[Issue 19743] Foreach loops desugar into code that breaks `return scope`

[Issue 19743] [dip1000] unclear error message when escaping variable through foreach `ref`

Nov 04, 2022

Dec 17, 2022

Dec 13

November 04, 2022

[Issue 19743] [dip1000] unclear error message when escaping variable through foreach `ref`

Posted by Dennis

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Dennis

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https://issues.dlang.org/show_bug.cgi?id=19743

Dennis <dkorpel@live.nl> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic, safe
                 CC|                            |dkorpel@live.nl
            Summary|Foreach loops desugar into  |[dip1000] unclear error
                   |code that breaks `return    |message when escaping
                   |scope`                      |variable through foreach
                   |                            |`ref`
           Severity|normal                      |enhancement

--- Comment #1 from Dennis <dkorpel@live.nl> ---
Haystack in your example is an `int[9]`, a value type without pointers, so (return) scope doesn't apply to it. When you `return &val;`, you're escaping a pointer to the expired stack frame containing your `haystack` parameter, so an error is in place.

To make `find` valid, your haystack should be passed by `ref`, and then it compiles:

```
int* find(return ref int[9] haystack, int needle) @safe
{
    foreach (ref val; haystack)
        if (val == needle)
            return &val;

    return null;
}

void main() @safe
{
    int[9] haystack = [0, 10, 5, 6, 2, 3, 9, 4, 5];
    int* foundVal = haystack.find(3);
}
```

However, "scope variable __r2 may not be returned" is not a good diagnostic, so I'm changing this issue to be about that.

--

https://issues.dlang.org/show_bug.cgi?id=19743 Iain Buclaw <ibuclaw@gdcproject.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Priority|P1 |P4 --

https://issues.dlang.org/show_bug.cgi?id=19743 --- Comment #2 from dlangBugzillaToGithub <robert.schadek@posteo.de> --- THIS ISSUE HAS BEEN MOVED TO GITHUB https://github.com/dlang/dmd/issues/19542 DO NOT COMMENT HERE ANYMORE, NOBODY WILL SEE IT, THIS ISSUE HAS BEEN MOVED TO GITHUB --

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