# Let's debug: missing RBI data race diagnostics

**URL:** <https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910>\
**Category:** Compiler\
**Tags:** concurrency, diagnostics\
**Created:** [March 28, 2025, 5:02pm UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910 "2025-03-28T17:02:04Z")\
**Posts on this page:** 11\
**Page:** 1

<div class="post-metadata">

**Author:** ![jamieQ](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/jamieq/32/5117_2.png) [@jamieQ](https://forums.swift.org/u/jamieQ)\
**Post date:** [March 28, 2025, 5:02pm UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/1 "2025-03-28T17:02:04Z")

</div>

the following is a compilation of notes and thoughts from researching code that should produce RBI diagnostics, but is not currently doing so. it is somewhat long and meandering, but can hopefully serve as documentation for any future efforts to investigate similar problems. i don't claim to have a great mental model of everything going on here, so do feel free to correct any mistakes i've made!

* * *

### Context

i've been looking into a class of issues with region based isolation (RBI) diagnostics that seem to admit code with data races. here's an example that is fairly representative of the problem:

```swift
@MainActor // *
func mutableLocalCaptureDataRace() {
  var x = 0

  Task.detached { x = 1 }

  x = 2
}
// * note: the @MainActor annotation may not actually be necessary to
// reproduce this problem, but it was present during my investigation

```

this currently compiles without a concurrency error or warning in the 6.0 (and 6.1) compilers, under both 5 & 6 language modes.

using the RBI logging\[1\], we can inspect the output of the analysis to get a better sense of what's going on. [here are](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/2) the 'interesting' bits of the output for the `mutableLocalCaptureDataRace` function (note: this excludes the Task closure as it is analyzed independently).

per my current understanding of the relevant code, the initial step of the analysis processes the SILGen instructions to determine the 'trackable' values required for the dataflow phase. additionally, each instruction is translated from SIL into zero or more 'partition operations', depending on the specific instruction and translation semantics. the 'trackable state' collected during this phase also records various pieces of information about elements, like whether it represents a non-sendable value, the isolation region it's in, etc.

here's a portion of the pre-dataflow phase log output where the `partial_apply` instruction for the Task closure is handled:

```swift
// SIL:
  // function_ref closure #1 in mutableLocalCaptureDataRace()
  %11 = function_ref @$s6output27mutableLocalCaptureDataRaceyyFyyYacfU_ : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %14
  %12 = copy_value %1 : ${ var Int } // user: %14
  %13 = enum $Optional<any Actor>, #Optional.none!enumelt // user: %14
  %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16

// RBI:
Visiting: // function_ref closure #1 in mutableLocalCaptureDataRace()
  %11 = function_ref @$s6output27mutableLocalCaptureDataRaceyyFyyYacfU_ : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %14
    Semantics: assign_fresh
Visiting: %12 = copy_value %1 : ${ var Int } // user: %14
    Semantics: look_through
Visiting: %13 = enum $Optional<any Actor>, #Optional.none!enumelt // user: %14
    Semantics: assign_fresh
Visiting: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
    Semantics: special
 ┌─┬─╼ %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
 │ └─╼ line:5:17
 ├─────╼ require %%1: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
 │ └╼ assign %%10 = %%1: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
 └─────╼ Used Values
          └╼ State: %%1. TrackableValueState[id: 1][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
             Rep Value: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
             Type: ${ var Int }
          └╼ State: %%10. TrackableValueState[id: 10][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
             Rep Value: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
             Type: $@isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <()>

```

the logs indicate that handling the isolated `partial_apply` has 'special' translation semantics, and in this case the instruction is translated into both the 'require' and 'assign' partition operations. the 'require' is for the tracked value with id `%%1`, which in this instance is the mutable local variable `x`. IIUC, a 'require' here means that during the dataflow, the tracked value's region will be _required_ to not have been marked as 'sent' when this partition operation is encountered. if that's not the case, a diagnostic should (generally) be produced.

then we have the output from the call to `Task.detached` which looks like:

```swift
//SIL:
  // function_ref static Task<>.detached(priority:operation:)
  %15 = function_ref @$sScTss5NeverORs_rlE8detached8priority9operationScTyxABGScPSg_xyYaYAcntFZ : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %16
  %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19

// RBI:
Visiting: // function_ref static Task<>.detached(priority:operation:)
  %15 = function_ref @$sScTss5NeverORs_rlE8detached8priority9operationScTyxABGScPSg_xyYaYAcntFZ : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %16
    Semantics: assign_fresh
Visiting: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
    Semantics: apply
 ┌─┬─╼ %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
 │ └─╼ line:5:8
 ├─────╼ require %%10: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
 │ └╼ send %%10: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
 └─────╼ Used Values
          └╼ State: %%10. TrackableValueState[id: 10][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
             Rep Value: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
             Type: $@isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <()>

