Copy trait defeats generator optimization #62952
Description
The following example (playground):
#![feature(generators, generator_trait)] use std::ops::Generator; #[derive(Clone)] struct Foo([u8; 1024]); fn overlap_foo() -> impl Generator<Yield = (), Return = ()>{ static || { let x = Foo([0u8; 1024]); yield; drop(x); let y = Foo([0u8; 1024]); yield; drop(y); } } #[derive(Clone, Copy)] struct Bar([u8; 1024]); fn overlap_bar() -> impl Generator<Yield = (), Return = ()>{ static || { let x = Bar([0u8; 1024]); yield; drop(x); let y = Bar([0u8; 1024]); yield; drop(y); } } fn main() { dbg!(std::mem::size_of_val(&overlap_foo())); dbg!(std::mem::size_of_val(&overlap_bar())); }Outputs:
[gen-explicit-drop.rs:34] std::mem::size_of_val(&overlap_foo()) = 1028 [gen-explicit-drop.rs:35] std::mem::size_of_val(&overlap_bar()) = 2052 The only difference between Foo and Bar is that Foo is Copy. This defeats the generator optimization implemented in #61922 which considers any local that has been moved from as a candidate for overlap.
Technically, this is semantically consistent with the existing overlap behavior for generators. But making a type Copy and getting worse performance is a footgun that we should fix.
This can be fixed with a pass that turns unnecessary copies into moves in MIR.