why static lifetime is needed in the main method return type signature

code_example_study/examples/result_question_mark_and_unwrap.rs

@@ -65,4 +65,61 @@ fn main() -> Result<(), &'static str> {
 //
 // In this case, the `&str` reference inside `Err` has the same lifetime as the `message` variable because it's a reference to the data owned by `message`.
 //
-// So, in your example, the `&str` inside `Err` has a `'static` lifetime because it's a reference to a string literal, which has a `'static` lifetime by default. This is a common pattern in Rust when you need to return or store string literals or other static data in a context that requires a reference.
+// So, in your example, the `&str` inside `Err` has a `'static` lifetime because it's a reference to a string literal, which has a `'static` lifetime by default. This is a common pattern in Rust when you need to return or store string literals or other static data in a context that requires a reference.
+
+
+//S
+// 'static lifetime is implicitly applied to the string literal, then
+//
+// What are the differences between
+//
+//
+// Copy code
+// let a: &str = "abc";
+// let b: &'static str = "abc";
+
+
+
+//The difference between `let a: &str = "abc";` and `let b: &'static str = "abc";` lies in the way the lifetime of the reference is specified.
+//
+// In the first case, `let a: &str = "abc";`, the compiler infers the lifetime of the reference `&str` from the context. Since `"abc"` is a string literal with a `'static` lifetime, the compiler automatically assigns the `'static` lifetime to the reference `a`. This is an example of lifetime elision, where the compiler can infer the lifetime based on the context and the programmer doesn't need to explicitly specify it.
+//
+// In the second case, `let b: &'static str = "abc";`, the programmer explicitly annotates the reference `b` with the `'static` lifetime. This tells the compiler that `b` is a reference to a string with a `'static` lifetime, which is guaranteed to live for the entire duration of the program.
+//
+// Both declarations are valid and result in the same thing: a reference to the string literal `"abc"` with a `'static` lifetime. However, explicitly annotating the lifetime with `'static` can sometimes be necessary or preferred in certain situations:
+//
+// 1. **Disambiguation**: If the compiler cannot infer the correct lifetime due to ambiguity or complex code, explicit annotation may be required to resolve the ambiguity.
+//
+// 2. **Clarity**: Explicitly annotating the lifetime can make the code more readable and self-documenting, especially in larger codebases or when working with complex lifetime scenarios.
+//
+// 3. **Consistency**: Some teams or coding styles may prefer to always explicitly annotate lifetimes for consistency and to avoid relying on lifetime elision.
+//
+// 4. **Generic Code**: When writing generic code that works with references of different lifetimes, explicit lifetime annotations are often necessary to specify the relationships between lifetimes.
+//
+// In general, for simple cases like string literals or when the lifetime can be easily inferred from the context, it's common to rely on lifetime elision and omit the explicit lifetime annotation. However, in more complex scenarios or when working with generic code, explicit lifetime annotations can improve code clarity and maintainability.
+
+
+
+//S
+// In 'fn main() -> Result<(), &'static str> {', why it doesn't work without static lifetime?
+//
+// The following doesn't work
+//
+//
+// Copy code
+// fn main() -> Result<(), &str> {
+
+
+//The reason why `fn main() -> Result<(), &str> {` doesn't work is because the `&str` type doesn't have a `'static` lifetime constraint. The `main` function is the entry point of a Rust program, and it needs to return a value that has a `'static` lifetime, which means it must live for the entire duration of the program's execution.
+//
+// When you define `fn main() -> Result<(), &str> {`, you're saying that the `Err` variant of the `Result` can contain a reference to a string (`&str`), but without any lifetime constraint. This means that the string could have any lifetime, including a non-`'static` lifetime.
+//
+// The problem with this is that the `main` function needs to return a value that can be stored in the binary and accessed at any point during the program's execution. If the `Err` variant contains a reference to a string with a non-`'static` lifetime, it could potentially point to memory that has already been freed or deallocated, leading to undefined behavior or a program crash.
+//
+// By adding the `'static` lifetime constraint, `fn main() -> Result<(), &'static str> {`, you're explicitly stating that the `Err` variant can only contain references to strings that have a `'static` lifetime. This guarantees that the string data will be stored in the read-only data section of the binary and will remain valid for the entire duration of the program's execution.
+//
+// The `'static` lifetime is a special lifetime that represents the entire lifetime of the program. It's used for values that are statically allocated and live for the entire duration of the program's execution, such as string literals, static variables, and constants.
+//
+// If you try to use a non-`'static` lifetime for the `main` function's return type, the compiler will complain because it cannot guarantee that the returned value will be valid for the entire lifetime of the program.
+//
+// In summary, the `'static` lifetime constraint is necessary for the `main` function's return type to ensure that the returned value is valid and accessible for the entire duration of the program's execution, avoiding potential undefined behavior or crashes.