Fix issue 21209 - scope attribute inference fails on foreach

[dkorpel]

Scope inference fails as soon as you do:

auto foo(int[] arr) {
    int[] tmp = arr; // tmp must be marked explicitly `scope`
}

A foreach on an array gets rewritten to:

int[] __r83 = arr[];
ulong __key84 = 0LU;
for (; __key84 < __r83.length; __key84 += 1LU)
{
	int c = __r83[__key84];
	// body
}

This fix explicitly marks the temporary __r83 scope. It can't escape since it's hidden.

This does not fix foreach on an AliasSeq (which gets lowered to a special "unrolled" with the same temporary problem). If this fix gets accepted, I'll open a follow-up issue for that case, which is a bit harder since the temporary is not hidden, so you cannot just mark it scope.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21209	normal	scope attribute inference with does not work well with foreach

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#12620"

[dkorpel]

I still have to account for foreach(ref a; arr) {return a;} I see

[Geod24]

Well, foreach (scope foo; a) doesn't even parse, so there's that too.

[RazvanN7]

Does this also fix foreach on a struct that defines an opApply?

[dkorpel]

Does this also fix foreach on a struct that defines an opApply?

It only changes foreach on an array. The lowering to opApply is already scope compatible it seems.

@safe:
void foo(T)(T s) {
    static int x; x++; // force impure for issue 20150
    foreach(node; s) {cast(void) node;}
}

struct S {
    int* ptr;
    int opApply(int delegate(scope S) @safe dg) scope {return 0;}
}

void main() {
    scope S t;
    foo(t);
}

[Geod24]

The lowering to opApply is already scope compatible it seems.

Pretty sure it just ignores it. Try to escape something from the foreach.

[dkorpel]

Pretty sure it just ignores it. Try to escape something from the foreach.

Then it says "cannot pass argument __foreachbody3 of type int delegate(S node) nothrow @nogc @safe to parameter int delegate(scope S) @safe dg"

[dkorpel]

Follow-up issue is here: https://issues.dlang.org/show_bug.cgi?id=21990