Add debug-refcell feature to libcore

[Aaron1011]

See https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Attaching.20backtraces.20to.20RefCell/near/226273614
for some background discussion

This PR adds a new off-by-default feature debug-refcell to libcore.
When enabled, this feature stores additional debugging information in
RefCell. This information is included in the panic message when
borrow() or borrow_mut() panics, to make it easier to track down the
source of the issue.

Currently, we store the caller location for the earliest active borrow.
This has a number of advantages:

• There is only a constant amount of overhead per RefCell
• We don't need any heap memory, so it can easily be implemented in core
• Since we are storing the earliest active borrow, we don't need any
  extra logic in the Drop implementation for Ref and RefMut

Limitations:

• We only store the caller location, not a full Backtrace. Until
  we get support for Backtrace in libcore, this is the best tha we can
  do.
• The captured location is only displayed when borrow() or
  borrow_mut() panics. If a crate calls try_borrow().unwrap()
  or try_borrow_mut().unwrap(), this extra information will be lost.

To make testing easier, I've enabled the debug-refcell feature by
default. I'm not sure how to write a test for this feature - we would
need to rebuild core from the test framework, and create a separate
sysroot.

Since this feature will be off-by-default, users will need to use
xargo or cargo -Z build-std to enable this feature. For users using
a prebuilt standard library, this feature will be disabled with zero
overhead.

I've created a simple test program:

use std::cell::RefCell;

fn main() {
    let _ = std::panic::catch_unwind(|| {
        let val = RefCell::new(true);
        let _first = val.borrow();
        let _second = val.borrow();
        let _third = val.borrow_mut();
    });

    let _ = std::panic::catch_unwind(|| {
        let val  = RefCell::new(true);
        let first = val.borrow_mut();
        drop(first);

        let _second = val.borrow_mut();

        let _thid = val.borrow();
    });
}

which produces the following output:

thread 'main' panicked at 'already borrowed: BorrowMutError at refcell_test.rs:6:26', refcell_test.rs:8:26
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'main' panicked at 'already mutably borrowed: BorrowError at refcell_test.rs:16:27', refcell_test.rs:18:25

[rust-highfive]

r? @Mark-Simulacrum

(rust-highfive has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

Implementation seems reasonable - I'm not sure how much we want to do this, but I could easily see it being quite useful in practice.

r? @m-ou-se for T-libs

[Aaron1011]

Ancedotally, I've already used this to fix a bug (ruffle-rs/ruffle#3322) where the borrow() and borrow_mut() calls were very far apart in the call stack.

[m-ou-se]

• The captured location is only displayed when borrow() or
  borrow_mut() panics. If a crate calls try_borrow().unwrap()
  or try_borrow_mut().unwrap(), this extra information will be lost.

Why not add the location to the BorrowError and BorrowMutError types instead? Then you don't need to modify the borrow() and borrow_mut() functions at all, as the .expect() will include the information in the panic message. And it'll make try_borrow().unwrap() (etc.) also show the information.

[Aaron1011]

@m-ou-se I've addressed your comments. I've only modified the Debug impls of BorrowError/BorrowMutError - I've left the Display impls alone for now.

[Aaron1011]

The assertion failure is due to the fact that I've enabled this by default to make testing easier. I'll disable the feature by default before this is merged.

[m-ou-se]

This looks good!

r=me with the feature disabled by default.

[Aaron1011]

@bors r=m-ou-se

[bors]

📌 Commit a23273e has been approved by m-ou-se

[bors]

⌛ Testing commit a23273e with merge 9b6339e...

[bors]

☀️ Test successful - checks-actions
Approved by: m-ou-se
Pushing 9b6339e to master...