Why can I declare a str variable as mutable if Rust str is immutable?

Question

According to: https://web.mit.edu/rust-lang_v1.25/arch/amd64_ubuntu1404/share/doc/rust/html/book/first-edition/strings.html

rust str is immutable, and cannot be used when mutability is required.

However, the following program compiles and works

fn main() {
    let mut mystr = "foo";   
    mystr = "bar";    
    {
         mystr = "baz";      
    }
    println!("{:?}", mystr);  
}

Can someone explain the mutability of str in Rust?

I expect let mut mystr = "foo"; to result in compilation error since str in Rust is immutable. But it compiles.

You are not mutating str here, you're just replacing one &str with another &str. — HJVT
– HJVT, Commented Jan 11, 2023 at 21:27
I've never coded rust, but doesn't mut only apply to the variable, not the contents? e.g. you can re-assign the variable to a different string, but you cannot change the string itself? (pretty much like final works in Java). Never coded rust, so I might be totally wrong. — knittl
– knittl, Commented Jan 11, 2023 at 21:28

Miiao · Accepted Answer · 2023-01-11 21:47:01Z

2

You did not change the string itself. &str is basically (*const u8, usize) - a pointer to the buffer and a length. While mutating a variable with type &str, you’re just replacing one pointer with another and not mutating the original buffer. Immutability of a string literal means that the buffer is actually linked to your binary (and, as I remember, is contained in .rodata), so you cannot change it’s contents. To actually mutate a string, use a heap-allocated one - String.

answered Jan 11, 2023 at 21:47

Miiao

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