component_model 0.16.0

# Task 004: Configuration File Support

## 🎯 **Objective**

Integrate component model with popular configuration formats (TOML, YAML, JSON) and the `config` crate to provide seamless configuration loading with environment variable overrides and profile support.

## 📋 **Current State**

Users must manually handle configuration loading:
```rust
// Manual approach
let config_str = std::fs::read_to_string( "config.toml" )?;
let parsed : ConfigData = toml::from_str( &config_str )?;

let mut app_config = AppConfig::default();
app_config.assign( parsed.database.host );
app_config.assign( parsed.database.port );
// ... lots of manual mapping
```

## 🎯 **Target State**

Seamless configuration loading with component model:
```rust
#[ derive( ComponentModel, Config ) ]
struct AppConfig
{
  #[ config( env = "DATABASE_HOST" ) ]
  database_host : String,
  
  #[ config( env = "DATABASE_PORT", default = "5432" ) ]
  database_port : u16,
  
  #[ config( profile = "production" ) ]
  ssl_enabled : bool,
}

// Load from file with environment overrides
let config = AppConfig::from_config_file( "app.toml" )
  .with_env_overrides()
  .with_profile( "production" )
  .build()?;

// Or build programmatically
let config = AppConfig::default()
  .impute( "localhost" )       // database_host
  .impute( 5432u16 )          // database_port
  .impute( true )             // ssl_enabled
  .load_from_env()          // Override with env vars
  .validate()?;             // Run validation
```

## 📝 **Detailed Requirements**

### **Core Configuration API**

#### **Config Derive**
```rust
#[ proc_macro_derive( Config, attributes( config ) ) ]
pub fn derive_config( input : TokenStream ) -> TokenStream
{
  // Generate configuration loading methods
}
```

#### **Configuration Loading Methods**
```rust
impl AppConfig
{
  // File loading
  fn from_config_file< P : AsRef< Path > >( path : P ) -> ConfigBuilder< Self >;
  fn from_toml< P : AsRef< Path > >( path : P ) -> Result< Self, ConfigError >;
  fn from_yaml< P : AsRef< Path > >( path : P ) -> Result< Self, ConfigError >;
  fn from_json< P : AsRef< Path > >( path : P ) -> Result< Self, ConfigError >;
  
  // Environment loading
  fn from_env() -> Result< Self, ConfigError >;
  fn from_env_with_prefix( prefix : &str ) -> Result< Self, ConfigError >;
  
  // Builder pattern
  fn config() -> ConfigBuilder< Self >;
}

pub struct ConfigBuilder< T >
{
  // Builder state
}

impl< T > ConfigBuilder< T >
{
  fn from_file< P : AsRef< Path > >( self, path : P ) -> Self;
  fn from_env( self ) -> Self;
  fn with_profile( self, profile : &str ) -> Self;
  fn with_overrides< F >( self, f : F ) -> Self where F : Fn( &mut T );
  fn build( self ) -> Result< T, ConfigError >;
}
```

### **Attribute System**

#### **Field Attributes**
```rust
#[ derive( ComponentModel, Config ) ]
struct DatabaseConfig
{
  // Environment variable mapping
  #[ config( env = "DB_HOST" ) ]
  host : String,
  
  // Default value
  #[ config( default = "5432" ) ]
  port : u16,
  
  // Profile-specific values
  #[ config( profile = "production", default = "true" ) ]
  #[ config( profile = "development", default = "false" ) ]
  ssl_required : bool,
  
  // Nested configuration
  #[ config( nested ) ]
  connection_pool : PoolConfig,
  
  // Custom deserializer
  #[ config( deserialize_with = "parse_duration" ) ]
  timeout : Duration,
}
```

#### **Container Attributes**
```rust
#[ derive( ComponentModel, Config ) ]
#[ config( prefix = "APP" ) ]              // Environment prefix
#[ config( file = "app.toml" ) ]           // Default config file
#[ config( profiles = [ "dev", "prod" ] ) ]  // Available profiles
struct AppConfig
{
  // fields...
}
```