```

we can see that in the translation of the `apply` instruction, both a 'require' and 'send' partition operation are created. the 'send' will mark all trackable state values in the same region as `%%10` to become marked as 'sent', and so subsequent 'require' operations on any of them should typically produce diagnostics.

now if we focus on the logging around the subsequent assignment `x = 2`, we have from the pre-dataflow logs:

```swift
// SIL:
  %24 = begin_access [modify] [dynamic] %2 : $*Int // users: %25, %26
  store %23 to [trivial] %24 : $*Int // id: %25
  end_access %24 : $*Int // id: %26

// RBI:
Visiting: %24 = begin_access [modify] [dynamic] %2 : $*Int // users: %25, %26
    Semantics: look_through
Visiting: store %23 to [trivial] %24 : $*Int // id: %25
    Semantics: store
Visiting: end_access %24 : $*Int // id: %26
    Semantics: ignored

```

the logging indicates that the SIL `store` instruction has 'store' semantics, and if we look at [the logic](https://github.com/swiftlang/swift/blob/783118ec69e531dc4e791823879ebac1d8bcc87f/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L2611) for this translation, it appears that a store to a non-sendable destination should be translated into either an assign or merge partition operation, both of which should eventually add some additional 'require' partition operations if trackable state is involved. i believe this means that in the dataflow portion of the analysis, we would expect to see a 'require' operation processed after the earlier 'send' produced for the `Task.detached` instructions.

however, if we inspect the output of the dataflow logs, they appear to end with the 'send' operation associated with the `Task.detached` call:

```swift
// Relevant trackable state:
%%0: TrackableValue. State: TrackableValueState[id: 0][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
    Rep Value: %0 = alloc_box ${ var Int }, var, name "x" // users: %28, %1
%%1: TrackableValue. State: TrackableValueState[id: 1][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
    Rep Value: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
%%10: TrackableValue. State: TrackableValueState[id: 10][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
    Rep Value: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16

// Dataflow:
Block: bb0
    Visiting Preds!
Applying: assign_fresh %%0: %0 = alloc_box ${ var Int }, var, name "x" // users: %28, %1
    Before: []
    After: [(0)]
Applying: require %%0: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
    Before: [(0)]
    After: [(0)]
Applying: assign %%1 = %%0: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
    Before: [(0)]
    After: [(0 1)]
Applying: require %%1: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
    Before: [(0 1)]
    After: [(0 1)]
Applying: assign %%10 = %%1: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
    Before: [(0 1)]
    After: [(0 1 10)]
Applying: require %%10: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
    Before: [(0 1 10)]
    After: [(0 1 10)]
Applying: send %%10: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
    Before: [(0 1 10)]
    After: [{0 1 10}]
    Working Partition: [{0 1 10}]
    Exit Partition: [{0 1 10}]
    Updated Partition: yes
// ⁉️ where's the `require` for the write to `x`?

```

### Potential explanation

after digging into the implementation\[2\], this is what i think is going on:

1. during the initial pass through the SIL instructions to collect 'trackable state', when we reach the `begin_access` instruction for the second write to `x`:

```swift
%24 = begin_access [modify] [dynamic] %2 : $*Int // users: %25, %26

```

we enter [the method](https://github.com/swiftlang/swift/blob/63c24afde6505419d4d89d4db81066516b0a569c/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L2465) for translating instructions with 'lookthrough' semantics.
2. the 'destination' value, which in this case is the `begin_access` instruction, is [attempted to be tracked](https://github.com/swiftlang/swift/blob/63c24afde6505419d4d89d4db81066516b0a569c/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L2465).
3. after a few method hops, this logic ends up in [getUnderlyingTrackedValueHelper](https://github.com/swiftlang/swift/blob/63c24afde6505419d4d89d4db81066516b0a569c/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L870)
4. it fails the test to check for non-sendability [here](https://github.com/swiftlang/swift/blob/63c24afde6505419d4d89d4db81066516b0a569c/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L873-L874) (first goes [here](https://github.com/swiftlang/swift/blob/d36b06747a54689a09ca6771b04798fc42b3e701/lib/SILOptimizer/Utils/SILIsolationInfo.cpp#L1256), and then [here](https://github.com/swiftlang/swift/blob/d36b06747a54689a09ca6771b04798fc42b3e701/lib/SIL/IR/SILType.cpp#L1252)). this seems to be because the type of the destination value resolves to `Int`, which is sendable.
5. since the value appears to be sendable, a new 'underlying tracked value' is [created and returned](https://github.com/swiftlang/swift/blob/63c24afde6505419d4d89d4db81066516b0a569c/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L874). this eventually shows up in the log output as:

```swift
%%18: TrackableValue. State: TrackableValueState[id: 18][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
Rep Value: %24 = begin_access [modify] [dynamic] %2 : $*Int // users: %25, %26

