Default compiler flags (or cli options)

[bcardiff]

I would like to be able to run the compiler specs against some specific flags.
This would help to test some opt-in features behind flags (overflow, mt, etc).

1. I would like to agree on the scope:
   1.a) This change could be either be scoped only the compiler_specs (changing https://github.com/crystal-lang/crystal/blob/0.28.0/spec/spec_helper.cr#L160) or,
   1.b) to the whole compiler (changing https://github.com/crystal-lang/crystal/blob/0.28.0/src/compiler/crystal/compiler.cr#L198).

2. I would like to agree on the interface
   2.a) Use an env variable like CRYSTAL_COMPILER_FLAGS that will contain a space separated list of flags to be used.
   2.b) Use an env variable to mimic any CLI option of the compiler via an env variable like CRYSTAL_COMPILER_OPTS. The implementation will need to parse them (instead of just split spaces).

I would be happy with 1.b + 2.a.

[asterite]

Does this need to be in the compiler or in the spec suite? You can probably do bin/crystal spec spec/compiler_spec.cr -Dflag1 -Dflag2, right? And that can be simplified/manipulated in a Makefile, I think.

[bcardiff]

I want to inject the flags in the Crystal::Compiler instances that are created in the compiler_specs. Your snippet will affect the compiled suite, but not each test.

[asterite]

Oooh... I see.