### **Integration with Popular Crates**

#### **Config Crate Integration**
```rust
impl AppConfig
{
  fn from_config_crate() -> Result< Self, ConfigError >
  {
    let settings = config::Config::builder()
      .add_source( config::File::with_name( "config" ) )
      .add_source( config::Environment::with_prefix( "APP" ) )
      .build()?;
      
    Self::from_config_settings( settings )
  }
  
  fn from_config_settings( settings : config::Config ) -> Result< Self, ConfigError >
  {
    let mut instance = Self::default();
    
    // Use component model to assign values from config
    if let Ok( host ) = settings.get_string( "database.host" )
    {
      instance.assign( host );
    }
    // ... etc
    
    Ok( instance )
  }
}
```

#### **Figment Integration** (Rocket's config system)
```rust
#[ cfg( feature = "figment" ) ]
impl Configurable for AppConfig
{
  fn from_figment( figment : figment::Figment ) -> Result< Self, figment::Error >
  {
    let mut config = Self::default();
    
    // Extract values and use component assignment
    let extracted = figment.extract::< ConfigData >()?;
    config.apply_config_data( extracted );
    
    Ok( config )
  }
}
```

### **Environment Variable Support**

#### **Automatic Mapping**
```rust
// Field name to environment variable mapping
struct Config
{
  database_host : String,    // -> DATABASE_HOST
  api_key : String,          // -> API_KEY  
  worker_count : usize,      // -> WORKER_COUNT
}

// With prefix
#[ config( prefix = "APP" ) ]
struct Config
{
  database_host : String,    // -> APP_DATABASE_HOST
}
```

#### **Custom Environment Mapping**
```rust
#[ derive( Config ) ]
struct Config
{
  #[ config( env = "DB_URL" ) ]
  database_url : String,
  
  #[ config( env = "PORT", default = "8080" ) ]
  server_port : u16,
}
```

### **Profile Support**

#### **Profile-Specific Values**
```rust
// config.toml
[default]
debug = false
workers = 1

[development]
debug = true
workers = 1

[production]
debug = false
workers = 8
ssl_required = true

// Usage
let config = AppConfig::from_config_file( "config.toml" )
  .with_profile( "production" )
  .build()?;
```

## 🗂️ **File Changes**

### **New Files**
- `component_model_config/` - New crate for configuration support
- `component_model_config/src/lib.rs` - Main configuration API
- `component_model_config/src/config_derive.rs` - Config derive implementation
- `component_model_config/src/formats/` - Format-specific loaders (TOML, YAML, JSON)
- `component_model_config/src/env.rs` - Environment variable support
- `component_model_config/src/profiles.rs` - Profile management
- `component_model_config/src/builder.rs` - Configuration builder
- `examples/config_example.rs` - Comprehensive configuration example

### **Modified Files**
- `Cargo.toml` - Add new workspace member
- `component_model/Cargo.toml` - Add config dependency (feature-gated)
- `component_model/src/lib.rs` - Re-export config functionality

## ⚡ **Implementation Steps**

### **Phase 1: Core Configuration (Week 1)**
1. Create `component_model_config` crate
2. Implement basic file loading for TOML/JSON/YAML
3. Create `Config` derive macro with basic functionality
4. Add environment variable mapping

### **Phase 2: Advanced Features (Week 2)**
1. Implement profile support
2. Add configuration builder pattern
3. Create integration with `config` crate
4. Add validation integration

### **Phase 3: Polish & Documentation (Week 2-3)**
1. Comprehensive examples and documentation
2. Error handling improvement
3. Performance optimization
4. Integration testing with real-world configs

