Conditional Compilation: Feature Flags for Customizable Rust Applications

Conditional compilation is a powerful feature in Rust that allows you to include or exclude specific code blocks based on predefined conditions, most commonly feature flags. This enables you to create flexible applications that can be tailored for different environments or use cases without maintaining multiple codebases.

Problem Description

Your task is to implement conditional compilation in a Rust application to enable or disable specific functionalities based on feature flags. You will be creating a simple "Greeter" application that can either print a basic greeting or an extended, more verbose greeting depending on which feature is enabled.

What needs to be achieved:

1. Define two distinct feature flags: basic_greeting and verbose_greeting.
2. Create a greet function that behaves differently based on these features.
3. Demonstrate how to activate these features during compilation.

Key requirements:

• When the basic_greeting feature is enabled (and verbose_greeting is not), the greet function should print a simple "Hello, World!".
• When the verbose_greeting feature is enabled (and basic_greeting is not), the greet function should print a more detailed message, for example, "Welcome to our amazing application! We hope you have a pleasant day.".
• If both features are enabled, the verbose_greeting should take precedence.
• If neither feature is enabled, the program should ideally compile but not print any greeting (or print a default message indicating no features are enabled, as per your design choice).
• Your solution should use Rust's #[cfg(...)] attribute for conditional compilation.

Expected behavior:

• Compilation with cargo build --features basic_greeting should result in an executable that prints "Hello, World!".
• Compilation with cargo build --features verbose_greeting should result in an executable that prints the verbose greeting.
• Compilation with cargo build --features basic_greeting,verbose_greeting should result in an executable that prints the verbose greeting.
• Compilation with cargo build (no features specified) should result in an executable that prints nothing or a default message.

Important edge cases to consider:

• The behavior when multiple features are enabled. The requirement is for verbose_greeting to take precedence.
• The behavior when no features are enabled.

Examples

Example 1:

• Compilation Command: cargo build --features basic_greeting
• Execution of resulting binary:

  Hello, World!
  

• Explanation: The basic_greeting feature is enabled, so the code under #[cfg(feature = "basic_greeting")] is included and executed.

Example 2:

• Compilation Command: cargo build --features verbose_greeting
• Execution of resulting binary:

  Welcome to our amazing application! We hope you have a pleasant day.
  

• Explanation: The verbose_greeting feature is enabled, leading to the execution of the more detailed greeting code.

Example 3:

• Compilation Command: cargo build --features basic_greeting,verbose_greeting
• Execution of resulting binary:

  Welcome to our amazing application! We hope you have a pleasant day.
  

• Explanation: Both features are enabled. Since verbose_greeting has higher precedence (or is checked last in a logical order to override), the verbose greeting is printed.

Example 4:

• Compilation Command: cargo build
• Execution of resulting binary:

  (No output, or a message like "No greeting feature enabled.")
  

• Explanation: No feature flags are specified, so none of the conditional greeting code blocks are compiled or executed.

Constraints

• The solution must be written entirely in Rust.
• You must use the #[cfg(feature = "...")] attribute for conditional compilation.
• The Cargo.toml file must be updated to define the features section.
• The primary logic should reside within a single main.rs file for simplicity.
• No external crates are required for this challenge.

Notes

• Remember to define your features in the Cargo.toml file under the [features] section.
• The #[cfg(...)] attribute can be applied to functions, modules, and even individual statements.
• Consider the logical flow of your conditional statements to ensure the correct greeting is printed when multiple features might be active. A common pattern is to use #[cfg(all(...))] or #[cfg(any(...))] for more complex conditions, but for this problem, a simple else if structure within your greet function, combined with #[cfg] attributes, might be sufficient.
• Think about how you would handle the case where neither feature is enabled. You could have a final else block that prints nothing or a default message.