Warn on repeated non-trivial evaluations in templates

[arnetheduck]

Summary

When passing parameters to templates, subsitution may cause the parameter to be evaluated multiple times - warn when this happens

Description

proc f(): int =
  echo "hello"
  42

template test(a: int): int =
  a + a

echo test(f())

In the above template, a is evaluated twice which may be expensive - in the example, "hello" is printed twice which in most cases is both undesirable and inefficient.

In most cases, it's problem with the template declaration: it should probably have captured a in a local variable.

However, this is only a problem when a evaluates to something non-trivial - ie test(425) is not a problem and should not cause an evaluation warning.

Similarly, sometimes the multi-eval is desired, thus there should be a way to disable the warning locally - test(a{.dirty.}: int) would be one option (or some other keyword) - another option would be a call-site annotation.

Alternatives

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Examples

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Backwards Compatibility

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Links

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