Omnibus: Known low-pri remaining bugs in Extract Method

[RyanCavanaugh]

• Detect and disallow stranding of type parameters
• Disallow extraction of LHS of assignmentlike exprs (e.g. do not allow extracting x in x += 3 or x = 10)
• Need to sometimes insert whitespace before the substituted function call (e.g. when extracting [] from return[])
• Disallow extracting the empty statement
• Figure out what to do with decorators 😞
• Allow extraction of code in class methods into local functions if that code doesn't reference this
• Disallow extracting into a class scope if this doesn't bind to an instance of the class at the call site
• getEditsForAction can, and probably should, return the location of a Rename session to kick off
• Trailing trivia on an expression is duplicated, appearing both in the new function and at the call site
• If the leading trivia on an expression contains a newline, it will be inserted after return in the extracted method, implicitly creating an empty return statement followed by an unreachable expression statement

All credit @amcasey

[RyanCavanaugh]

Good luck @amcasey ✋

[amcasey]

Potential issue: variables that are only assigned (mutations are different) should not be passed in as parameters.

[amcasey]

Potential improvement: can we use destructuring to expose declarations in the extracted range (assuming there are no writes)?

[amcasey]

FYI @billti

[RyanCavanaugh]

Improvement: Allow extraction of a span containing a variable declaration if we can initialize the declarations:

var x = 10 + 10;
~~~~~~~~~~~~~~~

return x;

should be come

var x = newFunction();
return x;
...
function newFunction() {
  return 10 + 10;
}

[RyanCavanaugh]

Improvement: same as above, but for multiple variables (using destructuring)

[amcasey]

Example issue from the checklist above. Try extracting 2 from

// a
var q = /*b*/ //c
    /*d*/ 2 /*e*/ //f
    /*g*/; /*h*/ //i

Edit: covered by #18626.