rust_book/ch4/refs_and_borrow/src/main.rs

fn main() {
    // These lines wont work bc m1 and m2 are moved
    // let m1 = String::from("Hello");
    // let m2 = String::from("World");
    // greet(m1, m2);
    // let s = format!("{} {}", m1, m2);

    // This is suboptimal due to verboseness of solution
    let m1 = String::from("Hello");
    let m2 = String::from("World");
    let (m1_again, m2_again) = greet_ret(m1, m2);
    let s = format!("{} {}", m1_again, m2_again);
    println!("{s}");

    // references make a convenient usage of ownership
    // Can now use mm1 and mm2 later after the call to greet_ref
    // References are non-owning pointers.
    let mm1 = String::from("Hello");
    let mm2 = String::from("World");
    greet_ref(&mm1, &mm2);
    let ss = format!("{} {}", mm1, mm2);
    println!("{ss}");

    // -----------------------------------------------------------------------------
    // Dereferencing a pointer accesses it data

    // Rust often implicitly inserts references and dereferences throughout
    // the code; ergo, won't see * often in Rust code
    // This can be seen in str::len

    let mut x: Box<i32> = Box::new(1);
    let a: i32 = *x; // reads heap value, so a stores 1 not following x
    *x += 1; // *x points to value on heap, now adds 1 to x
    println!("{}", *x);

    let r1: &Box<i32> = &x; // r1 points to x on the stack
    let b: i32 = **r1; // two dereferences get the heap value
    println!("{}", b);

    let r2: &i32 = &*x; // r2 points to heap value directly. Making a new reference to the deferenced i32
    let c: i32 = *r2; // one dereference needed to read it

    // -----------------------------------------------------------------------------
    // Rust avoids simultaneous aliasing and mutation
    //
    // Aliasing: accessing same data through different variables
    // aliasing on its own okay. Combining with mutation allows "rug pulling" from other variables

    // Invalid code:
    // let mut v: Vec<i32> = vec![1, 2, 3];
    // let num: &i32 = &v[2];
    // v.push(4);
    // println!("3rd element is {}", *num); // undefined behavior here

    // Data in Rust should never be aliased and mutated at the same time

    // -----------------------------------------------------------------------------
    // References Change Permissions on Places
    //
    // Read (R): Data can be copied to another location
    // Write (W): Data can be mutated
    // Own (O): Dta can be moved or dropped
    // These permissions only exist in the compiler, not at runtime.
    // Default permission are RO, mut makes W
    //   References can temporarily remove these permissions.
    // Permissions are defined on places and not just variables
    //   A place being anything to the left of the = sign
    //     Variables like a, Dereferences like *a, array accesses like a[0]

    // -----------------------------------------------------------------------------
    // Borrow checker finds permissions violations

    // Anytime a place is used, Rust expects that place to have certain
    //   permissions depending on the operation

    // -----------------------------------------------------------------------------
    // mutable references provide unique and non-owning access to data

    let mut v: Vec<i32> = vec![1, 2, 3];
    let num: &mut i32 = &mut v[2];
    *num += 1;
    println!("Third element is {}", *num); // actually changes the 2nd element
    println!("Vector is now {:?}", v);

    // temporarily "downgrade" to a read-only reference
    let mut v: Vec<i32> = vec![1, 2, 3];
    let num: &mut i32 = &mut v[2];
    let num2: &i32 = &*num;
    println!("{} {}", *num, *num2);

    // -----------------------------------------------------------------------------
    // Permissions are returned at the end of a references lifetime

    let mut x = 1;
    let y = &x;
    let z = *y;
    x += z;
    println!("final x val: {x}");

    // -----------------------------------------------------------------------------
    // Data must outlive all of its references

    // -----------------------------------------------------------------------------
    // Summary

    // References provide ability to read/write data with consuming ownership.
    // References created with borrows (& and &mut) used dereferenced with * (often implictly)

    // Off to fixing ownership issues in ch4.3
}

fn greet(g1: String, g2: String) {
    println!("{} {}!", g1, g2);
}

fn greet_ref(g1: &String, g2: &String) {
    println!("{} {}!", g1, g2);
}

fn greet_ret(g1: String, g2: String) -> (String, String) {
    println!("{} {}!", g1, g2);
    (g1, g2)
}

fn ascii_capitalize(v: &mut Vec<char>) {
    let c = &v[0]; // *v has permissions R, W taken away
    if c.is_ascii_lowercase() {
        let up = c.to_ascii_uppercase(); // *v has permissions R, W is given
        v[0] = up;
    } else {
        // *v has R, W is given
        println!("Already Capitalized {:?}", v);
    }
}

fn first(strings: &Vec<String>) -> &String {
    // This function introduces the flow permission, F.
    // F doesn't change throughout the body of a function
    let s_ref = &strings[0];
    s_ref
}

// Doesn't know whether &String is a reference to either strings or default
// fn first_or(strings: &Vec<String>, default: &String) -> &String {}