Less confusing placeholder when RefCell is exclusively borrowed

[dtolnay]

Based on ExpHP's comment in RefCell.borrow_mut get strange result:

it would perhaps be nicer if it didn't put something that could be misinterpreted as a valid string value

The previous Debug implementation would show:

RefCell { value: "<borrowed>" }

The new one is:

RefCell { value: <borrowed> }

[rust-highfive]

r? @aturon

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[sfackler]

LGTM. I think Mutex might have something similar?

[dtolnay]

Here is what I ran to check for anything similar. RefCell is the only one outside of tests.

rg '\.field\("[^"]*", &"'

Mutex prints <locked> correctly without quotes but is wrong for other reasons. I filed #44771 to follow up. :(

[dtolnay]

@bors r=sfackler rollup

[bors]

📌 Commit f9d92d2 has been approved by sfackler