Go's := syntax is error-prone with multiple targets.md

Problem

The := syntax is error-prone when dealing with multiple targets and variables of enclosing scope:

// often a mistake:
var a int
{
  a, b := foo()    // creates a new 'a' in this scope instead of using 'a' of outer scope
}

// the fix
var a int
{
  var b *Bar
  a, b = foo()
}

Not only is the fix verbose, it defeats a nicety of inferred typing that it often spares us from having to remember precise return type(s), e.g. whether a returned type is a pointer or not:

a := bar()       // whether bar() returns an *X or just an X, we often don't need to care

Normally, Go disallows redeclarations, but it makes a hacky exception for the := syntax, such that we can't look at an := assignment and know for sure which of the target variables are being declared:

x, y, z := foo()    // only one of x, y, and z must be new, but just looking here doesn't tell us which variables are new

Another problem is that we can't mix 'name' targets and 'non-name' targets:

a, b[0] := foo()          // compile error: b[0] is a non-name target

Proposal

Instead of marking an assignment itself as an implicit declaration, mark the individual target variables. Let's say for now we'll use ' as a suffix marker:

x', y, z' = foo()         // assign to new variable 'x', existing variable 'y', and new variable 'z'

With this syntax, I don't see a good reason to disallow mixing in non-name targets:

a', b[0] = foo()          // ok

It would also be nice if we could optionally specify types of one or more of the newly declared target variables to make them different from the corresponding return type:

a' Fruit, b = bar()       // first return type of bar() is Banana, but we store it in a Fruit interface variable

Lastly, while it's great that Go requires explicit casts in most cases, it would sometimes be nice to cast the individual return values in a multi-assignment. In fact, it would sometimes be nice if we could perform any arbitrary operation on each return value. A solution would be to allow an expression after each target where _ stands in for the return value:

// the first return value is cast to int64 and the result assigned to new variable 'a'
// the second return value is added to 3 and the result assigned to existing variable 'b'
a' int64(_), b _ + 3 = foo()

If arbitrary expressions in these positions make the code too cluttered, perhaps restrict the expressions to being just single function/method calls.

Type-independent zero value literal

Another small thing I'd like in Go is a literal that stands-in for any zero value. Let's say # is this special literal:

var f Foo     // struct type Foo
f = #         // f = Foo{}

This literal would be particularly handy when returning zero values along with a non-nil error. Instead of having to write specific zero values...

return SomeStruct{}, "", err

...we could just write:

return #, #, err

@alejoloaiza Yeah, I'm not proposing getting rid of any syntax, just adding new stuff. I think these additions are compatible with the current grammar, but if not, I'm not married to these particular symbols.