{.compileTime.} tuple destructuring - crash on 0.20, bad codegen on 0.19

[mratsim]

I'm encountering a couple of issues when experimenting with a deep learning compiler proof of concept in Nim.

Unfortunately the repro code is about 1000 lines but for my experiments purposes everything his kept in a single file so is reproducible with a simple nim c foo.nim.
Edit: Found a minimal repro code, see my 3rd comment

The following code works (e11), the only caveat is the API I highlighted which uses global {.compileTime.} variables.

So in another experiment (e12) I kept the backend procs but added a couple of procs to try another API, the extra generate macro generates a block with compile-time function evaluation that should be equivalent to e11.

# Output of generate in e12 according to `toStrLit`
block generated_foobar_286080:
  let a {.compileTime.} = input(0)
  let b {.compileTime.} = input(1)
  let c {.compileTime.} = input(2)
  let (bar {.compileTime.}, baz {.compileTime.}, buzz {.compileTime.}) = static(
      foobar(a, b, c))

The strange thing is that:

• On 0.20.x the compiler crashes with SIGSEGV after toStrLit
• On 0.19.6 compilation is successful but running the binary crashes at runtime with

Traceback (most recent call last)
e12_syntax_revamp.nim(1070) e12_syntax_revamp
gc.nim(186)              asgnRefNoCycle
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

The C code also includes the code for my AstNode variant type even though it is supposed to only exist at compile-time.

[mratsim]

Here is the compiler stacktrace on current devel

Hint: tables [Processing]
block generated_foobar_341131:
  let a {.compileTime.} = input(0)
  let b {.compileTime.} = input(1)
  let c {.compileTime.} = input(2)
  let (bar {.compileTime.}, baz {.compileTime.}, buzz {.compileTime.}) = static(
      foobar(a, b, c))
Traceback (most recent call last)
/home/beta/Programming/Nim/Nim/compiler/nim.nim(98) nim
/home/beta/Programming/Nim/Nim/compiler/nim.nim(75) handleCmdLine
/home/beta/Programming/Nim/Nim/compiler/cmdlinehelper.nim(92) loadConfigsAndRunMainCommand
/home/beta/Programming/Nim/Nim/compiler/main.nim(194) mainCommand
/home/beta/Programming/Nim/Nim/compiler/main.nim(90) commandCompileToC
/home/beta/Programming/Nim/Nim/compiler/modules.nim(147) compileProject
/home/beta/Programming/Nim/Nim/compiler/modules.nim(88) compileModule
/home/beta/Programming/Nim/Nim/compiler/passes.nim(197) processModule
/home/beta/Programming/Nim/Nim/compiler/passes.nim(86) processTopLevelStmt
/home/beta/Programming/Nim/Nim/compiler/sem.nim(603) myProcess
/home/beta/Programming/Nim/Nim/compiler/sem.nim(571) semStmtAndGenerateGenerics
/home/beta/Programming/Nim/Nim/compiler/semstmts.nim(2184) semStmt
/home/beta/Programming/Nim/Nim/compiler/semexprs.nim(980) semExprNoType
/home/beta/Programming/Nim/Nim/compiler/semexprs.nim(2712) semExpr
/home/beta/Programming/Nim/Nim/compiler/semstmts.nim(2124) semStmtList
/home/beta/Programming/Nim/Nim/compiler/semexprs.nim(2609) semExpr
/home/beta/Programming/Nim/Nim/compiler/semexprs.nim(962) semDirectOp
/home/beta/Programming/Nim/Nim/compiler/semexprs.nim(854) afterCallActions
/home/beta/Programming/Nim/Nim/compiler/sem.nim(468) semMacroExpr
/home/beta/Programming/Nim/Nim/compiler/sem.nim(411) semAfterMacroCall
/home/beta/Programming/Nim/Nim/compiler/semexprs.nim(2711) semExpr
/home/beta/Programming/Nim/Nim/compiler/semexprs.nim(2431) semBlock
/home/beta/Programming/Nim/Nim/compiler/semexprs.nim(2712) semExpr
/home/beta/Programming/Nim/Nim/compiler/semstmts.nim(2124) semStmtList
/home/beta/Programming/Nim/Nim/compiler/semexprs.nim(2715) semExpr
/home/beta/Programming/Nim/Nim/compiler/semstmts.nim(576) semVarOrLet
/home/beta/Programming/Nim/Nim/compiler/vm.nim(2022) setupCompileTimeVar
/home/beta/Programming/Nim/Nim/compiler/vm.nim(1996) evalConstExprAux
/home/beta/Programming/Nim/Nim/compiler/transf.nim(1159) transformExpr
/home/beta/Programming/Nim/Nim/compiler/transf.nim(1057) processTransf
/home/beta/Programming/Nim/Nim/compiler/transf.nim(1009) transform
/home/beta/Programming/Nim/Nim/compiler/transf.nim(123) transformSons
/home/beta/Programming/Nim/Nim/compiler/transf.nim(1037) transform
/home/beta/Programming/Nim/Nim/compiler/transf.nim(123) transformSons
/home/beta/Programming/Nim/Nim/compiler/transf.nim(909) transform
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
FAILURE

[mratsim]

Minimal repro code shows that it's the tuple destructuring of {.compileTime.} variables that causes the issues:

• bad codegen (undeclared idents) on 0.19.6 (I don't have the asgnRefNoCycle with this minimal code)
• crash on 0.20.x

type Foo = ref object
  val: int

proc divmod(a, b: Foo): (Foo, Foo) =
  (
    Foo(val: a.val div b.val),
    Foo(val: a.val mod b.val)
  )

block:
  let a {.compileTime.} = Foo(val: 2)
  let b {.compileTime.} = Foo(val: 3)
  let (c {.compileTime.}, d {.compileTime.}) = divmod(a, b)

[mratsim]

Tagging low-priority as we can always assign and then destructure one by one as a workaround.

let cd {.compileTime.} = divmod(a, b)
let c = cd.c
let d = cd.d

[deech]

Once I'm done with my PR mixing {.compileTime.} into tuple destructuring will be disallowed, semVarOrLet is a mess.

[wltsmrz]

BTW, not sure if it's exactly the same error but I'm running into compileTime tuple unpacking error

Error: illformed AST: (a, b) = gorgeEx("make -C " & dep_path & " " & "lib")

Allow or disallow, the important thing is that a programmer can determine "Tuple unpacking is not allowed in compile time context" from error message IMO