```

which indicates that the `is_sendable` flag is true.
6. now, when we get to the translation logic for the `store` instruction, we [check](https://github.com/swiftlang/swift/blob/63c24afde6505419d4d89d4db81066516b0a569c/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L2614) if the store's destination is trackable state (destination in this case is the `begin_access` inst), and we fail because the state is marked as `is_sendable`.
7. the `store` is thus ignored, and produces no partition operations that would validate `x` isn't used after being sent

### Possible remediation

i think it's odd that the result of translating an instruction with 'look through' semantics would produce a new underlying trackable value. it would seem to me that a `begin_access` should generally track whatever backs the accessed value. there is [logic to this effect](https://github.com/swiftlang/swift/blob/63c24afde6505419d4d89d4db81066516b0a569c/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L876-L879) for handling `ProjectBoxInst`, for example.

adding an additional condition to the underlying value helper [early exit check](https://github.com/swiftlang/swift/blob/63c24afde6505419d4d89d4db81066516b0a569c/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L873) that requires the instruction _not_ be a look through instruction, still does not produce a diagnostic. however, this change does enable the `translateSILStore()` logic to end up in [translateSILMerge](https://github.com/swiftlang/swift/blob/63c24afde6505419d4d89d4db81066516b0a569c/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L2631), which seems like an improvement.

it seems that in the [merge translation logic](https://github.com/swiftlang/swift/blob/63c24afde6505419d4d89d4db81066516b0a569c/lib/SILOptimizer/Analysis/RegionAnalysis.cpp#L2537-L2551), we currently only add a partition 'require' for the destination value if both it and at least one of its operands are 'trackable'. if we change the code to always add a 'require' for the destination value as long as it's trackable, then we get a diagnostic to the effect of:

```swift
18 | var x = 0
19 | 
20 | Task.detached { x = 1 }
   | |- error: sending value of non-Sendable type '() async -> ()' risks causing data races
   | `- note: Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(priority:operation:)' risks causing races in between local and caller code
21 | 
22 | x = 2
   | `- note: access can happen concurrently
23 | }
24 | 

```

i have not yet attempted drafting a PR or looked too deeply into what might break with such a change, but i'm curious if a fix along the lines proposed seems reasonable to those more familiar with the code, or any interested parties (cc @Michael_Gottesman, @hborla). i will also note that @Nickolas_Pohilets recently put together a [draft PR](https://github.com/swiftlang/swift/pull/80088) to address a similar issue, which takes a slightly different approach to addressing the problem (changing the sendability check logic vs altering 'look through' tracking semantics).

### References

1. example code in [godbolt](https://swift.godbolt.org/z/d47GaPaTq). note that at the time of writing, it is using a compiler from the current 6.1 dev branch so that the RBI logs will show up.

* * *

1. using the flags `-Xllvm -sil-regionbasedisolation-log=verbose` in a compiler built with assertions enabled 

2. read: adding copious print statements into the compiler

---

<div class="post-metadata">

**Author:** ![jamieQ](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/jamieq/32/5117_2.png) [@jamieQ](https://forums.swift.org/u/jamieQ)\
**Post date:** [March 28, 2025, 5:03pm UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/2 "2025-03-28T17:03:14Z")

</div>

and, as i hit the character limit in the original post by including these, here are excerpts of the RBI logs for the relevant function:

