I decided to turn on Expression Adjustment Hints inlay hints in Rust Analyzer, then I get confused about line 2. why is metadata being dereferenced then borrowed again. Since is_dir() is acting on its reference ( pub fn is_dir(&self) -> bool ) and the parameter metadata is already borrowed ( metadata: &Metadata ), isn't it redundant to dereference and borrow again? But since Rust Analyzer is displaying the inlay hints as (&*metadata).is_dir(), I am wondering is it really like that under the hood?
It's a reborrow.
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Calling functions with a reference can involve a reborrow, which is what the &* or &mut * is. It's usually not a significant change for shared references (&, which implement Copy), but crucial for exclusive references (&mut, which do not).
For example this would not compile if not for reborrows.
fn example(v: &mut Vec<i32>) {
// reborrows
v.push(0);
// ownership of `v` was not lost
println!("{v:?}");
}
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And the thing that a reborrow does is produce a reference whose lifetime is shorter than the original reference. Formally, &'a T and &'b T are different types and it is a specific feature of the language that you are allowed to use a long-lived reference as if it were a short-lived one without explicitly reborrowing all the time.
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