Misleading error "Sending 'asdf' risks causing data races" when var asdf is Sendable

[nickasd]

Description

In Swift 6, the code below results in a compiler error

Sending 'asdf' risks causing data races

which is misleading, since the problem is not the variable asdf itself, but the object to which it is being assigned to, namely a, which is not Sendable.

Reproduction

class A {
    var description = ""
}

class B {
    let a = A()
    
    func b() {
        let asdf = ""
        Task { @MainActor in
            a.description = asdf // Sending 'asdf' risks causing data races
        }
    }
}

Expected behavior

An error like

Task or actor isolated value cannot be sent

This is in fact the error displayed when changing

a.description = asdf

to

a.description = "asdf"

but it is displayed at the line

Task { @MainActor in

Instead, it would be useful if the compiler indicated what particular value cannot be sent.

Environment

swift-driver version: 1.115 Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)
Target: arm64-apple-macosx15.0

Additional information

No response

[GeroHerkenrath]

Some further info:
If the above class A is isolated to the same actor as the Task closure (@MainActor in above example) or adopts @unchecked Sendable, the shown error message is exactly the same, as the underlying problem seems to be that the line a.description = asdf actually captures self (i.e. the instance of B), which is also not sendable.
Thus, a better error message imo would be

Task or actor isolated value 'self' cannot be sent

In this case the correct code would require to capture a explicitly, i.e. write Task { @MainActor [a] in, which could even perhaps be a fix-it suggestion.
And of course if A is not matching the isolation of the Task's closure or is not @unchecked Sendable, the explicit capture should change the error message to

Task or actor isolated value 'a' cannot be sent

In this case there is no easy fix as either A or B must allow sending somehow.

[ir-fuel]

@nickasd A few hours before your post I raised the same issue on the Swift forums:

https://forums.swift.org/t/how-to-solve-sending-url-risks-causing-data-races

[gottesmm]

Great bug. I have a fix for it.

[gottesmm]

Fixed in c3d4458