> **🪵 expand for pre-dataflow logs 🪵**
>
> ```swift
> ╾──────────────────────────────╼
> Performing pre-dataflow scan to gather flow insensitive information $s6output27mutableLocalCaptureDataRaceyyF:
> ╾──────────────────────────────╼
> PartialApplyReachability::add.
> Value: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> User: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> Propagating Captures Result!
> (BitNo, Value):
> 0: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> (Block,GenBits):
> bb0.
> Entry: 0[]
> Gen: 1[0]
> Exit: 1[0]
> Initializing Function Args:
> None.
> ╾──────────────────────────────╼
> Compiling basic block for function $s6output27mutableLocalCaptureDataRaceyyF: bb0
> ╾──────────────────────────────╼
> sil_scope 1 { loc "<source>":2:6 parent @$s6output27mutableLocalCaptureDataRaceyyF : $@convention(thin) () -> () }
> sil_scope 2 { loc "<source>":3:7 parent 1 }
> sil_scope 3 { loc "<source>":3:11 parent 1 }
> bb0:
> %0 = alloc_box ${ var Int }, var, name "x" // users: %28, %1
> %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> %2 = project_box %1 : ${ var Int }, 0 // users: %24, %7
> %3 = integer_literal $Builtin.IntLiteral, 0 // user: %6
> %4 = metatype $@thin Int.Type // user: %6
> // function_ref Int.init(_builtinIntegerLiteral:)
> %5 = function_ref @$sSi22_builtinIntegerLiteralSiBI_tcfC : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %6
> %6 = apply %5(%3, %4) : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %7
> store %6 to [trivial] %2 : $*Int // id: %7
> %8 = metatype $@thin Task<(), Never>.Type // user: %16
> %9 = alloc_stack $Optional<TaskPriority> // users: %18, %17, %16, %10
> inject_enum_addr %9 : $*Optional<TaskPriority>, #Optional.none!enumelt // id: %10
> // function_ref closure #1 in mutableLocalCaptureDataRace()
> %11 = function_ref @$s6output27mutableLocalCaptureDataRaceyyFyyYacfU_ : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %14
> %12 = copy_value %1 : ${ var Int } // user: %14
> %13 = enum $Optional<any Actor>, #Optional.none!enumelt // user: %14
> %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> // function_ref static Task<>.detached(priority:operation:)
> %15 = function_ref @$sScTss5NeverORs_rlE8detached8priority9operationScTyxABGScPSg_xyYaYAcntFZ : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %16
> %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
> destroy_addr %9 : $*Optional<TaskPriority> // id: %17
> dealloc_stack %9 : $*Optional<TaskPriority> // id: %18
> destroy_value %16 : $Task<(), Never> // id: %19
> %20 = integer_literal $Builtin.IntLiteral, 2 // user: %23
> %21 = metatype $@thin Int.Type // user: %23
> // function_ref Int.init(_builtinIntegerLiteral:)
> %22 = function_ref @$sSi22_builtinIntegerLiteralSiBI_tcfC : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %23
> %23 = apply %22(%20, %21) : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %25
> %24 = begin_access [modify] [dynamic] %2 : $*Int // users: %25, %26
> store %23 to [trivial] %24 : $*Int // id: %25
> end_access %24 : $*Int // id: %26
> end_borrow %1 : ${ var Int } // id: %27
> destroy_value %0 : ${ var Int } // id: %28
> %29 = tuple () // user: %30
> return %29 : $() // id: %30
> ╾──────────────────────────────╼
> Results:
> Visiting: %0 = alloc_box ${ var Int }, var, name "x" // users: %28, %1
> Semantics: assign_fresh
> ┌─┬─╼ %0 = alloc_box ${ var Int }, var, name "x" // users: %28, %1
> │ └─╼ line:3:7
> ├─────╼ assign_fresh %%0: %0 = alloc_box ${ var Int }, var, name "x" // users: %28, %1
> └─────╼ Used Values
> └╼ State: %%0. TrackableValueState[id: 0][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
> Rep Value: %0 = alloc_box ${ var Int }, var, name "x" // users: %28, %1
> Type: ${ var Int }
> Visiting: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> Semantics: assign
> ┌─┬─╼ %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> │ └─╼ line:3:7
> ├─────╼ require %%0: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> │ └╼ assign %%1 = %%0: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> └─────╼ Used Values
> └╼ State: %%0. TrackableValueState[id: 0][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
> Rep Value: %0 = alloc_box ${ var Int }, var, name "x" // users: %28, %1
> Type: ${ var Int }
> └╼ State: %%1. TrackableValueState[id: 1][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
