Method call resolution behavioral changes on custom DSTs in Rust 1.70 -> 1.71

[ZhennanWu]

I tried this code:

use std::any::TypeId;
use std::any::Any;
use std::hint::black_box;
struct A<T:?Sized+'static> {
    a: i32,
    b: T
}

impl<T:?Sized+'static> A<T> {
    fn bb(&self) -> TypeId {
        self.b.type_id()
    }
}

pub fn main() {
    let mut a0 = A{a: 8, b: 9 as i32};
    let mut a: &mut A<dyn Any> = &mut a0;
    println!("{:?}",a.bb());
    println!("{:?}",a.b.type_id());
    println!("{:?}",std::any::TypeId::of::<i32>());
}

I expected to see this happen: The three printed TypeIds should be identical, as with the behavior in Rust <=1.70

Instead, this happened:

Program returned: 0
TypeId { t: 2368704253749761200 }
TypeId { t: 5817408772836814867 }
TypeId { t: 5817408772836814867 }

Meta

https://godbolt.org/z/8vvr37Ynf switch between rust versions.

I could not find reference in the 1.71 RELEASES that could explain such behavioral changes.

[apiraino]

if my bisection is correct, this surfaced in commit 5546cb6

cc @saethlin (makes sense?)

[saethlin]

I do not believe that is the correct PR to point at. Bisecting anything related to MIR opts can be tricky because you need to stabilize the MIR opt pipeline against changes like in that PR which don't actually implement any new behavior, they just change the meaning of MIR opt levels.

Try bisecting with -Zmir-opt-level=3? The above PR just changes the opt level 3 inliner behavior to be the default.

[compiler-errors]

Reproduces since before 2021-01-01 with -Zmir-opt-level=3

[compiler-errors]

This has to do with the fact that this impl: https://doc.rust-lang.org/std/any/trait.Any.html#impl-Any-for-T ... applies even when T = dyn Any.

Therefore in a polymorphic function like bb, we're eagerly inlining type_id from that impl even though during monomorphization we'd prefer the <dyn Any as Any>::type_id built-in candidate instead.

This seems problematic.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high