1.99 beta crater regression: "type annotations needed"

[theemathas]

This regression was found by the 1.99 crater run.

[INFO] [stdout] error[E0282]: type annotations needed for `&_`
[INFO] [stdout]    --> src/sync/mcast.rs:172:65
[INFO] [stdout]     |
[INFO] [stdout] 172 |     let reader = MpMcastReader::new(DynRef::new(queue.clone(), |q| q.as_ref()));
[INFO] [stdout]     |                                                                 ^  - type must be known at this point
[INFO] [stdout]     |
[INFO] [stdout] help: consider giving this closure parameter an explicit type, where the type for type parameter `Q` is specified
[INFO] [stdout]     |
[INFO] [stdout] 172 |     let reader = MpMcastReader::new(DynRef::new(queue.clone(), |q: &Q| q.as_ref()));
[INFO] [stdout]     |                                                                  ++++
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] For more information about this error, try `rustc --explain E0282`.

Affects the Banyc/primitive crate, and its 7 dependents, all by the same author.

• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.primitive/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.auth/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.git_runner/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.math/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.metrics/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.monitor_table/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.mux/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.udp_listener/log.txt

[fmease]

In-progress minimization:

use std::sync::Arc;

pub struct DynRef<T, U> {
    value: T,
    convert: fn(&T) -> &U,
}
impl<T, U> DynRef<T, U> {
    pub const fn new(_: T, _: fn(&T) -> &U) -> Self {
        loop {}
    }
}

pub fn f<T>() -> MpMcastReader<T, Arc<()>> {
    let reader = MpMcastReader::new(DynRef::new(Arc::new(()), |q| q.as_ref()));
    reader
}

pub struct MpMcastReader<T, Q> {
    _marker: (T, Q),
}

impl<T, Q> MpMcastReader<T, Q> {
    pub fn new(_: DynRef<Q, ()>) -> Self {
        loop {}
    }
}

Further minimized:

struct Arc<T>(T);

impl<T> std::convert::AsRef<()> for Arc<T> {
    fn as_ref(&self) -> &() {
        loop {}
    }
}

struct DynRef<T, U> {
    value: T,
    convert: fn(&T) -> &U,
}
impl<T, U> DynRef<T, U> {
    fn new(_: T, _: fn(&T) -> &U) -> Self {
        loop {}
    }
}

fn f<T>() -> (T, Arc<()>) {
    let reader = new(DynRef::new(Arc(()), |q| q.as_ref()));
    reader
}

fn new<T, Q>(_: DynRef<Q, ()>) -> (T, Q) {
    loop {}
}

Further:

struct Arc<T>(T);

impl<T> Arc<T> {
    fn as_ref(&self) -> &() {
        loop {}
    }
}

struct DynRef<T, U> {
    value: T,
    convert: fn(&T) -> &U,
}

impl<T, U> DynRef<T, U> {
    fn new(_: T, _: fn(&T) -> &U) -> Self {
        loop {}
    }
}

fn f<T>() -> (T, Arc<()>) {
    let reader = new(DynRef::new(Arc(()), |q| q.as_ref()));
    reader
}

fn new<T, Q>(_: DynRef<Q, ()>) -> (T, Q) {
    loop {}
}

Further:

fn f<T>() -> (T, X<()>) {
    let v = h(g(X(()), |w| w.m()));
    v
}

fn g<T, U>(_: T, _: fn(&T) -> &U) -> Y<T, U> {
    loop {}
}

fn h<T, Q>(_: Y<Q, ()>) -> (T, Q) {
    loop {}
}

struct X<T>(T);

impl<T> X<T> {
    fn m(&self) -> &() {
        loop {}
    }
}

struct Y<T, U> {
    a: T,
    b: fn(&T) -> &U,
}

[theemathas]

Bisection using fmease's minimized reproducer:

searched toolchains 96eb6d756bb98a82ae056c384575ff059bf80e8f through 0844f35a32c98882d77e39da8c2a872ec74615cd


********************************************************************************
Regression in 7088e4b63a9516ebfbfe2ab2d999cf01a528ac14
********************************************************************************

Attempting to search unrolled perf builds
ERROR: couldn't find perf build comment

Regressed in #158447. cc @jdonszelmann (author), @lcnr (reviewer)