> Rep Value: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> Type: ${ var Int }
> Visiting: %2 = project_box %1 : ${ var Int }, 0 // users: %24, %7
> Semantics: look_through
> Visiting: %3 = integer_literal $Builtin.IntLiteral, 0 // user: %6
> Semantics: assign_fresh
> Visiting: %4 = metatype $@thin Int.Type // user: %6
> Semantics: ignored
> Visiting: // function_ref Int.init(_builtinIntegerLiteral:)
> %5 = function_ref @$sSi22_builtinIntegerLiteralSiBI_tcfC : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %6
> Semantics: assign_fresh
> Visiting: %6 = apply %5(%3, %4) : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %7
> Semantics: apply
> Visiting: store %6 to [trivial] %2 : $*Int // id: %7
> Semantics: store
> Visiting: %8 = metatype $@thin Task<(), Never>.Type // user: %16
> Semantics: ignored
> Visiting: %9 = alloc_stack $Optional<TaskPriority> // users: %18, %17, %16, %10
> Semantics: assign_fresh
> Visiting: inject_enum_addr %9 : $*Optional<TaskPriority>, #Optional.none!enumelt // id: %10
> Semantics: ignored
> Visiting: // function_ref closure #1 in mutableLocalCaptureDataRace()
> %11 = function_ref @$s6output27mutableLocalCaptureDataRaceyyFyyYacfU_ : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %14
> Semantics: assign_fresh
> Visiting: %12 = copy_value %1 : ${ var Int } // user: %14
> Semantics: look_through
> Visiting: %13 = enum $Optional<any Actor>, #Optional.none!enumelt // user: %14
> Semantics: assign_fresh
> Visiting: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> Semantics: special
> ┌─┬─╼ %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> │ └─╼ line:5:17
> ├─────╼ require %%1: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> │ └╼ assign %%10 = %%1: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> └─────╼ Used Values
> └╼ State: %%1. TrackableValueState[id: 1][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
> Rep Value: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> Type: ${ var Int }
> └╼ State: %%10. TrackableValueState[id: 10][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
> Rep Value: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> Type: $@isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <()>
> Visiting: // function_ref static Task<>.detached(priority:operation:)
> %15 = function_ref @$sScTss5NeverORs_rlE8detached8priority9operationScTyxABGScPSg_xyYaYAcntFZ : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %16
> Semantics: assign_fresh
> Visiting: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
> Semantics: apply
> ┌─┬─╼ %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
> │ └─╼ line:5:8
> ├─────╼ require %%10: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
> │ └╼ send %%10: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
> └─────╼ Used Values
> └╼ State: %%10. TrackableValueState[id: 10][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
> Rep Value: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> Type: $@isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <()>
> Visiting: destroy_addr %9 : $*Optional<TaskPriority> // id: %17
> Semantics: ignored
> Visiting: dealloc_stack %9 : $*Optional<TaskPriority> // id: %18
> Semantics: ignored
> Visiting: destroy_value %16 : $Task<(), Never> // id: %19
> Semantics: ignored
> Visiting: %20 = integer_literal $Builtin.IntLiteral, 2 // user: %23
> Semantics: assign_fresh
> Visiting: %21 = metatype $@thin Int.Type // user: %23
> Semantics: ignored
> Visiting: // function_ref Int.init(_builtinIntegerLiteral:)
> %22 = function_ref @$sSi22_builtinIntegerLiteralSiBI_tcfC : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %23
> Semantics: assign_fresh
> Visiting: %23 = apply %22(%20, %21) : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %25
> Semantics: apply
> Visiting: %24 = begin_access [modify] [dynamic] %2 : $*Int // users: %25, %26
> Semantics: look_through
> Visiting: store %23 to [trivial] %24 : $*Int // id: %25
> Semantics: store
> Visiting: end_access %24 : $*Int // id: %26
> Semantics: ignored
> Visiting: end_borrow %1 : ${ var Int } // id: %27
> Semantics: ignored
> Visiting: destroy_value %0 : ${ var Int } // id: %28
> Semantics: ignored
> Visiting: %29 = tuple () // user: %30
> Semantics: assign
> Visiting: return %29 : $() // id: %30
> Semantics: require
> 
> ```

