[Issue 16220] New: sync

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16220

          Issue ID: 16220
           Summary: sync
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: petar.p.kirov at gmail.com

The current implementation of `synchronized` leaves much to be desired:
1. It adds the overhead of a monitor field to every class, even when it's never
used. Also makes object creation / finalization in druntime more complex.

2. It requires that all monitors/mutexes are implemented as classes, derived
from Object.Monitor, which also excludes struct-based implementations.

3. It does not allow custom implementation - e.g. `struct` objects can't be
synchronized.

4. Can't be `nothrow`/`@nogc`/etc. by design - because `Object.Monitor` tries
to be maximally inclusive, similarly to the Object.op* attribute problem.

Instead there should be a well defined, easily accessible protocol (e.g.
`opLock`, `opUnlock`, `opTryLock` non-static member functions) that is
recognized by the compiler (similar to how it already recognizes both struct
and class based input ranges) which would be used whenever `obj` in
`synchronized (obj)` implements it.

Another option would be to allow users to implement the `__monitor` field
themselves, provided that it has the required methods (at least `lock` and
`unlock` or `opLock`/`opUnlock`).

A third option would be to allow user overloads (non-extern) of
`_d_monitorenter` `_d_monitorexit` as free functions that are looked up
similarly to how `std.math.pow` is for the `^^` operator.

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