cargo-rr 0.1.2

A light wrapper around rr, the time-travelling debugger
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# cargo-rr

[![Crates.io](https://img.shields.io/crates/v/cargo-rr)][crates-io]
![MIT Licensed](https://img.shields.io/crates/l/cargo-rr)

A light wrapper around rr, the time-travelling debugger.

> Do you find yourself running the same test over and over in the debugger,
> trying to figure out how the code got in a bad state? We have a tool for you!
> Easy to install and setup, it will record an execution trace, and that gives
> magical new powers to gdb. Step backwards, run backwards, see where variables
> changed their value or when a function was last called on an object (using
> conditional breakpoints). [(source)][about-quote-source]

Suppose we ran a test and got a failure. We first re-run the test under `rr` to
record the entire execution (including everything else on your system the test
interacts with).

```bash
> cargo rr test my_test

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `2`', tests/tests.rs:100

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 2 filtered out; finished in 0.06s
```

Let's replay the recording.

```bash
> cargo rr replay

(rr) continue

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `42`', tests/tests.rs:100

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 2 filtered out; finished in 0.06s
```

We'll go backwards until we return to the failed assertion so we can have a look at `a` and `b`.

```bash
(rr) break tests/tests.rs:100
(rr) reverse-continue
Continuing.

Breakpoint 1, tests::main () at tests/tests.rs:100
100         assert_eq!(a, b);

(rr) print a
$1 = 1

(rr) print b
$2 = 42
```

I wonder where `a` is set to `1`?

```bash
(rr) watch -l a
(rr) reverse-continue

Old value = 1
New value = -1992923200
0x000055dcac485ed9 in tests::main () at tests/tests.rs:30
30          let a = some_calculation();

```

Note that since we're going in reverse the old and new values are backwards.
`-1992923200` is whatever used to be at the address `0x000055dcac485ed9`
before it was used for `a`.

[about-quote-source]: https://developer.chrome.com/blog/chromium-chronicle-13/
[crates-io]: https://crates.io/crates/cargo-rr