> **🪵 expand for dataflow logs 🪵**
>
> ```swift
> ╾──────────────────────────────╼
> Performing Dataflow!
> ╾──────────────────────────────╼
> Values!
> %%0: TrackableValue. State: TrackableValueState[id: 0][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
> Rep Value: %0 = alloc_box ${ var Int }, var, name "x" // users: %28, %1
> %%1: TrackableValue. State: TrackableValueState[id: 1][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
> Rep Value: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> %%2: TrackableValue. State: TrackableValueState[id: 2][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %2 = project_box %1 : ${ var Int }, 0 // users: %24, %7
> %%3: TrackableValue. State: TrackableValueState[id: 3][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %3 = integer_literal $Builtin.IntLiteral, 0 // user: %6
> %%4: TrackableValue. State: TrackableValueState[id: 4][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: // function_ref Int.init(_builtinIntegerLiteral:)
> %5 = function_ref @$sSi22_builtinIntegerLiteralSiBI_tcfC : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %6
> %%5: TrackableValue. State: TrackableValueState[id: 5][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %4 = metatype $@thin Int.Type // user: %6
> %%6: TrackableValue. State: TrackableValueState[id: 6][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %6 = apply %5(%3, %4) : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %7
> %%7: TrackableValue. State: TrackableValueState[id: 7][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %9 = alloc_stack $Optional<TaskPriority> // users: %18, %17, %16, %10
> %%8: TrackableValue. State: TrackableValueState[id: 8][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: // function_ref closure #1 in mutableLocalCaptureDataRace()
> %11 = function_ref @$s6output27mutableLocalCaptureDataRaceyyFyyYacfU_ : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %14
> %%9: TrackableValue. State: TrackableValueState[id: 9][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %13 = enum $Optional<any Actor>, #Optional.none!enumelt // user: %14
> %%10: TrackableValue. State: TrackableValueState[id: 10][is_no_alias: no][is_sendable: no][region_value_kind: disconnected].
> Rep Value: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> %%11: TrackableValue. State: TrackableValueState[id: 11][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: // function_ref static Task<>.detached(priority:operation:)
> %15 = function_ref @$sScTss5NeverORs_rlE8detached8priority9operationScTyxABGScPSg_xyYaYAcntFZ : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %16
> %%12: TrackableValue. State: TrackableValueState[id: 12][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %8 = metatype $@thin Task<(), Never>.Type // user: %16
> %%13: TrackableValue. State: TrackableValueState[id: 13][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
> %%14: TrackableValue. State: TrackableValueState[id: 14][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %20 = integer_literal $Builtin.IntLiteral, 2 // user: %23
> %%15: TrackableValue. State: TrackableValueState[id: 15][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: // function_ref Int.init(_builtinIntegerLiteral:)
> %22 = function_ref @$sSi22_builtinIntegerLiteralSiBI_tcfC : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %23
> %%16: TrackableValue. State: TrackableValueState[id: 16][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %21 = metatype $@thin Int.Type // user: %23
> %%17: TrackableValue. State: TrackableValueState[id: 17][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %23 = apply %22(%20, %21) : $@convention(method) (Builtin.IntLiteral, @thin Int.Type) -> Int // user: %25
> %%18: TrackableValue. State: TrackableValueState[id: 18][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %24 = begin_access [modify] [dynamic] %2 : $*Int // users: %25, %26
> %%19: TrackableValue. State: TrackableValueState[id: 19][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
> Rep Value: %29 = tuple () // user: %30
> Block: bb0
> Visiting Preds!
> Applying: assign_fresh %%0: %0 = alloc_box ${ var Int }, var, name "x" // users: %28, %1
> Before: []
> After: [(0)]
> Applying: require %%0: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> Before: [(0)]
> After: [(0)]
> Applying: assign %%1 = %%0: %1 = begin_borrow [var_decl] %0 : ${ var Int } // users: %27, %12, %2
> Before: [(0)]
> After: [(0 1)]
> Applying: require %%1: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> Before: [(0 1)]
> After: [(0 1)]
> Applying: assign %%10 = %%1: %14 = partial_apply [callee_guaranteed] [isolated_any] %11(%13, %12) : $@convention(thin) @async @substituted <τ_0_0> (@guaranteed Optional<any Actor>, @guaranteed { var Int }) -> @out τ_0_0 for <()> // user: %16
> Before: [(0 1)]
> After: [(0 1 10)]
> Applying: require %%10: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
> Before: [(0 1 10)]
> After: [(0 1 10)]
> Applying: send %%10: %16 = apply %15<(), Never>(%9, %14, %8) : $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Sendable, τ_0_1 == Never> (@in_guaranteed Optional<TaskPriority>, @sil_sending @owned @isolated(any) @async @callee_guaranteed @substituted <τ_0_0> () -> @out τ_0_0 for <τ_0_0>, @thin Task<τ_0_0, Never>.Type) -> @owned Task<τ_0_0, Never> // user: %19
> Before: [(0 1 10)]
> After: [{0 1 10}]
> Working Partition: [{0 1 10}]
> Exit Partition: [{0 1 10}]
> Updated Partition: yes
> 
> ```

