[Sema] Diagnose access to the underlying storage of a lazy variable

[theblixguy]

Since #21996, the name of the underlying storage variable of a lazy var has been changed to $__lazy_storage_$_{property_name}. You couldn't actually write an identifier in (Swift) source beginning with an $ before (except anonymous closure params like $0 and etc), but since the introduction of property wrappers, you can do that and this has inadvertently allowed access to the lazy var storage and made it possible to "reset" it.

So, detect use of $__lazy_storage_$_{property_name} identifier in (Swift) source and emit a tailored error diagnostic.

[harlanhaskins]

Good catch, thank you!

[rintaro]

Not 100% sure we should diagnose this in Lexer/Parser. What if a user actually declares __lazy_storage_$_foo?

@propertyWrapper
struct Wrapper {
  var wrappedValue: Int { 1 }
  var projectedValue: Int { 1 }
}

struct MyStruct {
  @Wrapper var __lazy_storage_$_foo
  func test() {
    _ = $__lazy_storage_$_foo  // Should be OK.
  }
}

I know It's fairly unrealistic, but still.

BTW, I noticed this crashes the compiler:

@propertyWrapper
struct Wrapper {
  var wrappedValue: Int { 1 }
  var projectedValue: Int { 1 }
}

struct MyStruct {
  @Wrapper var __lazy_storage_$_foo
  lazy var foo
}

[theblixguy]

Hmm, that's interesting. I think one solution might be to do this in Sema instead. We have isLazyStorageProperty() so we could use this to disallow access to the variable. I think we can do this in MiscDiagnostics.cpp.

BTW, I noticed this crashes the compiler:

It doesn't crash for me on master (I am on Swift version 5.3-dev (LLVM f8bd914aadc2e7b, Swift 69f543e908a2434)):

/Users/spare/Desktop/test.swift:9:3: error: lazy properties must have an initializer
  lazy var foo
  ^
/Users/spare/Desktop/test.swift:9:12: error: type annotation missing in pattern
  lazy var foo
           ^

but I'll update again and re-check.

[rintaro]

Ah, sorry. The crasher was missing the initializer.

@propertyWrapper
struct Wrapper {
  var wrappedValue: Int { 1 }
  var projectedValue: Int { 1 }
}

struct MyStruct {
  @Wrapper var __lazy_storage_$_foo
  lazy var foo = 1
}

[theblixguy]

Oh yeah, that reproduces for me! I previously some work in this area (#31915) so I'll take a look at it soon.

[theblixguy]

I have re-implemented this in Sema (in MiscDiagnostics). Does this approach seem okay?

[theblixguy]

@rintaro I have fixed the crash you reported in #33152 :)

[theblixguy]

Thanks!

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test