MIR inlining inlines default trait methods

[arielb1]

STR

$ cat mirline.rs
pub trait Foo {
    fn bar(&self) {}
}

impl Foo for () {
    fn bar(&self) { println!("Hello, World!"); }
}

pub fn main() {
    ().bar();
}
$ rustc mirline.rs -Z mir-opt-level=2
$ ./mirline

Expected Result

Code should print "Hello, World!"

Actual Result

Code does not print anything, because the empty default implementation is inlined.

cc @Aatch

[wesleywiser]

Is this just a matter of preventing the inliner from inlining trait default methods?

[arielb1]

1.17 is now beta.

[nikomatsakis]

triage: P-medium

Marking this as medium because it requires you to opt-in to the higher "MIR opt level", which is unstable, but we should definitely fix this.

[brson]

cc @Aatch @nikomatsakis just pinging for an update.

[arielb1]

1.17 is now stable. This is a nightly -> nightly regression affecting only -Z mir-opt-level.

[nikomatsakis]

triage: P-medium

[nikomatsakis]

I have been talking to @qmx, who is going to take a look at implementing this. I'll leave some mentoring instructions shortly!

[nikomatsakis]

So #44383 will fix this by just not inlining trait calls -- the simplest possible fix. I just opened #44389 as an enhancement issue to track a more complete fix.