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

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On Saturday, 12 January 2019 at 15:34:02 UTC, Seb wrote:
> 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!

Thank you so much for working on this. There's nothing more 
infuriating than my program crashing due to a failed assert, then 
having to edit and recompile it just to put in some debug 
printing. It's not a big problem when I take the time to do 
things properly and put the relevant information into the assert 
message, but when I'm quickly hacking on some code it's annoying 
to do it for every single assert (I generally use asserts and DBC 
religiously to ensure preconditions and assumptions actually 
hold).

This is one of those little things that is so small but is also 
extremely annoying, so thank you again for taking the time to fix 
it.