---

<div class="post-metadata">

**Author:** ![Nickolas\_Pohilets](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/nickolas_pohilets/32/10659_2.png) [@Nickolas\_Pohilets](https://forums.swift.org/u/Nickolas_Pohilets)\
**Post date:** [March 30, 2025, 8:24pm UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/3 "2025-03-30T20:24:11Z")

</div>

I was trying to fix exactly the same issue. Idea behind my fix attempt was to treat pointer values as first-class, but I'm not familiar with the sufficiently to judge if that is the right approach. It is more of an experiment. You might be right, maybe we need instead to treat `begin_borrow` the same way `project_box` is handled in `getUnderlyingTrackedValueHelper()`.

But regardless of that, [change in `translateSILMerge()`](https://github.com/swiftlang/swift/pull/80088/files#diff-a04da4fbf48e0bae7d4d758cec3817ce3342b7bd57a9e1ddedc34e5172887624R2542) seems to be needed as well.

---

<div class="post-metadata">

**Author:** ![jamieQ](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/jamieq/32/5117_2.png) [@jamieQ](https://forums.swift.org/u/jamieQ)\
**Post date:** [March 30, 2025, 10:08pm UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/4 "2025-03-30T22:08:12Z")

</div>

after looking into it more, i've begun to suspect that even the handling of `project_box` may not be quite right (at least, per my current understanding). in addition to the change you made to `translateSILMerge()`, i tried adding an additional clause to the sendable value early exit in `getUnderlyingTrackedValueHelper()` basically like:

```cpp
auto *di = value->getDefiningInstruction();
if ((di && !isStaticallyLookThroughInst(di)) &&
    !SILIsolationInfo::isNonSendableType(value))
  return UnderlyingTrackedValueInfo(value);

```

however, that then caused the `transfernonsendable.swift` tests to fail, specifically [the one](https://github.com/swiftlang/swift/blob/cba4aeb1b428ba5cdd287f1d03b627a26a915575/test/Concurrency/transfernonsendable.swift#L1332-L1334) that checks that it's okay to send a tuple with a mix of non-sendable and sendable fields as long as the non-sendable elements aren't used after send. so just skipping that sendable type check for 'look through' instructions seemingly isn't sufficient, or would require additional changes elsewhere.

so, going back to the original example, i noticed that the debug logs had this line:

```swift
%%2: TrackableValue. State: TrackableValueState[id: 2][is_no_alias: no][is_sendable: yes][region_value_kind: disconnected].
    Rep Value: %2 = project_box %1 : ${ var Int }, 0 // users: %24, %7

```

upon reflection, i found this confusing because `project_box` is supposed to be 'look through', so why would it show up as a 'representative value' of some trackable state? shouldn't the representative be the thing being projected (the `alloc_box` in this case)?

my current theory is that we perhaps need to do a more thorough inspection of the use-def chain for the look through instructions that are dealing with addresses. in this [WIP PR](https://github.com/swiftlang/swift/pull/80393/files) i've hoisted the use-def walking logic to occur before the sendability test in such cases, and that does appear to resolve the issue without regressing the existing tests (though still hasn't passed CI for seemingly unrelated reasons). i'm still not sure if such a change really makes sense though...

> [@Nickolas\_Pohilets](#):
>
> But regardless of that, [change in `translateSILMerge()`](https://github.com/swiftlang/swift/pull/80088/files#diff-a04da4fbf48e0bae7d4d758cec3817ce3342b7bd57a9e1ddedc34e5172887624R2542) seems to be needed as well.

yeah i wonder if this change should perhaps be made regardless of whether it fixes any currently-known bugs...

---

<div class="post-metadata">

**Author:** ![Michael\_Gottesman](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/michael_gottesman/32/280_2.png) [@Michael\_Gottesman](https://forums.swift.org/u/Michael_Gottesman)\
**Post date:** [April 1, 2025, 6:43am UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/5 "2025-04-01T06:43:06Z")

</div>

> [@jamieQ](#):
>
> ```swift
> @MainActor // *
> func mutableLocalCaptureDataRace() {
> var x = 0
> 
> Task.detached { x = 1 }
> 
> x = 2
> }
> 
> ```

So I took a little look at this. There is an issue in translateMerge where we are not requiring the destination operand if all of the src are Sendable. There is also a further issue around how we are currently handling Sendable addresses from non-Sendable boxes. I am right now thinking about the problem. The issue is more complex than the WIP PR. Give me a little bit to detangle it.

---

<div class="post-metadata">

**Author:** ![Nickolas\_Pohilets](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/nickolas_pohilets/32/10659_2.png) [@Nickolas\_Pohilets](https://forums.swift.org/u/Nickolas_Pohilets)\
**Post date:** [April 5, 2025, 6:41am UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/6 "2025-04-05T06:41:14Z")

</div>

@Michael_Gottesman, any updates on this?

I want to submit [Proposal to unban weak let bindings by nickolas-pohilets · Pull Request #2771 · swiftlang/swift-evolution · GitHub](https://github.com/swiftlang/swift-evolution/pull/2771) for review, but this bug would cause a lot of confusion during review process. So I think it is better to fix it first.

---

<div class="post-metadata">

**Author:** ![Michael\_Gottesman](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/michael_gottesman/32/280_2.png) [@Michael\_Gottesman](https://forums.swift.org/u/Michael_Gottesman)\
**Post date:** [April 10, 2025, 10:29pm UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/7 "2025-04-10T22:29:42Z")

</div>

This is the fix for it: [[rbi] Teach RBI how to handle non-Sendable bases of Sendable values by gottesmm · Pull Request #80745 · swiftlang/swift · GitHub](https://github.com/swiftlang/swift/pull/80745). Like I mentioned above, it required changing a bit of the "machine".

---

<div class="post-metadata">

**Author:** ![CrystDragon](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/crystdragon/32/10461_2.png) [@CrystDragon](https://forums.swift.org/u/CrystDragon)\
**Post date:** [April 11, 2025, 7:27am UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/8 "2025-04-11T07:27:00Z")

</div>

I believe this issue is related to OP's case: [A simple concurrency safety breach of a var being captured by reference · Issue #76929 · swiftlang/swift · GitHub](https://github.com/swiftlang/swift/issues/76929). It's a long standing regression.

BTW, thanks for sharing your compilation, it's very informative.

