Issue 8765, 9255 - Show informative assert errors

[wilzbach]

Rebased #5189 (orginates from: #1203 via #1426)

[dlang-bot]

Thanks for your pull request, @wilzbach!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	8765	normal	assert should print the source code for the condition when no message argument present

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#7575"

[PetarKirov]

@nomad-software it looks like your testing library needs to be updated to not rely on the default assert error message.

[ibuclaw]

Could this be inlined into its one and only caller?

[ibuclaw]

Right, this should be done before semantic, but is using toChars correct here? What happens with multiline expressions?

[JinShil]

Could exp.copy() be used instead? But ultimately you're going to want to print the expression.

[timotheecour]

isn't exp.copy() shallow, so that the copy will still be affected by semantic analysis?

[JinShil]

Issue 8765 - assert should print the source code for the condition when no message argument present

I side with @WalterBright and his comments in the Bugzilla issue. I think if the developer chose to leave off the assert message, then that was their deliberate choice, and I don't think the compiler should override that choice. If we want to print the expression by default, then I recommend introducing an assert overload or a different assertXXX function.

Furthermore, this does have the potential to be a disruptive change for test suites that analyze assert outputs.

I vote to close Issue 8765 with won't fix, and instead encourage an alternate solution as I mentioned above.

UPDATE: After reading through the comments of the previous PRs that attempted to address this issue, I believe that my opinion above will likely be very unpopular. If the community at large finds this enhancement useful, I will concede.

[timotheecour]

• an assert(__FILE__.isAbsolute) failure will print the expanded version of FILE, eg:
  "foo/main.d".isAbsolute failed instead of __FILE__.isAbsolute failed
  Likewise with other special tokens.
• Also, a problem with this is the original formatting of source code is lost, which may be undesirable (it gets pretty-printed)

#7831 solves a more general problem via ARGS_STRING; assert could either be implemented in terms of ARGS_STRING or reuse code from that PR in functionParameters modulo a small refactoring to extract out this block of code:

auto temp = sc._module.srcfile;
...
argname = txt[0..end];

[wilzbach]

#8517 supersedes this.