[SR-10286] Swift 5 Metadata documentation is out of date

[swift-ci]

Previous ID	SR-10286
Radar	None
Original Reporter	plinth666 (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, Documentation, Runtime
Assignee	None
Priority	Medium

md5: dab758796316c8ab4c094a6309ade70a

Issue Description:

The documentation on the type metadata layout for Swift 5 needs to be updated.

If I run the following code:

func printFromMemory (_ ty: Any.Type) {

{{ let p = unsafeBitCast(ty, to: UnsafePointer<Int>.self)}}

{{ let s = String (p.pointee, radix: 16)}}

{{ print (s)}}

{{}}}

public enum OnIt {

{{ case ItsOff, ItsOn}}

{{}}}

public struct Foo {

{{ public var x: Int}}

{{}}}{{ }}

public class Bar {

{{ public init () { }}}

{{}}}{{ }}

public protocol Proto {

{{ func nothing()}}

{{}}}{{ }}

print ("class")

printFromMemory (Bar.self)

print ("struct")

printFromMemory (Foo.self)

print ("optional")

printFromMemory(Int?.self)

print ("enum")

printFromMemory (OnIt.self)

print ("tuple")

printFromMemory(type(of🙁3, false)))

print ("function")``printFromMemory((()->()).self)

print ("protocol")``printFromMemory(Proto.self)

I get the following output :

class

1d8001000032c9

struct

200

optional

202

enum

201

tuple

301

function

302

protocol

303

This is clearly not the Kind field as described in the document but appears to have the bits spread out over the bottom 2 bytes? What is the layout now?

[belkadan]

cc @mikeash, @jckarter

[jckarter]

In the meantime, until we update the docs, the definitive source of the layouts is the runtime headers in include/swift/ABI. For a higher-level example of what's available, you can look at the swift-reflection-dump tool's implementation.