Is using an enum constructor worth a lazy evaluation?

#1

This is a simplified version of an example that triggers clippy's unnecessary_lazy_evaluations lint:

#![warn(clippy::unnecessary_lazy_evaluations)]

pub enum Foo {
    A(String),
    B(Box<Foo>),
}

fn bar(s: &str) -> Option<Foo> {
    if s.len() < 42 {
        Some(Foo::B(Box::new(Foo::A("hello".to_owned()))))
    } else {
        None
    }
}


pub fn foo(s: String) -> Foo {
    bar(&s).unwrap_or_else(|| Foo::A(s))
}

It suggests that the last line be replaced with bar(&s).unwrap_or(Foo::A(s)). Is that a good idea? Assuming that everything except bar gets inlined and optimized, does building a value using an enum constructor count as work that ought to be delayed in the case that bar returns Some(...)? I tried looking at the MIR for this in the playground but it doesn't look like it's being optimized.

#2

Using an enum constructor is basically free, so yes replace it with unwrap_or

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#3

Isn't it a memcpy? In my real example it's also in an Ok so it seems like you would want to resolve both of those at the same time.

#4

Yes it's a memcpy, but so are moves (like moving the s: String into the closure). Keep in mind that using a closure is constructing an anonymous type and moving things inside.

You can read more about closures on by blog

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#5

Is a closure actually constructed though? I would expect this code to actually be compiled to a match, because unwrap_or_else is being applied to a closure that only exists once in the code, so monomorphization will kick in and this specialized unwrap_or_else will almost certainly be inlined.

#6

The closure definitely gets constructed, however there is something else going here.
I just checked godbot and got results I wasn't expecting Compiler Explorer. It looks like LLVM can't elide the drop glue for Foo! This is because allocating memory using Box::new and to_owned could in theory panic, and so then the drop glue would be run. Then it can't figure out that Foo::A is the only variant constructed.

Depending on if this matters to you, I would silence clippy and make a comment with this godbolt link. Using a closure doesn't construct Foo, until we can prove that panics don't happen so it's all good.

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