[Issue 20104] New: core.atomic has no exchange function

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20104

          Issue ID: 20104
           Summary: core.atomic has no exchange function
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: turkeyman at gmail.com

Stomic exchange is a critical operation, and it's not present in the library.

You need this to implement a spin lock; the single most basic atomic primitive.

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