vm: variable declaration code in AssignStmt and ValueDecl should be synced (issues with map index / type assertion tuple assignment)

[thehowl]

The Go spec states:

A variable declaration creates one or more variables, binds corresponding identifiers to them, and gives each a type and an initial value. [...]

A short variable declaration [...] is shorthand for a regular variable declaration with initializer expressions but no types:

"var" IdentifierList "=" ExpressionList .

There is one notable exception, which is that the shorthand := allows for "redeclarations" on the LHS, so long as at least one name isn't being redeclared. The other difference is that the var syntax can be used to declare global variables, while := can't.

Currently, the GnoVM has two very different handling of these two statements, which according to the go spec should work the same (at least for their use in function bodies):

gno/gnovm/pkg/gnolang/preprocess.go

Line 1538 in 5a976ef

if n.Op == DEFINE {

gno/gnovm/pkg/gnolang/preprocess.go

Line 1804 in 5a976ef

case *ValueDecl:

The changes don't concern only this TRANS_LEAVE block, though it is one where the difference is most significant.

The practical consequence is that there are key differences in behaviour when using the var syntax or the := syntax; for instance, "tuple" map indexes and type assertions are not supported in the var syntax:

package hello

func main() {
  m := map[string]string{"hey": "1"}
  // a, ok := m["hey"]
  var a, ok = m["hey"]
  println(a, ok)
}

https://play.gno.land/p/wvIG65jLGmT

package hello

func main() {
  v := (interface{})(11)
  // i, ok := v.(int)
  var i, ok = v.(int)
  println(i, ok)
}

https://play.gno.land/p/FMC5fHKjlgg

You can easily note in the playground how, switching the statements to the corresponding := statements, the statement will work again.

To avoid further bugs, we should try to unify as much as possible the code for preprocessing both expressions, so that there is no surprising behaviour at runtime for two variables declared with the different syntaxes

[hthieu1110]

Just a first comment: seems that "tuple" map indexes and type assertions are now supported in the var syntax, I don't have any error when executing the code mentioned above.

I'm working on refactoring code.

[mvertes]

Fixed by #3017