `ConfigError::InvalidEnvPlaceholder { line, char }`, where line and char
+/// are 1-based positions of the $ that started the placeholder.
+///
+/// For efficiency, if the input contains no `${`, the function
+/// returns a `borrowed Cow::Borrowed` without allocating
+pub fn expand_env_placeholders<'a>(input: &'a str) -> Result<Cow<'a, str>, ConfigError> {
+    if !input.contains("${") {
+        return Ok(Cow::Borrowed(input));
+    }
+
+    fn bump(ch: char, line: &mut usize, col: &mut usize) {
+        if ch == '\n' {
+            *line += 1;
+            *col = 1;
+        } else {
+            *col += 1;
+        }
+    }
+
+    fn is_valid_env_name(name: &str) -> bool {
+        let mut it = name.chars();
+        let Some(first) = it.next() else {
+            return false;
+        };
+        if !(first == '_' || first.is_ascii_alphabetic()) {
+            return false;
+        }
+        it.all(|c| c == '_' || c.is_ascii_alphanumeric())
+    }
+
+    let mut out = String::with_capacity(input.len());
+    let mut it = input.char_indices().peekable();
+
+    let mut line: usize = 1;
+    let mut col: usize = 1;
+
+    while let Some((_i, ch)) = it.next() {
+        let start_line = line;
+        let start_col = col;
+
+        match ch {
+            '\\' => {
+                let mut look = it.clone();
+                if matches!(look.next(), Some((_, '$'))) && matches!(look.next(), Some((_, '{'))) {
+                    // съедаем '$' и '{'
+                    let (_, d) = it.next().unwrap();
+                    let (_j, b) = it.next().unwrap();
+
+                    // учёт позиции
+                    bump('\\', &mut line, &mut col);
+                    bump(d, &mut line, &mut col);
+                    bump(b, &mut line, &mut col);
+
+                    out.push_str("${");
+                    continue;
+                }
+
+                bump('\\', &mut line, &mut col);
+                out.push('\\');
+            }
+            '$' => {
+                if !matches!(it.peek(), Some((_, '{'))) {
+                    bump('$', &mut line, &mut col);
+                    out.push('$');
+                    continue;
+                }
+                let (brace_idx, brace) = it.next().unwrap();
+
+                bump('$', &mut line, &mut col);
+                bump(brace, &mut line, &mut col);
+                let name_start = brace_idx + brace.len_utf8();
+                let mut name_end: Option<usize> = None;
+
+                while let Some((k, c)) = it.next() {
+                    match c {
+                        '}' => {
+                            name_end = Some(k);
+                            bump('}', &mut line, &mut col);
+                            break;
+                        }
+                        '\n' => {
+                            return Err(ConfigError::InvalidEnvPlaceholder {
+                                line: start_line,
+                                char: start_col,
+                            });
+                        }
+                        _ => bump(c, &mut line, &mut col),
+                    }
+                }
+
+                let name_end = name_end.ok_or(ConfigError::InvalidEnvPlaceholder {
+                    line: start_line,
+                    char: start_col,
+                })?;
+
+                let name = &input[name_start..name_end];
+
+                if name.is_empty() || !is_valid_env_name(name) {
+                    return Err(ConfigError::InvalidEnvPlaceholder {
+                        line: start_line,
+                        char: start_col,
+                    });
+                }
+
+                let Some(val) = std::env::var_os(name) else {
+                    return Err(ConfigError::MissingEnvVar(name.to_string()));
+                };
+
+                out.push_str(&val.to_string_lossy());
+            }
+
+            _ => {
+                bump(ch, &mut line, &mut col);
+                out.push(ch);
+            }
+        }
+    }
+
+    Ok(Cow::Owned(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 +38,17 @@ fn load_configuration(config_path: &PathBuf) -> Option<Config> {
         Err(ConfigError::Io(message, ..)) => {
             error!("Failed to load configuration: {}", message);
         }
+        Err(ConfigError::InvalidEnvPlaceholder { line, char }) => {
+            error!(
+                "Failed to load configuration: invalid environment placeholder at {line}:{char}"
+            );
+        }
+        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);
         }

server.toml

@@ -0,0 +1,68 @@
+bind = "0.0.0.0:25565"
+welcome_message = "Welcome to PicoLimbo!"
+action_bar = "Welcome to PicoLimbo!"
+default_game_mode = "spectator"
+hardcore = false
+fetch_player_skins = false
+reduced_debug_info = false
+allow_unsupported_versions = false
+allow_flight = false
+
+[forwarding]
+method = "NONE"
+secret = "${PATH}"
+
+[world]
+spawn_position = [
+    0.0,
+    320.0,
+    0.0,
+]
+spawn_rotation = [
+    0.0,
+    0.0,
+]
+dimension = "end"
+time = "day"
+
+[world.experimental]
+view_distance = 2
+schematic_file = ""
+lock_time = false
+
+[world.boundaries]
+enabled = true
+min_y = -64
+teleport_message = "<red>You have reached the bottom of the world.</red>"
+
+[server_list]
+reply_to_status = true
+max_players = 20
+message_of_the_day = "A Minecraft Server"
+show_online_player_count = true
+server_icon = "server-icon.png"
+
+[compression]
+threshold = -1
+level = 6
+
+[tab_list]
+enabled = true
+header = "<bold>Welcome to PicoLimbo</bold>"
+footer = "<green>Enjoy your stay!</green>"
+player_listed = true
+
+[boss_bar]
+enabled = false
+title = "<bold>Welcome to PicoLimbo!</bold>"
+health = 1.0
+color = "pink"
+division = 0
+
+[title]
+enabled = false
+title = "<bold>Welcome!</bold>"
+subtitle = "Enjoy your stay"
+fade_in = 10
+stay = 70
+fade_out = 20