tests/ui/replace_box.rs

#![warn(clippy::replace_box)]

fn with_default<T: Default>(b: &mut Box<T>) {
    *b = Box::new(T::default());
    //~^ replace_box
}

fn with_sized<T>(b: &mut Box<T>, t: T) {
    *b = Box::new(t);
    //~^ replace_box
}

fn with_unsized<const N: usize>(b: &mut Box<[u32]>) {
    // No lint for assigning to Box<T> where T: !Default
    *b = Box::new([42; N]);
}

macro_rules! create_default {
    () => {
        Default::default()
    };
}

macro_rules! create_zero_box {
    () => {
        Box::new(0)
    };
}

macro_rules! same {
    ($v:ident) => {
        $v
    };
}

macro_rules! mac {
    (three) => {
        3u32
    };
}

fn main() {
    let mut b = Box::new(1u32);
    b = Default::default();
    //~^ replace_box
    b = Box::default();
    //~^ replace_box

    // No lint for assigning to the storage
    *b = Default::default();
    *b = u32::default();

    // No lint if either LHS or RHS originates in macro
    b = create_default!();
    b = create_zero_box!();
    same!(b) = Default::default();

    b = Box::new(5);
    //~^ replace_box

    b = Box::new(mac!(three));
    //~^ replace_box

    // No lint for assigning to Box<T> where T: !Default
    let mut b = Box::<str>::from("hi".to_string());
    b = Default::default();

    // No lint for late initializations
    #[allow(clippy::needless_late_init)]
    let bb: Box<u32>;
    bb = Default::default();
}

fn issue15951() {
    struct Foo {
        inner: String,
    }

    fn embedded_body() {
        let mut x = Box::new(());
        let y = x;
        x = Box::new(());

        let mut x = Box::new(Foo { inner: String::new() });
        let y = x.inner;
        x = Box::new(Foo { inner: String::new() });
        //~^ replace_box
    }

    let mut x = Box::new(Foo { inner: String::new() });
    let in_closure = || {
        x = Box::new(Foo { inner: String::new() });
        //~^ replace_box
    };
}

static R: fn(&mut Box<String>) = |x| *x = Box::new(String::new());
//~^ replace_box

fn field() {
    struct T {
        content: String,
    }

    impl T {
        fn new() -> Self {
            Self { content: String::new() }
        }
    }

    struct S {
        b: Box<T>,
    }

    let mut s = S { b: Box::new(T::new()) };
    let _b = s.b;
    s.b = Box::new(T::new());

    // Interestingly, the lint and fix are valid here as `s.b` is not really moved
    let mut s = S { b: Box::new(T::new()) };
    _ = s.b;
    s.b = Box::new(T::new());
    //~^ replace_box

    let mut s = S { b: Box::new(T::new()) };
    s.b = Box::new(T::new());
    //~^ replace_box

    struct Q(Box<T>);
    let mut q = Q(Box::new(T::new()));
    let _b = q.0;
    q.0 = Box::new(T::new());

    let mut q = Q(Box::new(T::new()));
    _ = q.0;
    q.0 = Box::new(T::new());
    //~^ replace_box

    // This one is a false negative, but it will need MIR analysis to work properly
    let mut x = Box::new(String::new());
    x = Box::new(String::new());
    x;
}