block return expressions should behave like return statement

[timotheecour]

links

• D20201125T155521
• fix #14339, #13511, #14420: fixes limited VM support for addr nim-lang/Nim#16002 (comment)
• Doc Comment in Template Causes Compilation Failure nim-lang/Nim#13511 (comment)
• VM and js backend require result for lent or var annotations to work · Issue #14420 · nim-lang/Nim
  (was fixed but the point remains)
• Error: ignored invalid for loop for it triggered when changing a # to a ## nim-lang/Nim#9230
• Q: treat result as a normal variable? nim-lang/compilerdev#14

TODO

• check if codegen differs

scratch

proc genImplicitReturn(c: var Con) =
  if c.owner.kind in {skProc, skFunc, skMethod, skIterator, skConverter} and resultPos < c.owner.ast.len:
    gen(c, c.owner.ast[resultPos])

proc genReturn(c: var Con; n: PNode) =
  if n[0].kind != nkEmpty:
    gen(c, n[0])
  else:
    genImplicitReturn(c)
  genBreakOrRaiseAux(c, 0, n)

  of nkBlockStmt, nkBlockExpr:
    result = copyNode(n)
    result.add n[0]
    var bodyScope = nestedScope(s)
    result.add if n[1].typ.isEmptyType or willProduceStmt:
                 processScope(c, bodyScope, processCall(n[1], bodyScope))
               else:
                 processScopeExpr(c, bodyScope, n[1], processCall)

proc transformBlock(c: PTransf, n: PNode): PNode =
  var labl: PSym
  if c.inlining > 0:
    labl = newLabel(c, n[0])
    idNodeTablePut(c.transCon.mapping, n[0].sym, newSymNode(labl))
  else:
    labl =
      if n[0].kind != nkEmpty:
        n[0].sym  # already named block? -> Push symbol on the stack
      else:
        newLabel(c, n)
  c.breakSyms.add(labl)
  result = transformSons(c, n)
  discard c.breakSyms.pop
  result[0] = newSymNode(labl)