Fix issue #2907.

[smklein]

Update the "borrow box" lint to avoid recommending the following
conversion:

  // Old
  pub fn f(&mut Box<T>) {...}

  // New
  pub fn f(&mut T) {...}

Given a mutable reference to a box, functions may want to change
"which" object the Box is pointing at.

This change avoids recommending removing the "Box" parameter
for mutable references.

changelog: Don't trigger [borrow_box] lint on &mut Box references

[smklein]

Hey all! This is my first commit to clippy - lemme know if I'm heading down the wrong direction or whatever. I'm happy to drastically change things, I just wanted to get in the flow of poking around :)

As a future improvement, it does seem like this lint could be once again enabled for mutable types if we have sufficient context to know that the object location won't change. Maybe something like:

// Might modify pointer, so clippy won't recommend &mut T.
pub fn f(&mut Box<T>) {...};

// This probably *could* be &mut T, right?
pub fn f(&mut Pin<Box<T>>) {...};

[flip1995]

Thanks! Great contribution, especially the comments in the test files for context!

@bors r+

[bors]

📌 Commit 0ef5dee has been approved by flip1995

[bors]

⌛ Testing commit 0ef5dee with merge 43b4623...

[bors]

💔 Test failed - checks-action_test

[flip1995]

@bors retry

[bors]

⌛ Testing commit 0ef5dee with merge 2efc2d6...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: flip1995
Pushing 2efc2d6 to master...