---

<div class="post-metadata">

**Author:** ![jamieQ](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/jamieq/32/5117_2.png) [@jamieQ](https://forums.swift.org/u/jamieQ)\
**Post date:** [April 12, 2025, 12:07am UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/9 "2025-04-12T00:07:57Z")

</div>

thanks! i tested on the main branch since the PR was already merged, and while it does appear the changes address some of the issues (yay!), code that is structurally nearly identical (at least from a source perspective) but with slight variations on the isolation crossings seems to still fail to produce the expected diagnostics:

```swift
// this is now diagnosed:

@MainActor
func mutableLocalCaptureDataRace() {
  var x = 0

  Task.detached { x = 1 }
       |- error: sending value of non-Sendable type '() async -> ()' risks causing data races [#SendingRisksDataRace]
       `- note: Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(priority:operation:)' risks causing races in between local and cal

  x = 2
    `- note: access can happen concurrently
}

// but this is not:

nonisolated func mutableLocalCaptureDataRace2() {
  var x = 0

  Task { @MainActor in x = 1 } // or could use `Task.detached` here

  x = 2
}

// and neither is this:

@globalActor
actor GA { static let shared = GA() }

@MainActor
func mutableLocalCaptureDataRace3() {
  var x = 0

  Task.detached { @GA in x = 1 }

  x = 2
}

```

technically it seems the use after send _is_ seen during the diagnostic walk (you can see it reported in the logs), but it's hitting the [error suppression condition here](https://github.com/swiftlang/swift/blob/b2787835ef5f479e2a113d2fcf2c9df7598d8652/lib/SILOptimizer/Mandatory/SendNonSendable.cpp#L2699-L2702):

```cpp
  void handleLocalUseAfterSend(LocalUseAfterSendError error) const {
    const auto &partitionOp = *error.op;
    REGIONBASEDISOLATION_LOG(error.print(llvm::dbgs(), info->getValueMap()));

    // Ignore this if we are erroring on a mutable base of a Sendable value and
    // if when we sent the value's region was not closure captured.
    if (error.op->getOptions().containsOnly(
            PartitionOp::Flag::RequireOfMutableBaseOfSendableValue) &&
        !operandToStateMap.get(error.sendingOp).isClosureCaptured)
      return;

    sendingOpToRequireInstMultiMap.insert(
        error.sendingOp, RequireInst::forUseAfterSend(partitionOp.getSourceInst()));
  }

```

any idea offhand what may be going on there @Michael_Gottesman? FWIW i think the second example here is essentially a reduction of the issue you referenced @CrystDragon.

---

<div class="post-metadata">

**Author:** ![Nickolas\_Pohilets](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/nickolas_pohilets/32/10659_2.png) [@Nickolas\_Pohilets](https://forums.swift.org/u/Nickolas_Pohilets)\
**Post date:** [April 12, 2025, 12:11pm UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/10 "2025-04-12T12:11:50Z")

</div>

The two cases that I've described in [False negative in isolation checking · Issue #80014 · swiftlang/swift · GitHub](https://github.com/swiftlang/swift/issues/80014) don't reproduce anymore, but in the following scenario data race is still not detected:

```swift
// Compiled with -swift-version 6
func foo() {
    let x = X()
    doIt { [weak x] in
        x?.bar()
        x = nil // x is mutable, and should not be possible to capture in @Sendable closure
    }
}

func doIt(_ block: @escaping @Sendable () -> Void) {
}

final class X: Sendable {
    func bar() {}
}

```

---

<div class="post-metadata">

**Author:** ![Michael\_Gottesman](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/michael_gottesman/32/280_2.png) [@Michael\_Gottesman](https://forums.swift.org/u/Michael_Gottesman)\
**Post date:** [April 16, 2025, 7:54pm UTC](https://forums.swift.org/t/lets-debug-missing-rbi-data-race-diagnostics/78910/11 "2025-04-16T19:54:10Z")

</div>

```swift
// but this is not:

nonisolated func mutableLocalCaptureDataRace2() {
  var x = 0

  Task { @MainActor in x = 1 } // or could use `Task.detached` here

  x = 2
}

// and neither is this:

@globalActor
actor GA { static let shared = GA() }

@MainActor
func mutableLocalCaptureDataRace3() {
  var x = 0

  Task.detached { @GA in x = 1 }

  x = 2
}

```

I looked at this. This is due to a slight off by one error in a certain reachability analysis... not with the patch I did. I have a small fix.

> <https://github.com/swiftlang/swift/pull/80863>
>
> The specific issue was when we were walking instructions looking to see if there… was a partial apply escaping instruction, we were not including the user itself. That means that if the user was the partial apply escaping instruction, we would return that no escape occured.
> 
> rdar://149414471