[SR-11992] Check variable usage within closures

[maustinstar]

Summary

Variables that are never used or mutated are not diagnosed within closure bodies that are not at the top level. This is because closure bodies were never checked with VarDeclUsageChecker unless they were at the top level. This pull request adds swift::performClosureBodyDiagnostics which performs the same variable usage check as top level declarations and AFDs.

Examples:

Before this PR:

class Foo {
    lazy var x: Int = {
        var y = 42 // no warning
        var z = 42 // no warning
        return z
    }()
}

After this PR:

class Foo {
    lazy var x: Int = {
        var y = 42 // warning: initialization of variable 'y' was never used; consider replacing with assignment to '_' or removing it
        var z = 42 // warning: variable 'z' was never mutated; consider changing to 'let' constant
        return z
    }()
}

Links

Resolves SR-11992 and SR-13821

Tagging @rintaro.

[slavapestov]

Can we instead change those walkers to skip closure bodies, so that we can check them unconditionally here?

[maustinstar]

The walkers can't skip the closure bodies entirely because they look for uses of a variable within the closure. But, I could try to skip the tracking of any variables declared within the closures (I'm not sure yet how trivial this is, but I will look into it). Then later, walk through all closures and track variable declarations unconditionally.

Thank you for the feedback!

[CodaFi]

You can’t skip them in the walker itself, but you can skip them when you first enter the walker. E.g. instead of walking the entire Top Level Code Decl, you can iterate over its members and skip closures knowing they’ll be checked later.