## 🧪 **Testing Strategy**

### **Unit Tests**
```rust
#[ cfg( test ) ]
mod tests
{
  use super::*;
  use std::env;
  
  #[ test ]
  fn test_file_loading()
  {
    #[ derive( ComponentModel, Config, Debug, PartialEq ) ]
    struct TestConfig
    {
      name : String,
      port : u16,
    }
    
    // Create test config file
    let config_content = r#"
      name = "test-app"
      port = 8080
    "#;
    std::fs::write( "test_config.toml", config_content ).unwrap();
    
    let config = TestConfig::from_toml( "test_config.toml" ).unwrap();
    assert_eq!( config.name, "test-app" );
    assert_eq!( config.port, 8080 );
    
    std::fs::remove_file( "test_config.toml" ).unwrap();
  }
  
  #[ test ]
  fn test_env_override()
  {
    #[ derive( ComponentModel, Config ) ]
    struct TestConfig
    {
      #[ config( env = "TEST_HOST" ) ]
      host : String,
    }
    
    env::set_var( "TEST_HOST", "override.example.com" );
    
    let config = TestConfig::default()
      .load_from_env()
      .unwrap();
      
    assert_eq!( config.host, "override.example.com" );
    
    env::remove_var( "TEST_HOST" );
  }
  
  #[ test ]
  fn test_profile_selection()
  {
    let config_content = r#"
      [default]
      debug = false
      
      [development]
      debug = true
    "#;
    std::fs::write( "test_profile.toml", config_content ).unwrap();
    
    #[ derive( ComponentModel, Config ) ]
    struct TestConfig
    {
      debug : bool,
    }
    
    let config = TestConfig::from_config_file( "test_profile.toml" )
      .with_profile( "development" )
      .build()
      .unwrap();
      
    assert_eq!( config.debug, true );
    
    std::fs::remove_file( "test_profile.toml" ).unwrap();
  }
}
```

### **Integration Tests**
```rust
// tests/config_integration.rs
#[ test ]
fn test_real_world_config()
{
  let config_toml = r#"
    [database]
    host = "localhost"
    port = 5432
    
    [server]
    bind_addr = "127.0.0.1:8080"
    workers = 4
    
    [production]
    [production.database]
    host = "prod-db.example.com"
    
    [production.server]
    workers = 16
  "#;
  
  #[ derive( ComponentModel, Config ) ]
  struct DatabaseConfig
  {
    host : String,
    port : u16,
  }
  
  #[ derive( ComponentModel, Config ) ]
  struct ServerConfig
  {
    bind_addr : String,
    workers : usize,
  }
  
  #[ derive( ComponentModel, Config ) ]
  struct AppConfig
  {
    #[ config( nested ) ]
    database : DatabaseConfig,
    
    #[ config( nested ) ]
    server : ServerConfig,
  }
  
  std::fs::write( "app_test.toml", config_toml ).unwrap();
  
  // Test default profile
  let config = AppConfig::from_toml( "app_test.toml" ).unwrap();
  assert_eq!( config.database.host, "localhost" );
  assert_eq!( config.server.workers, 4 );
  
  // Test production profile
  let config = AppConfig::from_config_file( "app_test.toml" )
    .with_profile( "production" )
    .build()
    .unwrap();
    
  assert_eq!( config.database.host, "prod-db.example.com" );
  assert_eq!( config.server.workers, 16 );
  
  std::fs::remove_file( "app_test.toml" ).unwrap();
}
```

## 📊 **Success Metrics**

- [ ] Support for TOML, YAML, JSON configuration formats
- [ ] Seamless environment variable integration
- [ ] Profile-based configuration
- [ ] Integration with `config` crate
- [ ] Zero-overhead when features not used
- [ ] Clear error messages for configuration issues

## 🚧 **Potential Challenges**

1. **Format Compatibility**: Different formats have different capabilities
   - **Solution**: Common denominator approach with format-specific extensions

2. **Environment Variable Mapping**: Complex nested structures
   - **Solution**: Flattened dot-notation mapping with clear documentation

3. **Profile Merging**: Complex merge semantics
   - **Solution**: Clear precedence rules and merge strategy documentation

## 🔄 **Dependencies**

- **Requires**: 
  - Task 001 (Single Derive Macro) for attribute parsing
  - Task 003 (Validation) for config validation
- **Blocks**: None  
- **Related**: Task 002 (Popular Types) benefits from config loading

## 📅 **Timeline**

- **Week 1**: Core file loading and environment variables
- **Week 2**: Profiles, builder pattern, and config crate integration
- **Week 3**: Documentation, examples, and optimization

## 💡 **Future Enhancements**

- **Hot Reload**: Watch config files for changes
- **Remote Configuration**: Load from HTTP endpoints, databases
- **Configuration Schemas**: Generate JSON schemas from structs
- **Configuration UI**: Generate web UIs for configuration editing