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let greeting = "Good morning";

greeting is a string literal and they are immutable. The reason why it’s immutable is explained below.

In Rust, you can store part of the String and they are called String slices. For example,

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let str = String::from("Hello there");

let hello = &str[0..4];
let there = &str[6..10];

hello and there are String slices. They store references to the portion of the String.

Now let’s talk about why string literals are immutable. The string literals are literally slices of a string. Imagine that the value Good morning is stored on the heap(in a memory location called temp) then the greeting variable is basically temp[0..temp.len()] (can write as temp[..])

As I mentioned in the ownership chapter, if you have an immutable reference to a variable, you cannot take a mutable reference for the same.

Here is an example.

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fn main(){
	let str = String::from("Hello there");
	let hello = &str[0..4];
	let there = &str[6..10];

	/*this will result in an error because `hello`
	a `there` already have an immutable reference to `str`*/
	str.clear();
}