Add default_box_assignments lint

[bluebear94]

Adds a new lint that detects var = Default::default() when var is Box<T> and T implements Default.

changelog: new lint: [default_box_assignments]

• Followed [lint naming conventions][lint_naming]
• Added passing UI tests (including committed .stderr file)
• cargo test passes locally
• Executed cargo dev update_lints
• Added lint documentation
• Run cargo dev fmt

[rustbot]

r? @Alexendoo

rustbot has assigned @Alexendoo.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[samueltardieu]

This is a good start.

Can you include tests involving macros and make sure this gets handled? For example, what if Box::default() comes from a macro when called with certain parameters? This should not lint here. Or when the assignment happens in a macro with arguments coming from the user, they may not be aware that this will be assigned to the same variable.

[samueltardieu]

r? samueltardieu. @rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[bluebear94]

@rustbot ready

[samueltardieu]

This is looking good, although some things still need to be adjusted, but I like it so far!

You might also want to add tests to ensure that this deals correctly with generics (it should in the current state already), and also with unsized types to make sure they are (and stay) properly rejected:

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

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

fn with_unsized<const N: usize>(b: &mut Box<[u32]>) {
    *b = Box::new([42; N]);
}

[samueltardieu]

Can you please update the lint description? Even if we don't have a definite name for it, the description should be accurate so that we can start a FCP (final comment period) to let people a chance to comment on the lint.

[bluebear94]

OK, I’ve changed it to “assigning a newly created box to Box<T> is inefficient”.

[samueltardieu]

I've started the FCP on Zulip.