[SR-8546] Enforce @escaping for for all but the most trivial uses of captured variables.

[atrick]

Previous ID	SR-8546
Radar	rdar://problem/43355341
Original Reporter	@atrick
Type	Bug
Status	Resolved
Resolution	Done

Attachment: Download

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	@slavapestov
Priority	Medium

md5: 5633bcbc0929418f15d8bc331f63f2ed

relates to:

• SR-9043 Missing checks of inout/mutating captures

Issue Description:

This is a source compatibility bug.

A nested function used in an @escaping context must produce a type
checker error. In the example below, we expect a diagnostic:

error: nested function with an implicitly captured inout parameter can only be used as a non-escaping argument

Instead we get an error in diagnose exclusivity, but only in asserts builds, which is not good enough to protect against undefined behavior.

Applied argument must be @noescape function type: %21 = argument of bb3 : $@callee_guaranteed () -> ()
A partial_apply with @inout_aliasable may only be used as a @noescape function type argument.
UNREACHABLE executed at /s/s/swift/lib/SILOptimizer/Mandatory/DiagnoseStaticExclusivity.cpp:1085!

func doit(_ f: @escaping ()->()) { f() }

public func outerFoo1(x: inout Int) {
 func innerFoo1() {
  x = 1
 }
 func innerFoo2() {
  x = 2
 }
 doit(x == 0 ? innerFoo1 : innerFoo2)
}

[atrick]

@swift-ci create.

[rudkx]

The analysis being done for this at the moment seems like it's just fundamentally broken in that it looks at the direct arguments of the function to see if they are DeclRefs that capture inouts.

Here are two more (similar) examples that currently assert rather than getting flagged by type checking.

doit{ innerFoo1() } 

and

let fn = { innerFoo1() }
doit(fn) 

[atrick]

Here's another obvious type checker failure:

var globalf: ()->() = {}

func testEscapeToGlobal(x: inout Int) {
 globalf = { defer { x = 5 } }
}

These are miscompiles and source compatibility issues. The only reason that we're catching them now is that I added complex verification in the optimizer.

[slavapestov]

#23923