Reset location after block yielding in macro expansion

[makenowjust]

This pull request introduces #<loc:save> and #<loc:reset> new pragma comment, #<loc:save> saves current location and #<loc:reset> resets it. They are useful to keep original location when block yielding in macro expansion.

For example, this code makes broken error message:

macro foo
  a = {{ yield }}
  a.ok
end

foo { 1 }

It cannot be compiled and error message is:

in foo.cr:7: undefined method 'ok' for Int32

Error message points line 7 of foo.cr, but this code consists only 6 lines.

This bug comes from a location is not reset after block yielding in macro expansion. In fact, foo { 1 } expands to:

a =  begin #<loc:"foo.cr",6,7>1 end
a.ok

So, the parser reports a.ok is on line 7 of foo.cr.

This pull request also fixes it by wrapping #<loc:save> and #<loc:reeset> to yielding expanded code, such a like:

a =  #<loc:save>begin #<loc:"foo.cr",6,7>1 end#<loc:reset>
a.ok

It works well.

[asterite]

Super interesting! I actually had this idea too and eventually was going to implement it, though I imagined something more like a stack (push/pop). What do you think if we change it to #<loc:push> and #<loc:pop> and use a stack in the lexer? (I'd use a record to store all the info you are remembering now)

[makenowjust]

@asterite I'll try to implement #<loc:push> and #<loc:pop>. But, we cannot use record to remember old location. Because it is not immutable, it should be changed by lexer to report correct location.

[asterite]

Hmm... then I misunderstood something. I thought loc:save just saved the current location, then loc:reset reseted it. But it seems when you do loc:save then some tracking is done in many places... why is this needed?

[makenowjust]

For example, when given block is multiple lines:

macro foo
  a = {{ yield }}
  a.ok
end

foo do
  :foo
  :bar
end

It is expands to:

  a =  #<loc:save>begin #<loc:"foo.cr",7,3>:foo
#<loc:"foo.cr",8,3>:bar
end#<loc:reset>
a.ok

If #<loc:save> makes just copied and copied location's line number is not incremented, its location is wrong on #<loc:reset>.

[asterite]

Oooh... I understand now. Yes, the lexer needs to continue incrementing lines for the old line number. I think like you implemented it it's fine :-)

[makenowjust]

Thanks. I already implemented #<loc:push> and #<loc:pop> version, and now I run make spec on my environment. I'll write specs for #<loc:push> and #<loc:pop> (it is missing..), then I'll push it.

[asterite]

@makenowjust Great! Though now I'm not sure there's a use case for push/pop. I thought it was going to be useful to implement/fix this, but I'm not sure anymore. Can you think of a use case?

[makenowjust]

@asterite No, so I didn't implement location stack at first. However, I think #<loc:save> and #<loc:reset> is not good naming. Perhaps #<loc:push> and #<loc:pop> is better even if stack size is 1 only.

[asterite]

@makenowjust Thank you! And thank you for extracting a method for incrementing line numbers. I was actually going to ask if you wanted to do that, so it seems you somehow read my mind... :-P