My late christmas present for you: context-aware assertion error messages

[Seb]

tl;dr: I was annoyed for years that D's assert are so non 
informative AssertError, so here's my late Christmas present for 
the D community. With 2.085 DMD will gain an experimental flag 
for more informative assertion errors. Feedback on this 
experimental feature is welcome, s.t. we eventually can enable it 
by default.

Example:

---
void main()
{
     int a = 1, b = 2;
     assert(a == b); // ERROR: 1 != 2
}
---

```
> dmd -run onlineapp.d
core.exception.AssertError at onlineapp.d(4): 1 != 2
----------------
??:? _d_assert_msg [0xb150e350]
??:? _Dmain [0xb150d627]
```

Play online: https://run.dlang.io/is/GMfe9S
Full changelog: https://dlang.org/changelog/pending.html#assert


Where to start hacking:
- 
https://github.com/dlang/dmd/blob/00299e3b6ca9dcd5a5dc8bd50280b63d0bdc51f9/src/dmd/expressionsem.d#L5559
- 
https://github.com/dlang/druntime/blob/master/src/core/internal/dassert.d
- https://github.com/dlang/dmd/pull/8517

Q: Why not just introduce an `assertEqual` user function?
A:
- there's a lot of already written D code out there that uses 
`assert` [1]
- `assertEqual` wouldn't cut, it because then we would need to 
have an `assertLessThan` etc. too

Q: Why put this in the compiler and not in fluent-assert or 
similar library?
A:
- there's a lot of already written D code out there that uses 
`assert` [1]
- not everyone wants to use a library just to have somewhat 
decent assert messages
- libraries can only do sth. like `testedValue.should.equal(42)`, 
but the compiler can deal with `testedValue == 42`

[1] https://github.com/search?l=D&q=assert&type=Code

Error context
-------------

Also note that with 2.085 DMD will get -verrors=context


---
void foo()
{
     a = 1;
}
---


```
> dmd -verrors=context onlineapp.d
onlineapp.d(3): Error: undefined identifier a
     a = 1;
     ^
```

Full changelog: 
https://dlang.org/changelog/pending.html#error-context
Play online: https://run.dlang.io/is/8gsye1

Thanks to all the people (Jacob Carlborg, Petar Kirov, Nicolas 
Wilson, Rainer Schuetze, etc.) who helped me to get this feature 
into DMD!

Merry Christmas!