Comment by Brian (JIRA)

I just made a PR for SR-2115 which reported that unused warnings in top level statements was not working. In there I disabled top level variable warning because walking for all global variable usage was a bit more complicated. I would like to fix this too, but there were a number of red flags that made it seem worth talking through my approach to see what scenarios I'm missing.

• Split the diagnostics from `VarDeclUsageChecker` into a `VarDeclScopeDiagnostic` class, and have the VDUC use a VDSD instead. Move the reporting mechanism from the destructor to a method.

• Add a `VarDeclScopeDiagnostic` to `ASTContext`. `VarDeclUsageChecker` will fall back to this scope if the variable isn't defined in the current scope.

• Add some scope intelligence to VDSD where anything marked `public` would be considered R|W. Add this as a fixit to global variables.

• Trigger the module-wide diagnostics after all files are walked and type checked correctly.

Comment by Brian (JIRA)

I wanted two mention another issue with the `VarDeclUsageChecker`.

func outerFunction() {
  var noWarning: String? = "due to innerFunction not being typechecked"
  func innerFunction() {
    var warningGenerated: String? = ""
  }
}

There's another bug I was looking at that I can't find, where only the active condition in a `#if` statement would be checked, and the inactive blocks mark any decl ref as read. I think if an inactive block was type checked and didn't have any errors, the checker could be less conservative. Not positive this is important enough though.

Would (and should) implementing this allow unused globals to be optimized out? Currently, even under Whole Module Optimization, _main will initialize global let constants that are never used. For example, the following code:

let x = 1
func main() {}

Even with -O -wmo, this generates:

_main:
00000001000015a0        pushq   %rbp
00000001000015a1        movq    %rsp, %rbp
00000001000015a4        movq    $0x1, 0x3021(%rip)
00000001000015af        xorl    %eax, %eax
00000001000015b1        popq    %rbp
00000001000015b2        retq

The movq $0x1 should be possible to optimize away.