map shouldn't care whether proc arg is cdecl

[timotheecour]

reduced from a mysterious error I was seeing here https://travis-ci.org/nim-lang/Nim/jobs/402492005 for this PR: #8272

The code that caused it:

result = args.map(quoteShell).join(" ")

it worked if I wrote instead:

# with import sugar
result = args.map(a=>quoteShell(a)).join(" ")

After further investigating and reducing the error it boils down to:
map somehow can't be used with a proc that's marked as {.rtl.} (or, after reducing further from the definition of rtl in system/inclrtl, if it's marked as {. exportc: "nimrtl_$1", dynlib .})

#[ 
Error: type mismatch: got <openarray[string], proc (s: string): string{.cdecl, noSideEffect, gcsafe, locks: 0.}>
but expected one of:
proc map[T](s: var openArray[T]; op: proc (x: var T))
  for a 'var' type a variable needs to be passed, but 'args' is immutable
proc map[T, S](s: openArray[T]; op: proc (x: T): S): seq[S]
  first type mismatch at position: 2
  required type: proc (x: T): S{.closure.}
  but expression 'fun' is of type: proc (s: string): string{.cdecl, noSideEffect, gcsafe, locks: 0.}
  for a 'var' type a variable needs to be passed, but 'args' is immutable

expression: map(args, fun)
    result = args.map(fun).join(" ")
 ]#

include "system/inclrtl"

import sequtils

# This would also fail: 
# proc fun*(s: string): string {. exportc: "nimrtl_$1", dynlib .} = s
proc fun*(s: string): string {. rtl .} = s

proc funWrapper*(s: string): string = fun(s)

# OK
echo ["1"].map(funWrapper)
# could also be defined inline: echo ["1"].map(proc(s:string):auto=fun(s))

# Error: type mismatch:
echo ["1"].map(fun)

Isn't that a bug?

it makes things work locally but fail on travis/appveyor.
(or fail locally when run with nim c -o:app --define:createNimRtl --app:lib test.nim instead of nim c -o:app --app:lib test.nim (when something uses {.rtl.}))

(or fail when using nim c -o:app bugs/t50_app_lib_map.nim when something uses {. exportc: "nimrtl_$1", dynlib .} )

[EDIT] workaround in some cases see https://github.com/nim-lang/Nim/issue_comments#issuecomment-411508903

[timotheecour]

so the root problem is that --define:createNimRtl --app:lib implies {.rtl.} implies {. exportc: "nimrtl_$1", dynlib .} implies {.cdecl.}
and that's incompatible with map's 2nd argument op in lib/pure/collections/sequtils.nim:

proc map*[T, S](s: openArray[T], op: proc (x: T): S ): seq[S] =
  newSeq(result, s.len)
  for i in 0 ..< s.len:
    result[i] = op(s[i])

However this works if we use instead:

proc map3*[Fun, T](s: openArray[T], op: Fun ): auto =
  result = newSeq[type(op(s[0]))](s.len)
  for i in 0 ..< s.len:
    result[i] = op(s[i])

[timotheecour]

workaround in some cases: use mapIt(fun(it)) instead of map(fun) (now that #8543 was merged)

EDIT => blocked by another issue: #8577

[timotheecour]

@Araq can sigmatch ignore {.cdecl.}, assuming it's only used for name mangling of a declaration?

[Araq]

Oh no, .cdecl is really a different calling convention (a slower one on 32 bit Windows) and has little to do with mangling.

[timotheecour]

I was misled by this comment:

    # since we'll be loading the dynlib symbols dynamically, we must use
    # a calling convention that doesn't introduce custom name mangling
    # cdecl is the default - the user can override this explicitly

but the manual indeed says:

decl
The cdecl convention means that a procedure shall use the same convention as the C compiler. Under Windows the generated C procedure is declared with the __cdecl keyword.