[Issue 18516] New: Add -vnrvo switch for showing user when nrv is done

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18516

          Issue ID: 18516
           Summary: Add -vnrvo switch for showing user when nrv is done
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: ibuclaw at gdcproject.org

This optimization is practically undocumented, save for the glossary, and the
existing checks for NRVO in the D2 testsuite are really quite fragile as the
test for it is checking what is effectively undefined behaviour in normal
circumstances.

i.e:
What `p` is pointing to here is not known, as `tmp` is no longer in scope when
the assert() contract is executed.
---
static S* p;

S fn()
{
   S tmp;
   p = &tmp;
   return tmp;
}

S s = fn();
assert(p == &s);
---

So there's also a desire for a better way to test NRVO in the compiler, this
could be done by validating an AST dump of the backend codegen, but using the
existing TEST_OUTPUT support should be simpler.

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