Rust Configuration Manager

This challenge focuses on building a robust and flexible configuration management system in Rust. A good configuration system is crucial for applications, allowing them to adapt to different environments and user preferences without code changes. You will create a system that can load configuration from a file, parse it into strongly typed data structures, and provide access to these settings.

Problem Description

You need to design and implement a Rust library or module that handles application configuration. This system should be capable of:

1. Loading Configuration: Read configuration data from a specified file.
2. Parsing Configuration: Parse the loaded configuration into Rust data structures.
3. Type Safety: Ensure that configuration values are accessed with their correct types.
4. Default Values: Provide default values for configuration settings if they are not present in the file.
5. Error Handling: Gracefully handle potential errors like file not found, invalid file format, or missing required values.

Your solution should support a common configuration file format like TOML or YAML. For this challenge, we will use TOML.

Key Requirements

• Define a Rust struct that represents the application's configuration. This struct should include various data types (e.g., String, i32, bool, Vec<String>).
• Implement a function that takes a file path (as a &str) and returns a Result containing your configuration struct or an appropriate error type.
• The configuration loading process should merge values from the TOML file with predefined default values. If a setting exists in the file, it should override the default; otherwise, the default should be used.
• Handle cases where the configuration file is missing or malformed.
• Handle cases where specific, required configuration keys are missing from the file and do not have defaults.

Expected Behavior

When a configuration file is successfully loaded and parsed:

• The returned configuration struct should accurately reflect the values specified in the TOML file, overridden by any defaults where not specified.
• If a default value is defined for a key, and that key is missing from the TOML file, the default value should be used.

When errors occur:

• If the configuration file does not exist, an appropriate error should be returned.
• If the configuration file has invalid TOML syntax, an appropriate error should be returned.
• If a required configuration key is missing and has no default, an appropriate error should be returned.

Edge Cases to Consider

• An empty configuration file.
• A configuration file with only comments.
• Nested configuration structures within the TOML file.
• Different primitive data types in the TOML file.
• Arrays and nested arrays.

Examples

Example 1: Basic Loading with Overrides

Input:

config.toml file content:

title = "My Awesome App"
version = 2
enable_feature_x = true

Rust struct definition (conceptual):

#[derive(Debug, Default)]
struct AppConfig {
    title: String,
    version: i32,
    enable_feature_x: bool,
    log_level: String, // Has a default
}

Expected Output (when load_config("config.toml") is called):

Ok(AppConfig {
    title: "My Awesome App",
    version: 2,
    enable_feature_x: true,
    log_level: "info", // Default value
})

Explanation: The title, version, and enable_feature_x fields are read directly from config.toml. The log_level field is not present in the file, so its default value ("info") is used.

Example 2: Missing Optional Fields

Input:

config.toml file content:

version = 3

Rust struct definition (conceptual, same as Example 1):

#[derive(Debug, Default)]
struct AppConfig {
    title: String,
    version: i32,
    enable_feature_x: bool,
    log_level: String, // Has a default
}

Expected Output (when load_config("config.toml") is called):

Ok(AppConfig {
    title: "Default App Title", // Default value
    version: 3,
    enable_feature_x: false, // Default value
    log_level: "debug", // Another default value
})

Explanation: Only version is present in the file. title, enable_feature_x, and log_level are populated with their respective default values.

Example 3: Missing Required Fields (Illustrative - Your implementation might choose to panic or return a specific error)

Input:

config.toml file content:

enable_feature_x = true

Rust struct definition (conceptual, assuming title is required and has no default):

#[derive(Debug)] // No Default
struct AppConfig {
    title: String, // Required, no default
    version: i32,  // Has a default
    enable_feature_x: bool,
}

Expected Behavior: An error indicating that title is missing. The exact error type is up to your design, but it should clearly state the problem.

Explanation: The title field is expected but not found in the file, and it doesn't have a default value defined in the AppConfig struct.

Constraints

• The configuration file will be in TOML format.
• Your solution should use the toml crate for parsing TOML.
• The primary configuration loading function should return a Result<YourConfigStruct, YourErrorType>.
• You must define at least two distinct error variants (e.g., IoError, TomlParseError, MissingRequiredConfig).
• The implementation should be efficient enough for typical application startup times.
• You should handle basic primitive types (String, i32, f64, bool), arrays of primitives (Vec<String>, Vec<i32>, etc.), and potentially nested structures.

Notes

• Consider using serde for deserialization of TOML into Rust structs, as it integrates very well with toml.
• Think about how you want to define default values. Deriving Default for your configuration struct is a common and idiomatic approach.
• Your custom error type should provide clear and actionable information to the user or developer.
• For simplicity in this challenge, you can assume the configuration file path is valid UTF-8.
• Feel free to define a realistic AppConfig struct with several fields of different types to thoroughly test your implementation.