[SR-7396] [Exclusivity] SILGen loads entire struct when reading captured 'let' stored property

[devincoughlin]

Previous ID	SR-7396
Radar	rdar://problem/35561050
Original Reporter	@devincoughlin
Type	Bug

Additional Detail from JIRA

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

md5: 7e5c282beaaefdc99de4b06445d4bbb6

Issue Description:

When SILGen reads a captured ‘let’ stored property from a struct it loads the entire struct and then extracts the property value from the struct.

This causes unexpected exclusivity violations because to the programmer it looks like they are loading just the stored property.

Here is an example inspired by the Foundation overlay's Data Iterator:

func takesInoutAndClosure(_ p1: inout Int, _ p2: () -> ()) { }

public struct Iterator {
{{ private let _data: Int}}
{{ private var _buffer: Int}}

{{ public mutating func next() {}}
{{ takesInoutAndClosure(&_buffer) { // error: overlapping accesses to 'self._buffer'}}
{{ let _ = _data; // conflicting access (to all of ‘self’)}}
{{ }}}
{{ }}}
{{}}}

The closure in next() gets SILGen'd to:

// closure #​1 in Iterator.next()``sil private @$S1t8IteratorV4nextyyFyyXEfU_ : $@convention(thin) (@inout_aliasable Iterator) -> () {``// %0 // users: %2, %1``bb0(%0 : $*Iterator):{{ debug_value_addr %0 : $*Iterator, var, name "self", argno 1 // id: %1}}{{ %2 = begin_access [read] [unknown] %0 : $*Iterator // users: %4, %3}}{{ %3 = load [trivial] %2 : $*Iterator // user: %5}}{{ end_access %2 : $*Iterator // id: %4}}{{ %5 = struct_extract %3 : $Iterator, #Iterator._data}}{{ %6 = tuple () // user: %7}}{{ return %6 : $() // id: %7}}{{}}}

From John in an email:
> I think we could pretty easily change SILGen to only load the specific stored property here.

To prevent these exclusivity violations, the SILGen should instead look something like:

bb0(%0 : $*Iterator):
{{ %2 = begin_access [read] [unknown] %0 : $*Iterator}}
{{ %3 = struct_element_addr %6 : $*Iterator, #Iterator._data}}
{{ %4 = load [trivial] %23: $*Int}}
{{ end_access %2 : $*Iterator}}
{{ %6 = tuple ()}}
{{ return %6 : $()}}

In the Foundation overlay I've changed the let-bound variables in Data.Iterator to be 'var' to avoid the violation with #15823 We should revert that when this issue is fixed.