False positive on clippy::borrowed_box forced by Mockall

I tried this code:

#[mockall::automock]
trait Bar {
    fn foo(&self, value: Box<String>);
}

fn main() {
    let mut mock_bar = MockBar::default();
    mock_bar.expect_foo().with(mockall::predicate::function(|value: &Box<String>| **value == ""));
}

I expected clippy to give no warnings

Instead, this happened:

warning: you seem to be trying to use `&Box<T>`. Consider using just `&T`
 --> src\main.rs:8:69
  |
8 |     mock_bar.expect_foo().with(mockall::predicate::function(|value: &Box<String>| **value == ""));
  |                                                                     ^^^^^^^^^^^^ help: try: `&String`
  |
  = note: `#[warn(clippy::borrowed_box)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrowed_box

The Mockall library predicate uses &T where T is the type of a function's parameter. In this case T is Box so the type in the predicate's closure is forced to be &Box and can't be change to &String.

Meta

• cargo clippy -V: clippy 0.0.212 (bb37a0f9 2020-06-16)
• rustc -Vv:

rustc 1.44.1 (c7087fe00 2020-06-17)
binary: rustc
commit-hash: c7087fe00d2ba919df1d813c040a5d47e43b0fe7
commit-date: 2020-06-17
host: x86_64-pc-windows-msvc
release: 1.44.1
LLVM version: 9.0

Start with the provided Mockall reproducer and the borrowed_box lint, then run cargo clippy with the reported Rust version context to confirm the warning. Trace the lint's handling of closure parameter types and add a regression test if the relevant test location is found; done means this forced &Box<String> case no longer produces a false positive while ordinary borrowed-box cases remain covered.