[Issue 21821] New: Optimizer assumes immutables don't change, but they can in @system code

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21821

          Issue ID: 21821
           Summary: Optimizer assumes immutables don't change, but they
                    can in @system code
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: bugzilla at digitalmars.com

Consider this code from core.lifetime:

  void copyEmplacex(ref immutable(S) source, ref immutable(S) target) @system {
        import core.stdc.string : memcpy;
        memcpy(cast(S*) &target, cast(S*) &source, S.sizeof);
        (cast() target).__xpostblit();
  }

Note the last line casts away immutability, and the call to __xpostblit()
modifies the supposedly immutable `target`. The part of the optimizer that
fails this is the function Symbol_isAffected() in backend/symbol.d

The fix is to set a flag in `funcsym_p` when it is @safe, and enable the check
for immutability.

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